Determine if a string is collapsible: Difference between revisions

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Determine if a character string is collapsible.

And if so, collapse the string (by removing immediately repeated characters).

If a character string has immediately repeated character(s), the repeated characters are to be deleted (removed), but not the primary (1^st) character(s).

An immediately repeated character is any character that is immediately followed by an identical character (or characters). Another word choice could've been duplicated character, but that might have ruled out (to some readers) triplicated characters ··· or more.

{This Rosetta Code task was inspired by a newly introduced (as of around November 2019) PL/I BIF: collapse.}

Examples

In the following character string:

 The better the 4-wheel drive, the further you'll be from help when ya get stuck!

Only the 2^nd t, e, and l are repeated characters, indicated by underscores (above), even though they (those characters) appear elsewhere in the character string.

So, after collapsing the string, the result would be:

 The beter the 4-whel drive, the further you'l be from help when ya get stuck!

Another example: In the following character string:

 headmistressship

The "collapsed" string would be:

 headmistreship

Task

Write a subroutine/function/procedure/routine··· to locate repeated characters and collapse (delete) them from the character string. The character string can be processed from either direction.

Show all output here, on this page:

the original string and its length
the resultant string and its length
the above strings should be "bracketed" with <<< and >>> (to delineate blanks)
«««Guillemets may be used instead for "bracketing" for the more artistic programmers, shown used here»»»

Use (at least) the following five strings, all strings are length seventy-two (characters, including blanks), except the 1^st string:

 string
 number
        ╔╗
   1    ║╚═══════════════════════════════════════════════════════════════════════╗   ◄■■■■■■  a null string  (length zero)
   2    ║"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ║
   3    ║..1111111111111111111111111111111111111111111111111111111111111117777888║
   4    ║I never give 'em hell, I just tell the truth, and they think it's hell. ║
   5    ║                                                    --- Harry S Truman  ║   ◄■■■■■■  has many repeated blanks
        ╚════════════════════════════════════════════════════════════════════════╝

Other tasks related to string operations:

Metrics

Counting

Remove/replace

Anagrams/Derangements/shuffling

Find/Search/Determine

Formatting

Song lyrics/poems/Mad Libs/phrases

Tokenize

Sequences

11l

Translation of: Kotlin

F collapse(s)
   V cs = ‘’
   V last = Char("\0")
   L(c) s
      I c != last
         cs ‘’= c
         last = c
   R cs

V strings = [
   ‘’,
   ‘"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ’,
   ‘..1111111111111111111111111111111111111111111111111111111111111117777888’,
   ‘I never give 'em hell, I just tell the truth, and they think it's hell. ’,
   ‘                                                   ---  Harry S Truman  ’,
   ‘The better the 4-wheel drive, the further you'll be from help when ya get stuck!’,
   ‘headmistressship’,
   ‘aardvark’
]

L(s) strings
   V c = collapse(s)
   print(‘original  : length = ’s.len‘, string = <<<’s‘>>>’)
   print(‘collapsed : length = ’c.len‘, string = <<<’c‘>>>’)
   print()

Output:

original  : length = 0, string = <<<>>>
collapsed : length = 0, string = <<<>>>

original  : length = 72, string = <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
collapsed : length = 70, string = <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

original  : length = 72, string = <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
collapsed : length = 4, string = <<<.178>>>

original  : length = 72, string = <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
collapsed : length = 69, string = <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

original  : length = 72, string = <<<                                                   ---  Harry S Truman  >>>
collapsed : length = 17, string = <<< - Hary S Truman >>>

original  : length = 80, string = <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
collapsed : length = 77, string = <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>

original  : length = 16, string = <<<headmistressship>>>
collapsed : length = 14, string = <<<headmistreship>>>

original  : length = 8, string = <<<aardvark>>>
collapsed : length = 7, string = <<<ardvark>>>

8080 Assembly

bdos:	equ	5
puts:	equ	9
	org	100h
	jmp	main
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Collapse the $-terminated string at [HL]
colaps:	mov	b,m	; B = last character seen
	inx	h	; First character never collapses
	mov	d,h	; DE = output pointer
	mov	e,l
	mov	a,b	; Empty string?
	cpi	'$'
	rz		; Then do nothing
cloop:	mov	a,m	; Get character
	inx	h	; Advance pointer
	cmp	b	; Same as last one?
	jz	cloop	; Then keep scanning
	mov	b,a	; Otherwise, it is a new one
	stax	d	; Store it,
	inx	d	; and increment output pointer.
	cpi	'$'	; Reached the end?
	jnz	cloop	; If not, next character
	ret 
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Code to print the output in the required format
main:	lxi	h,strs	; List of string pointers
loop:	mov	e,m	; DE = string pointer
	inx	h
	mov	d,m
	inx	h
	mov	a,e	; If zero, end
	ora	d
	rz
	push	h	; Keep the list pointer
	push	d	; Keep the string pointer
	call	brstr	; Print original string in brackets
	pop	h	; Retrieve string pointer
	push	h
	call	colaps	; Collapse the string
	pop	d	; Retrieve string pointer
	call	brstr	; Print the collapsed string in brackets
	pop	h	; Retrieve the list pointer
	jmp	loop	; Next string
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Print string at DE in brackets with length
brstr:	push	d	; Store string pointer
	mvi	c,puts	; Print opening brackets
	lxi	d,open
	call	bdos
	pop	d	; Print original string
	push	d
	mvi	c,puts
	call	bdos
	mvi	c,puts	; Print closing brackets
	lxi	d,close 
	call	bdos
	pop	h
	call	strlen	; Find string length
	mov	a,b
	jmp	prnum	; Print string length
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Figure out the length of a string (max 255)
strlen:	mvi	b,0	; Counter
	mvi	a,'$'	; Terminator
sloop:	cmp	m	; Reached the end yet?
	rz		; If so, stop
	inr	b	; If not, increment counter,
	inx	h	; and pointer,
	jmp	sloop	; and check next byte.
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Print number in A as decimal
prnum:	lxi	d,num	; End of number string
prdgt:	mvi	b,-1	; Quotient
ploop:	inr	b	; Increment quotient
	sui	10	; Subtract 10
	jnc	ploop	; Subtract until negative
	adi	'0'+10	; Make ASCII digit
	dcx	d	; Store in number string
	stax	d
	mov	a,b	; If more digits,
	ana	a
	jnz	prdgt	; find next digit
	mvi	c,puts	; When done, print string
	jmp	bdos
	;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
	;;;	Formatting strings
open:	db	'<<<$'
close:	db	'>>> $'
	db	'***'
num:	db	13,10,'$'
	;;;	Input strings
strs:	dw	str1,str2,str3,str4,str5,0
str1:	db	'$'	; Empty string
str2:	db	'"If I were two-faced, would I be wearing '
	db	'this one?" --- Abraham Lincoln $'
str3:	db	'..111111111111111111111111111111111111111'
	db	'1111111111111111111111117777888$'
str4:	db	'I never give ',39,'em hell, I just tell the truth, '
	db	'and they think it',39,'s hell. $'
str5:	db	'                                              '
	db	'      --- Harry S Truman  $'

Output:

<<<>>> 0
<<<>>> 0
<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> 72
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> 70
<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> 72
<<<.178>>> 4
<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> 72
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> 69
<<<                                                    --- Harry S Truman  >>> 72
<<< - Hary S Truman >>> 17

Action!

PROC Collapse(CHAR ARRAY in,out)
  BYTE i,j
  CHAR c

  j=1 c=0
  FOR i=1 TO in(0)
  DO
    IF in(i)#c THEN
      c=in(i)
      out(j)=c
      j==+1
    FI
  OD
  out(0)=j-1
RETURN

PROC Test(CHAR ARRAY s)
  CHAR ARRAY c(100)
  BYTE CH=$02FC ;Internal hardware value for last key pressed

  Collapse(s,c)
  PrintF("<<<%S>>> (len=%B)%E",s,s(0))
  PrintF("<<<%S>>> (len=%B)%E",c,c(0))
  PutE()
  PrintE("Press any key to continue")
  PutE()

  DO UNTIL CH#$FF OD
  CH=$FF
RETURN

PROC Main()
  Test("")
  Test("""If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln ")
  Test("..1111111111111111111111111111111111111111111111111111111111111117777888")
  Test("I never give 'em hell, I just tell the truth, and they think it's hell. ")
  Test("                                                    --- Harry S Truman  ")
RETURN

Output:

Screenshot from Atari 8-bit computer

<<<>>> (len=0)
<<<>>> (len=0)

<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> (len=72)
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> (len=70)

<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (len=72)
<<<.178>>> (len=4)

<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (len=72)
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (len=69)

<<<                                                    --- Harry S Truman  >>> (len=72)
<<< - Hary S Truman >>> (len=17)

Ada

with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Collapsible is
   procedure Collapse (S : in String) is
      Res : String (1 .. S'Length);
      Len : Natural := 0;
   begin
      Put_Line ("Input  = <<<" & S & ">>>, length =" & S'Length'Image);
      for I in S'Range loop
         if Len = 0 or else S(I) /= Res(Len) then
            Len := Len + 1;
            Res(Len) := S(I);
         end if;
      end loop;
      Put_Line ("Output = <<<" & Res (1 .. Len) & ">>>, length =" & Len'Image);
   end Collapse;
begin
   Collapse ("");
   Collapse ("""If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln ");
   Collapse ("..1111111111111111111111111111111111111111111111111111111111111117777888");
   Collapse ("I never give 'em hell, I just tell the truth, and they think it's hell. ");
   Collapse ("                                                    --- Harry S Truman  ");
end Test_Collapsible;

Output:

Input  = <<<>>>, length = 0
Output = <<<>>>, length = 0
Input  = <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>, length = 72
Output = <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>, length = 70
Input  = <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>, length = 72
Output = <<<.178>>>, length = 4
Input  = <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>, length = 72
Output = <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>, length = 69
Input  = <<<                                                    --- Harry S Truman  >>>, length = 72
Output = <<< - Hary S Truman >>>, length = 17

ALGOL 68

BEGIN
    # returns a collapsed version of s                                    #
    #         i.e. s with adjacent duplicate characters removed           #
    PROC collapse = ( STRING s )STRING:
        IF s = ""
        THEN ""   # empty string                                          #
        ELSE      # non-empty string                                      #
            [ LWB s : UPB s ]CHAR result;
            INT     r pos   := LWB result;
            result[ r pos ] := s[ LWB s ];
            FOR s pos FROM LWB s + 1 TO UPB s DO
                IF result[ r pos ] /= s[ s pos ] THEN
                   r pos +:= 1;
                   result[ r pos ] := s[ s pos ]
                FI
            OD;
            result[ LWB result : r pos ]
        FI # callapse # ;   
    # task test cases                                                     #
    []STRING tests = ( ""
                     , """If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln "
                     , "..1111111111111111111111111111111111111111111111111111111111111117777888"
                     , "I never give 'em hell, I just tell the truth, and they think it's hell. "
                     , "                                                    --- Harry S Truman  "
                     );
    FOR t pos FROM LWB tests TO UPB tests DO
        STRING s = tests[ t pos ];
        STRING c = collapse( s );
        print( ( "       <<<", s, ">>> (length ", whole( ( UPB s + 1 ) - LWB s, 0 ), ")", newline ) );
        print( ( "result <<<", c, ">>> (length ", whole( ( UPB c + 1 ) - LWB c, 0 ), ")", newline ) )
    OD
END

Output:

       <<<>>> (length 0)
result <<<>>> (length 0)
       <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> (length 72)
result <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> (length 70)
       <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (length 72)
result <<<.178>>> (length 4)
       <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (length 72)
result <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (length 69)
       <<<                                                    --- Harry S Truman  >>> (length 72)
result <<< - Hary S Truman >>> (length 17)

APL

Works with: Dyalog APL

task←{
   ⍝ Collapse a string
   collapse←{(1,¯1↓⍵≠1⌽⍵)/⍵}

   ⍝ Given a function ⍺⍺, display a string in brackets,
   ⍝ along with its length, and do the same for the result
   ⍝ of applying ⍺⍺ to the string.
   display←{
      bracket←{(⍕⍴⍵),' «««',⍵,'»»»'}
      ↑(⊂bracket ⍵),(⊂bracket ⍺⍺ ⍵)
   }

   ⍝ Strings from the task
   s1←''
   s2←'"If I were two-faced, would I be wearing this one?"'
   s2,←' --- Abraham Lincoln '
   s3←'..1111111111111111111111111111111111111111111111111'
   s3,←'111111111111117777888'
   s4←'I never give ''em hell, I just tell the truth, '
   s4,←'and they think it''s hell. '
   s5←'                                             '
   s5,←'       --- Harry S Truman  '
   strs←s1 s2 s3 s4 s5

   ⍝ Collapse each string and display it as specified
   ↑collapse display¨ strs
}

Output:

72 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
70 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»  
                                                                                 
72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
4 «««.178»»»                                                                     
                                                                                 
72 «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
69 «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»   
                                                                                 
72 «««                                                    --- Harry S Truman  »»»
17 ««« - Hary S Truman »»»

Arturo

lines: [
    {::}
    {:"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln :}
    {:..1111111111111111111111111111111111111111111111111111111111111117777888:}
    {:I never give 'em hell, I just tell the truth, and they think it's hell. :}
    {:                                                    --- Harry S Truman  :}
]

loop lines 'line ->
    print squeeze line

Output:

"If I were two-faced, would I be wearing this one?" - Abraham Lincoln 
.178
I never give 'em hel, I just tel the truth, and they think it's hel. 
 - Hary S Truman

AutoHotkey

collapsible_string(str){
	for i, ch in StrSplit(str){
		if (ch <> prev)
			res .= ch
		prev := ch
	}
	return "original string:`t" StrLen(str) " characters`t«««" str "»»»`nresultant string:`t" StrLen(res) " characters`t«««" res "»»»"
}

Examples:

data := [""
	, """If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln "
	, "..1111111111111111111111111111111111111111111111111111111111111117777888"
	, "I never give 'em hell, I just tell the truth, and they think it's hell. "
	, "                                                    --- Harry S Truman  "]
for i, v in data
	MsgBox % := collapsible_string(v)
return

Outputs:

-----------------------------------------
original string:	0 characters	«««»»»
resultant string:	0 characters	«««»»»
-----------------------------------------
original string:	72 characters	«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
resultant string:	70 characters	«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
-----------------------------------------
original string:	72 characters	«««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
resultant string:	4 characters	«««.178»»»
-----------------------------------------
original string:	72 characters	«««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
resultant string:	69 characters	«««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
-----------------------------------------
original string:	72 characters	«««                                                    --- Harry S Truman  »»»
resultant string:	17 characters	««« - Hary S Truman »»»
-----------------------------------------

AWK

# syntax: GAWK -f DETERMINE_IF_A_STRING_IS_COLLAPSIBLE.AWK
BEGIN {
    for (i=1; i<=9; i++) {
      for (j=1; j<=i; j++) {
        arr[0] = arr[0] i
      }
    }
    arr[++n] = ""
    arr[++n] = "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
    arr[++n] = "..1111111111111111111111111111111111111111111111111111111111111117777888"
    arr[++n] = "I never give 'em hell, I just tell the truth, and they think it's hell. "
    arr[++n] = "                                                    --- Harry S Truman  "
    arr[++n] = "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
    arr[++n] = "headmistressship"
    for (i=0; i<=n; i++) {
      main(arr[i])
    }
    exit(0)
}
function main(str,  c,i,new_str,prev_c) {
    for (i=1; i<=length(str); i++) {
      c = substr(str,i,1)
      if (prev_c != c) {
        prev_c = c
        new_str = new_str c
      }
    }
    printf("old: %2d <<<%s>>>\n",length(str),str)
    printf("new: %2d <<<%s>>>\n\n",length(new_str),new_str)
}

Output:

old: 45 <<<122333444455555666666777777788888888999999999>>>
new:  9 <<<123456789>>>

old:  0 <<<>>>
new:  0 <<<>>>

old: 72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
new: 70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

old: 72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
new:  4 <<<.178>>>

old: 72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
new: 69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

old: 72 <<<                                                    --- Harry S Truman  >>>
new: 17 <<< - Hary S Truman >>>

old: 80 <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
new: 77 <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>

old: 16 <<<headmistressship>>>
new: 14 <<<headmistreship>>>

BaCon

DATA ""
DATA "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
DATA "..1111111111111111111111111111111111111111111111111111111111111117777888"
DATA "I never give 'em hell, I just tell the truth, and they think it's hell. "
DATA "                                                    --- Harry S Truman  "
DATA "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
DATA "headmistressship"

DOTIMES 7
    READ x$
    found = 0
    PRINT "<<<", x$, ">>> - length: ", LEN(x$)
    PRINT "<<<";
    FOR y = 1 TO LEN(x$)
        IF MID$(x$, y, 1) <> MID$(x$, y+1, 1) THEN
            PRINT MID$(x$, y, 1);
            INCR found
        ENDIF
    NEXT
    PRINT ">>> - length: ", found
DONE

Output:

<<<>>> - length: 0
<<<>>> - length: 0
<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> - length: 72
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> - length: 70
<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> - length: 72
<<<.178>>> - length: 4
<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> - length: 72
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> - length: 69
<<<                                                    --- Harry S Truman  >>> - length: 72
<<< - Hary S Truman >>> - length: 17
<<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>> - length: 80
<<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>> - length: 77
<<<headmistressship>>> - length: 16
<<<headmistreship>>> - length: 14

BASIC

10 READ N%
20 FOR A% = 1 TO N%
30 READ I$: GOSUB 100
40 PRINT LEN(I$); "<<<"; I$; ">>>"
50 PRINT LEN(O$); "<<<"; O$; ">>>"
55 PRINT
60 NEXT
70 END
100 REM Collapse I$ into O$
105 IF I$="" THEN O$=I$: RETURN
110 O$=SPACE$(LEN(I$))
120 P$=LEFT$(I$,1)
130 MID$(O$,1,1)=P$
140 O%=2
150 FOR I%=2 TO LEN(I$)
160 C$=MID$(I$,I%,1)
170 IF P$<>C$ THEN MID$(O$,O%,1)=C$: O%=O%+1: P$=C$
180 NEXT
190 O$=LEFT$(O$,O%-1)
200 RETURN
400 DATA 5: REM There are 5 strings
410 DATA ""
420 DATA "'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln "
430 DATA "..1111111111111111111111111111111111111111111111111111111111111117777888"
440 DATA "I never give 'em hell, I just tell the truth, and they think it's hell. "
450 DATA "                                                    --- Harry S Truman  "

Output:

 0 <<<>>>
 0 <<<>>>

 72 <<<'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln>>>
 70 <<<'If I were two-faced, would I be wearing this one?' - Abraham Lincoln >>>

 72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
 4 <<<.178>>>

 72 <<<I never give 'em hell, I just tell the truth, and they think it's hell.>>>
 69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

 72 <<<                                                    --- Harry S Truman>>>
 17 <<< - Hary S Truman >>>

BCPL

get "libhdr"

// Collapse a string
let collapse(in, out) = valof
$(  let o = 0
    for i = 1 to in%0
        unless i>1 & in%i = in%(i-1)
        $(  o := o + 1
            out%o := in%i
        $)
    out%0 := o
    resultis out
$)

// Print string with brackets and length
let brackets(s) be
    writef("%N: <<<%S>>>*N", s%0, s)

// Print original and collapsed version
let show(s) be
$(  let v = vec 1+255/BYTESPERWORD
    brackets(s)
    brackets(collapse(s, v))
    wrch('*N')
$)

let start() be
$(  show("")
    show("*"If I were two-faced, would I be wearing this one?*" --- Abraham Lincoln ")
    show("..1111111111111111111111111111111111111111111111111111111111111111111788")
    show("I never give 'em hell, I just tell the truth, and they think it's hell. ")
    show("                                                     --- Harry S Truman ")
$)

Output:

0: <<<>>>
0: <<<>>>

72: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72: <<<..1111111111111111111111111111111111111111111111111111111111111111111788>>>
4: <<<.178>>>

72: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72: <<<                                                     --- Harry S Truman >>>
17: <<< - Hary S Truman >>>

BQN

The base function is very simple: Keep adjacent unequal characters, make sure first character is always preserved.

Collapse←(≠↑1∾1↓≠⟜»)⊸/

>{ 
  c←Collapse 𝕩
  ⟨⟨≠𝕩 ⋄ "<<<"∾𝕩∾">>>"⟩≍⟨≠c ⋄ "<<<"∾c∾">>>"⟩⟩
}¨⟨
  "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
  "headmistressship"
  ""
  """If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln "
  "..1111111111111111111111111111111111111111111111111111111111111117777888"
  "I never give 'em hell, I just tell the truth, and they think it's hell. "
  "                                                    --- Harry S Truman  " 
⟩

┌─                                                                                                 
╵ ┌─                                                                                               
  ╵ 80 "<<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>"    
    77 "<<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>"       
                                                                                                ┘  
  ┌─                                                                                               
  ╵ 16 "<<<headmistressship>>>"                                                                    
    14 "<<<headmistreship>>>"                                                                      
                                ┘                                                                  
  ┌─                                                                                               
  ╵ 0 "<<<>>>"                                                                                     
    0 "<<<>>>"                                                                                     
               ┘                                                                                   
  ┌─                                                                                               
  ╵ 72 "<<<""If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln >>>"          
    70 "<<<""If I were two-faced, would I be wearing this one?"" - Abraham Lincoln >>>"            
                                                                                          ┘        
  ┌─                                                                                               
  ╵ 72 "<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>"            
    4  "<<<.178>>>"                                                                                
                                                                                        ┘          
  ┌─                                                                                               
  ╵ 72 "<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>"            
    69 "<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>"               
                                                                                        ┘          
  ┌─                                                                                               
  ╵ 72 "<<<                                                    --- Harry S Truman  >>>"            
    17 "<<< - Hary S Truman >>>"                                                                   
                                                                                        ┘          
                                                                                                  ┘

Bracmat

  ( colapse
  =   previous
    .   str
      $ ( vap
        $ ( (
            =
              .   !arg:!previous&
                | !arg:?previous
            )
          . !arg
          )
        )
  )
& ( testcolapse
  =   len
    .   (len=.@(!arg:? [?arg)&!arg)
      & out$(str$(««« !arg "»»» " len$!arg))
      & colapse$!arg:?arg
      & out$(str$(««« !arg "»»» " len$!arg))
  )
& testcolapse$
&   testcolapse
  $ "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
&   testcolapse
  $ "..1111111111111111111111111111111111111111111111111111111111111117777888"
&   testcolapse
  $ "I never give 'em hell, I just tell the truth, and they think it's hell. "
&   testcolapse
  $ "                                                    --- Harry S Truman  "

Output

«««»»» 0
«««»»» 0
«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» 72
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» 70
«««..1111111111111111111111111111111111111111111111111111111111111117777888»»» 72
«««.178»»» 4
«««I never give 'em hell, I just tell the truth, and they think it's hell. »»» 72
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»» 69
«««                                                    --- Harry S Truman  »»» 72
««« - Hary S Truman »»» 17

C

Identical implementation as in Determine_if_a_string_is_squeezable, as both tasks are very similar. The Lincoln quote contains backslashes to accommodate the double quotes via the command line. strcmpi is not part of the C Standard Library, thus comment out the definition in the code if testing on a system where it is already included.

#include<string.h>
#include<stdlib.h>
#include<stdio.h>

#define COLLAPSE 0
#define SQUEEZE 1

typedef struct charList{
    char c;
    struct charList *next;
} charList;

/*
Implementing strcmpi, the case insensitive string comparator, as it is not part of the C Standard Library.

Comment this out if testing on a compiler where it is already defined.
*/

int strcmpi(char* str1,char* str2){
    int len1 = strlen(str1), len2 = strlen(str2), i;

    if(len1!=len2){
        return 1;
    }

    else{
        for(i=0;i<len1;i++){
            if((str1[i]>='A'&&str1[i]<='Z')&&(str2[i]>='a'&&str2[i]<='z')&&(str2[i]-65!=str1[i]))
                return 1;
            else if((str2[i]>='A'&&str2[i]<='Z')&&(str1[i]>='a'&&str1[i]<='z')&&(str1[i]-65!=str2[i]))
                return 1;
            else if(str1[i]!=str2[i])
                return 1;
        }
    }

    return 0;
}

charList *strToCharList(char* str){
    int len = strlen(str),i;

    charList *list, *iterator, *nextChar;

    list = (charList*)malloc(sizeof(charList));
    list->c = str[0];
    list->next = NULL;

    iterator = list;

    for(i=1;i<len;i++){
        nextChar = (charList*)malloc(sizeof(charList));
        nextChar->c = str[i];
        nextChar->next = NULL;

        iterator->next = nextChar;
        iterator = nextChar;
    }

    return list;
}

char* charListToString(charList* list){
    charList* iterator = list;
    int count = 0,i;
    char* str;

    while(iterator!=NULL){
        count++;
        iterator = iterator->next;
    }

    str = (char*)malloc((count+1)*sizeof(char));
    iterator = list;

    for(i=0;i<count;i++){
        str[i] = iterator->c;
        iterator = iterator->next;
    }

    free(list);
    str[i] = '\0';

    return str;
}

char* processString(char str[100],int operation, char squeezeChar){
    charList *strList = strToCharList(str),*iterator = strList, *scout;

    if(operation==SQUEEZE){
        while(iterator!=NULL){
            if(iterator->c==squeezeChar){
                scout = iterator->next;

                while(scout!=NULL && scout->c==squeezeChar){
                        iterator->next = scout->next;
                        scout->next = NULL;
                        free(scout);
                        scout = iterator->next;
                }
            }
            iterator = iterator->next;
        }
    }

    else{
        while(iterator!=NULL && iterator->next!=NULL){
            if(iterator->c == (iterator->next)->c){
                scout = iterator->next;
                squeezeChar = iterator->c;

                while(scout!=NULL && scout->c==squeezeChar){
                        iterator->next = scout->next;
                        scout->next = NULL;
                        free(scout);
                        scout = iterator->next;
                }
            }
            iterator = iterator->next;
        }
    }

    return charListToString(strList);
}

void printResults(char originalString[100], char finalString[100], int operation, char squeezeChar){
    if(operation==SQUEEZE){
        printf("Specified Operation : SQUEEZE\nTarget Character : %c",squeezeChar);
    }

    else
        printf("Specified Operation : COLLAPSE");

    printf("\nOriginal %c%c%c%s%c%c%c\nLength : %d",174,174,174,originalString,175,175,175,(int)strlen(originalString));
    printf("\nFinal    %c%c%c%s%c%c%c\nLength : %d\n",174,174,174,finalString,175,175,175,(int)strlen(finalString));
}

int main(int argc, char** argv){
    int operation;
    char squeezeChar;

    if(argc<3||argc>4){
        printf("Usage : %s <SQUEEZE|COLLAPSE> <String to be processed> <Character to be squeezed, if operation is SQUEEZE>\n",argv[0]);
        return 0;
    }

    if(strcmpi(argv[1],"SQUEEZE")==0 && argc!=4){
        scanf("Please enter characted to be squeezed : %c",&squeezeChar);
        operation = SQUEEZE;
    }

    else if(argc==4){
        operation = SQUEEZE;
        squeezeChar = argv[3][0];
    }

    else if(strcmpi(argv[1],"COLLAPSE")==0){
        operation = COLLAPSE;
    }

    if(strlen(argv[2])<2){
        printResults(argv[2],argv[2],operation,squeezeChar);
    }

    else{
        printResults(argv[2],processString(argv[2],operation,squeezeChar),operation,squeezeChar);
    }
    
    return 0;
}

Output :

C:\My Projects\networks>a collapse "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
Specified Operation : COLLAPSE
Original «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
Length : 80
Final    «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»
Length : 77

C:\My Projects\networks>a collapse headmistressship
Specified Operation : COLLAPSE
Original «««headmistressship»»»
Length : 16
Final    «««headmistreship»»»
Length : 14

C:\My Projects\networks>a collapse ""
Specified Operation : COLLAPSE
Original «««»»»
Length : 0
Final    «««»»»
Length : 0

C:\My Projects\networks>a collapse "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln"
Specified Operation : COLLAPSE
Original «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln»»»
Length : 71
Final    «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln»»»
Length : 69

C:\My Projects\networks>a collapse ..1111111111111111111111111111111111111111111111111111111111111117777888
Specified Operation : COLLAPSE
Original «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
Length : 72
Final    «««.178»»»
Length : 4

C:\My Projects\networks>a collapse "I never give 'em hell, I just tell the truth, and they think it's hell."
Specified Operation : COLLAPSE
Original «««I never give 'em hell, I just tell the truth, and they think it's hell.»»»
Length : 71
Final    «««I never give 'em hel, I just tel the truth, and they think it's hel.»»»
Length : 68

C:\My Projects\networks>a collapse "                                                    --- Harry S Truman"
Specified Operation : COLLAPSE
Original «««                                                    --- Harry S Truman»»»
Length : 70
Final    ««« - Hary S Truman»»»
Length : 16

C#

using System;
using static System.Linq.Enumerable;

public class Program
{
    static void Main()
    {
        string[] input = {
            "",
            "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
            "headmistressship",
            "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
            "..1111111111111111111111111111111111111111111111111111111111111117777888",
            "I never give 'em hell, I just tell the truth, and they think it's hell. ",
            "                                                    --- Harry S Truman  "
        };
        foreach (string s in input) {
            Console.WriteLine($"old: {s.Length} «««{s}»»»");
            string c = Collapse(s);
            Console.WriteLine($"new: {c.Length} «««{c}»»»");
        }
    }

    static string Collapse(string s) => string.IsNullOrEmpty(s) ? "" :
        s[0] + new string(Range(1, s.Length - 1).Where(i => s[i] != s[i - 1]).Select(i => s[i]).ToArray());
}

Output:

old: 0 «««»»»
new: 0 «««»»»
old: 80 «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
new: 77 «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»
old: 16 «««headmistressship»»»
new: 14 «««headmistreship»»»
old: 72 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
new: 70 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
old: 72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
new: 4 «««.178»»»
old: 72 «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
new: 69 «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
old: 72 «««                                                    --- Harry S Truman  »»»
new: 17 ««« - Hary S Truman »»»

C++

The solution is a straightforward application of the standard library function "unique".

#include <string>
#include <iostream>
#include <algorithm>

template<typename char_type>
std::basic_string<char_type> collapse(std::basic_string<char_type> str) {
    auto i = std::unique(str.begin(), str.end());
    str.erase(i, str.end());
    return str;
}

void test(const std::string& str) {
    std::cout << "original string: <<<" << str << ">>>, length = " << str.length() << '\n';
    std::string collapsed(collapse(str));
    std::cout << "result string: <<<" << collapsed << ">>>, length = " << collapsed.length() << '\n';
    std::cout << '\n';
}

int main(int argc, char** argv) {
    test("");
    test("\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ");
    test("..1111111111111111111111111111111111111111111111111111111111111117777888");
    test("I never give 'em hell, I just tell the truth, and they think it's hell. ");
    test("                                                    --- Harry S Truman  ");
    return 0;
}

Output:

original string: <<<>>>, length = 0
result string: <<<>>>, length = 0

original string: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>, length = 72
result string: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>, length = 70

original string: <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>, length = 72
result string: <<<.178>>>, length = 4

original string: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>, length = 72
result string: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>, length = 69

original string: <<<                                                    --- Harry S Truman  >>>, length = 72
result string: <<< - Hary S Truman >>>, length = 17

Clojure

(defn collapse [s]
  (let [runs (partition-by identity s)]
    (apply str (map first runs))))

(defn run-test [s]
  (let [out (collapse s)]
    (str (format "Input: <<<%s>>> (len %d)\n" s (count s))
         (format "becomes: <<<%s>>> (len %d)\n" out (count out)))))

Output:

Input:   <<<>>> (len 0)
becomes: <<<>>> (len 0)
Input:   <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> (len 72)
becomes: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> (len 70)
Input:   <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (len 72)
becomes: <<<.178>>> (len 4)
Input:   <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (len 72)
becomes: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (len 69)
Input:   <<<                                                    --- Harry S Truman  >>> (len 72)
becomes: <<< - Hary S Truman >>> (len 17)

CLU

% Collapse a string 
collapse = proc (s: string) returns (string)
    out: array[char] := array[char]$[]
    last: char := '\000'
    
    for c: char in string$chars(s) do
        if c ~= last then
            last := c
            array[char]$addh(out,c)
        end
    end
    return (string$ac2s(out))
end collapse

% Show a string in brackets, with its length
brackets = proc (s: string)
    stream$putl(stream$primary_output(), 
                    int$unparse(string$size(s)) 
                 || " <<<"
                 || s
                 || ">>>")
end brackets

% Show a string and its collapsed version, and the corresponding lengths
show = proc (s: string)
    brackets(s)
    brackets(collapse(s))
    stream$putl(stream$primary_output(), "")
end show

% Try the examples from the task description
start_up = proc () 
    examples: array[string] := array[string]$[
        "",
        "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                    --- Harry S Truman  "
    ]
    
    for ex: string in array[string]$elements(examples) do
        show(ex)
    end
end start_up

Output:

0 <<<>>>
0 <<<>>>

72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
4 <<<.178>>>

72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72 <<<                                                    --- Harry S Truman  >>>
17 <<< - Hary S Truman >>>

Cowgol

include "cowgol.coh";
include "strings.coh";

# Collapse the string at in, and store the result in the given buffer
sub collapse(in: [uint8], out: [uint8]) is
    var ch := [in];
    in := @next in; 
    loop
        if ch == 0 then
            [out] := 0;
            return;
        elseif [in] != ch then
            [out] := ch;
            out := @next out;
            ch := [in];
        end if;
        in := @next in;
    end loop;
end sub;

# Given a string, collapse it and print all required output
sub show(str: [uint8]) is
    sub bracket_length(str: [uint8]) is
        print_i32(StrLen(str) as uint32);
        print(" <<<");
        print(str);
        print(">>>");
        print_nl();
    end sub;
    
    var buf: uint8[256];
    collapse(str, &buf[0]);
    bracket_length(str);
    bracket_length(&buf[0]);
    print_nl();
end sub;

# Strings from the task
var strings: [uint8][] := {
"",
"\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
"..1111111111111111111111111111111111111111111111111111111111111117777888",
"I never give 'em hell, I just tell the truth, and they think it's hell. ",
"                                                    --- Harry S Truman  "
};

# Collapse and print each string
var i: @indexof strings := 0;
while i < @sizeof strings loop
    show(strings[i]);
    i := i + 1;
end loop;

Output:

0 <<<>>>
0 <<<>>>

72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
4 <<<.178>>>

72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72 <<<                                                    --- Harry S Truman  >>>
17 <<< - Hary S Truman >>>

D

import std.stdio;

void collapsible(string s) {
    writeln("old: <<<", s, ">>>, length = ", s.length);

    write("new: <<<");
    char last = '\0';
    int len = 0;
    foreach (c; s) {
        if (c != last) {
            write(c);
            len++;
        }
        last = c;
    }
    writeln(">>>, length = ", len);

    writeln;
}

void main() {
    collapsible(``);
    collapsible(`"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln `);
    collapsible(`..1111111111111111111111111111111111111111111111111111111111111117777888`);
    collapsible(`I never give 'em hell, I just tell the truth, and they think it's hell. `);
    collapsible(`                                                    --- Harry S Truman  `);
}

Output:

old: <<<>>>, length = 0
new: <<<>>>, length = 0

old: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>, length = 72
new: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>, length = 70

old: <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>, length = 72
new: <<<.178>>>, length = 4

old: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>, length = 72
new: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>, length = 69

old: <<<                                                    --- Harry S Truman  >>>, length = 72
new: <<< - Hary S Truman >>>, length = 17

Delphi

Library: System.SysUtils

Translation of: D

program Determine_if_a_string_is_collapsible;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

procedure collapsible(s: string);
var
  c, last: char;
  len: Integer;
begin
  writeln('old: <<<', s, '>>>, length = ', s.length);
  write('new: <<<');
  last := #0;
  len := 0;
  for c in s do
  begin
    if c <> last then
    begin
      write(c);
      inc(len);
    end;
    last := c;
  end;
  writeln('>>>, length = ', len, #10);
end;

begin
  collapsible('');
  collapsible('"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ');
  collapsible('..1111111111111111111111111111111111111111111111111111111111111117777888');
  collapsible('I never give ''em hell, I just tell the truth, and they think it''s hell. ');
  collapsible('                                                    --- Harry S Truman  ');
  readln;
end.

EasyLang

func$ collapse s$ .
   for c$ in strchars s$
      if c$ <> cc$
         r$ &= c$
      .
      cc$ = c$
   .
   return r$
.
s$[] &= ""
s$[] &= "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
s$[] &= "..1111111111111111111111111111111111111111111111111111111111111117777888"
s$[] &= "I never give 'em hell, I just tell the truth, and they think it's hell. "
s$[] &= "                                                    --- Harry S Truman  "
for s$ in s$[]
   print "«««" & s$ & "»»» (" & len s$ & ")"
   r$ = collapse s$
   print "«««" & r$ & "»»» (" & len r$ & ")"
   print ""
.

F#

// Collapse a String. Nigel Galloway: June 9th., 2020
//As per the task description a function which 'determines if a character string is collapsible' by testing if any consecutive characters are the same.
let isCollapsible n=n|>Seq.pairwise|>Seq.tryFind(fun(n,g)->n=g)
//As per the task description a function which 'if the string is collapsable, collapses the string (by removing immediately repeated characters).
let collapse n=match isCollapsible n with
                Some _->let i=Seq.head n
                        let fN=let mutable g=i in (fun n->if n=g then false else g<-n; true)
                        let g=System.String([|yield i;yield! Seq.tail n|>Seq.filter fN|])
                        printfn "<<<%s>>> (length %d) colapses to <<<%s>>> (length %d)" n n.Length g g.Length
               |     _->printfn "<<<%s>>> (length %d) does not colapse" n n.Length

collapse ""
collapse "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
collapse "..1111111111111111111111111111111111111111111111111111111111111117777888"
collapse "I never give 'em hell, I just tell the truth, and they think it's hell. "
collapse "                                                    --- Harry S Truman  "
collapse "withoutConsecutivelyRepeatedCharacters"

Output:

<<<>>> (length 0) does not colapse
<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> (length 72) collapses to <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> (length 70)
<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (length 72) collapses to <<<.178>>> (length 4)
<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (length 72) collapses to <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (length 69)
<<<                                                    --- Harry S Truman  >>> (length 72) collapses to <<< - Hary S Truman >>> (length 17)
<<<withoutConsecutivelyRepeatedCharacters>>> (length 38) does not collapse

Factor

USING: formatting io kernel sbufs sequences strings ;
IN: rosetta-code.string-collapse

: (collapse) ( str -- str )
    unclip-slice 1string >sbuf
    [ over last over = [ drop ] [ suffix! ] if ] reduce >string ;

: collapse ( str -- str ) [ "" ] [ (collapse) ] if-empty ;

: .str ( str -- ) dup length "«««%s»»» (length %d)\n" printf ;

: show-collapse ( str -- )
    [ "Before collapse: " write .str ]
    [ "After  collapse: " write collapse .str ] bi nl ;

: collapse-demo ( -- )
    {
        ""
        "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
        "..1111111111111111111111111111111111111111111111111111111111111117777888"
        "I never give 'em hell, I just tell the truth, and they think it's hell. "
        "                                                   ---  Harry S Truman  "
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
        "headmistressship"
        "aardvark"
        "😍😀🙌💃😍😍😍🙌"
    } [ show-collapse ] each ;

MAIN: collapse-demo

Output:

(Using some extra test cases from the Go entry.)

Before collapse: «««»»» (length 0)
After  collapse: «««»»» (length 0)

Before collapse: «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» (length 72)
After  collapse: «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» (length 70)

Before collapse: «««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (length 72)
After  collapse: «««.178»»» (length 4)

Before collapse: «««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (length 72)
After  collapse: «««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (length 69)

Before collapse: «««                                                   ---  Harry S Truman  »»» (length 72)
After  collapse: ««« - Hary S Truman »»» (length 17)

Before collapse: «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»» (length 80)
After  collapse: «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»» (length 77)

Before collapse: «««headmistressship»»» (length 16)
After  collapse: «««headmistreship»»» (length 14)

Before collapse: «««aardvark»»» (length 8)
After  collapse: «««ardvark»»» (length 7)

Before collapse: «««😍😀🙌💃😍😍😍🙌»»» (length 8)
After  collapse: «««😍😀🙌💃😍🙌»»» (length 6)

FreeBASIC

Translation of: BASIC

Const numCad = 5
Data ""
Data "'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln "
Data "..1111111111111111111111111111111111111111111111111111111111111117777888"
Data "I never give 'em hell, I just tell the truth, and they think it's hell. "
Data "                                                    --- Harry S Truman  "

Dim Shared As String cadIN, cadOUT

Sub Collapse
    Dim As String a, b
    If cadIN = "" Then cadOUT = cadIN: Exit Sub
    cadOUT = Space(Len(cadIN))
    a = Left(cadIN,1)
    Mid(cadOUT,1,1) = a
    Dim As Integer txtOUT = 2
    For i As Integer = 2 To Len(cadIN)
        b = Mid(cadIN,i,1)
        If a <> b Then Mid(cadOUT,txtOUT,1) = b: txtOUT += 1: a = b
    Next i
    cadOUT = Left(cadOUT,txtOUT-1)
End Sub

For j As Integer = 1 To numCad
    Read cadIN
    Collapse
    Print " <<<"; cadIN; ">>> (longitud "; Len(cadIN); _
    !")\n    se pliega a:\n <<<"; cadOUT; ">>> (longitud "; Len(cadOUT); !")\n"
Next j
Sleep

Output:

 <<<>>> (longitud  0)
    se pliega a:
 <<<>>> (longitud  0)

 <<<'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln >>> (longitud  72)
    se pliega a:
 <<<'If I were two-faced, would I be wearing this one?' - Abraham Lincoln >>> (longitud  70)

 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (longitud  72)
    se pliega a:
 <<<.178>>> (longitud  4)

 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (longitud  72)
    se pliega a:
 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (longitud  69)

 <<<                                                    --- Harry S Truman  >>> (longitud  72)
    se pliega a:
 <<< - Hary S Truman >>> (longitud  17)

Frink

collapse[str] := str =~ %s/(.)\1+/$1/g

lines = ["",
         """"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln """,
"..1111111111111111111111111111111111111111111111111111111111111117777888",
"I never give 'em hell, I just tell the truth, and they think it's hell. ",
"                                                    --- Harry S Truman  "]

for line = lines
   println[collapse[line]]

Output:


"If I were two-faced, would I be wearing this one?" - Abraham Lincoln 
.178
I never give 'em hel, I just tel the truth, and they think it's hel. 
 - Hary S Truman

FutureBasic

local fn CollapseString( inString as CFStringRef ) as CFStringRef
  CFMutableStringRef outString = fn MutableStringWithCapacity(0)
  long index
  unichar prevChar = 0, currChar
  
  for index = 0 to len(inString) - 1
    currChar = fn StringCharacterAtIndex( inString, index )
    if ( currChar != prevChar ) then MutableStringAppendFormat( outString, @"%C", currChar )
    prevChar = currChar
  next
end fn = outString

window 1, @"Collapse String", (0,0,600,300)

CFStringRef string, collapsedString

string = @""
collapsedString = fn CollapseString( string )
printf @"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

string = @"\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
collapsedString = fn CollapseString( string )
printf@"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

string = @"..1111111111111111111111111111111111111111111111111111111111111117777888"
collapsedString = fn CollapseString( string )
printf@"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

string = @"I never give 'em hell, I just tell the truth, and they think it's hell. "
collapsedString = fn CollapseString( string )
printf@"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

string = @"                                                    --- Harry S Truman  "
collapsedString = fn CollapseString( string )
printf@"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

string = @"\"AAAAAll that glitters is not goldDDDD.\" - William Shakespeare"
collapsedString = fn CollapseString( string )
printf@"<<<%@>>> %ld\n<<<%@>>> %ld\n",string,len(string),collapsedString,len(collapsedString)

HandleEvents

Output:

<<<>>> 0
<<<>>> 0

<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> 72
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> 70

<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> 72
<<<.178>>> 4

<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> 72
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> 69

<<<                                                    --- Harry S Truman  >>> 72
<<< - Hary S Truman >>> 17

<<<"AAAAAll that glitters is not goldDDDD." - William Shakespeare>>> 62
<<<"Al that gliters is not goldD." - Wiliam Shakespeare>>> 52

Go

package main

import "fmt"

// Returns collapsed string, original and new lengths in
// unicode code points (not normalized).
func collapse(s string) (string, int, int) {
    r := []rune(s)
    le, del := len(r), 0
    for i := le - 2; i >= 0; i-- {
        if r[i] == r[i+1] {
            copy(r[i:], r[i+1:])
            del++
        }
    }
    if del == 0 {
        return s, le, le
    }
    r = r[:le-del]
    return string(r), le, len(r)
}
    
func main() {
    strings:= []string {
        "",
        `"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln `,
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌",
    }
    for _, s := range strings {
        cs, olen, clen := collapse(s)
        fmt.Printf("original : length = %2d, string = «««%s»»»\n", olen, s)
        fmt.Printf("collapsed: length = %2d, string = «««%s»»»\n\n", clen, cs)
    }
}

Output:

original : length =  0, string = «««»»»
collapsed: length =  0, string = «««»»»

original : length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
collapsed: length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

original : length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
collapsed: length =  4, string = «««.178»»»

original : length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
collapsed: length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

original : length = 72, string = «««                                                   ---  Harry S Truman  »»»
collapsed: length = 17, string = ««« - Hary S Truman »»»

original : length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
collapsed: length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

original : length = 16, string = «««headmistressship»»»
collapsed: length = 14, string = «««headmistreship»»»

original : length =  8, string = «««aardvark»»»
collapsed: length =  7, string = «««ardvark»»»

original : length =  8, string = «««😍😀🙌💃😍😍😍🙌»»»
collapsed: length =  6, string = «««😍😀🙌💃😍🙌»»»

Groovy

Translation of: Java

class StringCollapsible {
    static void main(String[] args) {
        for ( String s : [
                "",
                "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
                "..1111111111111111111111111111111111111111111111111111111111111117777888",
                "I never give 'em hell, I just tell the truth, and they think it's hell. ",
                "                                                    --- Harry S Truman  ",
                "122333444455555666666777777788888888999999999",
                "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
                "headmistressship"]) {
            String result = collapse(s)
            System.out.printf("old:  %2d <<<%s>>>%nnew:  %2d <<<%s>>>%n%n", s.length(), s, result.length(), result)
        }
    }

    private static String collapse(String input) {
        StringBuilder sb = new StringBuilder()
        for ( int i = 0 ; i < input.length() ; i++ ) {
            if ( i == 0 || input.charAt(i-1) != input.charAt(i) ) {
                sb.append(input.charAt(i))
            }
        }
        return sb.toString()
    }
}

Output:

old:   0 <<<>>>
new:   0 <<<>>>

old:  72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
new:  70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

old:  72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
new:   4 <<<.178>>>

old:  72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
new:  69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

old:  72 <<<                                                    --- Harry S Truman  >>>
new:  17 <<< - Hary S Truman >>>

old:  45 <<<122333444455555666666777777788888888999999999>>>
new:   9 <<<123456789>>>

old:  80 <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
new:  77 <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>

old:  16 <<<headmistressship>>>
new:  14 <<<headmistreship>>>

Haskell

import Text.Printf (printf)
import Data.Maybe (catMaybes)
import Control.Monad (guard)

input :: [String]
input = [ ""
        , "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
        , "headmistressship"
        , "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
        , "..1111111111111111111111111111111111111111111111111111111111111117777888"
        , "I never give 'em hell, I just tell the truth, and they think it's hell. "
        , "                                                    --- Harry S Truman  "
        , "😍😀🙌💃😍😍😍🙌"
        ]

collapse :: Eq a => [a] -> [a]
collapse = catMaybes . (\xs -> zipWith (\a b -> guard (a /= b) >> a) (Nothing : xs) (xs <> [Nothing])) . map Just

main :: IO ()
main = 
  mapM_ (\(a, b) -> printf "old: %3d «««%s»»»\nnew: %3d «««%s»»»\n\n" (length a) a (length b) b) 
  $ ((,) <*> collapse) <$> input

Output:

old:   0 «««»»»
new:   0 «««»»»

old:  80 «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
new:  77 «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

old:  16 «««headmistressship»»»
new:  14 «««headmistreship»»»

old:  72 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
new:  70 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

old:  72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
new:   4 «««.178»»»

old:  72 «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
new:  69 «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

old:  72 «««                                                    --- Harry S Truman  »»»
new:  17 ««« - Hary S Truman »»»

old:   8 «««😍😀🙌💃😍😍😍🙌»»»
new:   6 «««😍😀🙌💃😍🙌»»»

Note that we can also directly define a predicate, and a rewrite, in terms of Data.List group,

import Data.List (group)

isCollapsible :: String -> Bool
isCollapsible = any ((1 <) . length) . group

collapsed :: String -> String
collapsed = map head . group

or, without imports, in terms of pattern matching:

isCollapsible :: String -> Bool
isCollapsible [] = False
isCollapsible [_] = False
isCollapsible (h:t@(x:_)) = h == x || isCollapsible t

collapsed :: String -> String
collapsed [] = []
collapsed [x] = [x]
collapsed (h:t@(x:_))
  | h == x = collapsed t
  | otherwise = h : collapsed t

J

   STRINGS =: <;._2]0 :0

"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln 
..1111111111111111111111111111111111111111111111111111111111111117777888
I never give 'em hell, I just tell the truth, and they think it's hell. 
                                                    --- Harry S Truman  
)

   collapse =: (#~ (1 , 2 ~:/\ ])) ::(''"_)  NB. copy dissimilar neighbors
   assert 1 2 3 2 3 1 2 1 -: collapse 1 2 3 2 2 2 2 3 1 1 2 2 1   NB. test

   task =: ,&(<@:(;~ (_6 + #)))&('<<<' , '>>>' ,~ ]) collapse  NB. assemble the output

   task&> STRINGS   NB. operate on the data
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|+-+------+                                                                         |+-+------+                                                                       |
||0|<<<>>>|                                                                         ||0|<<<>>>|                                                                       |
|+-+------+                                                                         |+-+------+                                                                       |
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|+--+------------------------------------------------------------------------------+|+--+----------------------------------------------------------------------------+|
||72|<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>|||70|<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>||
|+--+------------------------------------------------------------------------------+|+--+----------------------------------------------------------------------------+|
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|+--+------------------------------------------------------------------------------+|+-+----------+                                                                   |
||72|<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>|||4|<<<.178>>>|                                                                   |
|+--+------------------------------------------------------------------------------+|+-+----------+                                                                   |
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|+--+------------------------------------------------------------------------------+|+--+---------------------------------------------------------------------------+ |
||72|<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>|||69|<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>| |
|+--+------------------------------------------------------------------------------+|+--+---------------------------------------------------------------------------+ |
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|+--+------------------------------------------------------------------------------+|+--+-----------------------+                                                     |
||72|<<<                                                    --- Harry S Truman  >>>|||17|<<< - Hary S Truman >>>|                                                     |
|+--+------------------------------------------------------------------------------+|+--+-----------------------+                                                     |
+-----------------------------------------------------------------------------------+---------------------------------------------------------------------------------+

Java

//  Title:  Determine if a string is collapsible

public class StringCollapsible {

    public static void main(String[] args) {
        for ( String s : new String[] {
              "", 
              "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ", 
              "..1111111111111111111111111111111111111111111111111111111111111117777888", 
              "I never give 'em hell, I just tell the truth, and they think it's hell. ", 
              "                                                    --- Harry S Truman  ",
              "122333444455555666666777777788888888999999999",
              "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
              "headmistressship"}) {
            String result = collapse(s);
            System.out.printf("old:  %2d <<<%s>>>%nnew:  %2d <<<%s>>>%n%n", s.length(), s, result.length(), result);
        }
    }
    
    private static String collapse(String in) {
        StringBuilder sb = new StringBuilder();
        for ( int i = 0 ; i < in.length() ; i++ ) {
            if ( i == 0 || in.charAt(i-1) != in.charAt(i) ) {
                sb.append(in.charAt(i));
            }
        }
        return sb.toString();
    }

}

Output:

old:   0 <<<>>>
new:   0 <<<>>>

old:  72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
new:  70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

old:  72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
new:   4 <<<.178>>>

old:  72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
new:  69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

old:  72 <<<                                                    --- Harry S Truman  >>>
new:  17 <<< - Hary S Truman >>>

old:  45 <<<122333444455555666666777777788888888999999999>>>
new:   9 <<<123456789>>>

old:  80 <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
new:  77 <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>

old:  16 <<<headmistressship>>>
new:  14 <<<headmistreship>>>

JavaScript

String.prototype.collapse = function() {
  let str = this;
  for (let i = 0; i < str.length; i++) {
    while (str[i] == str[i+1]) str = str.substr(0,i) + str.substr(i+1);
  }
  return str;
}

// testing
let strings = [
  '',
  '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
  '..1111111111111111111111111111111111111111111111111111111111111117777888',
  `I never give 'em hell, I just tell the truth, and they think it's hell. `,
  '                                                    --- Harry S Truman  '
];
for (let i = 0; i < strings.length; i++) {
  let str = strings[i], col = str.collapse();
  console.log(`«««${str}»»» (${str.length})`);
  console.log(`«««${col}»»» (${col.length})`);
}

Output:


«««»»» (0)
«««»»» (0)
«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» (72)
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» (70)
«««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (72)
«««.178»»» (4)
«««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (72)
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (69)
«««                                                    --- Harry S Truman  »»» (72)
««« - Hary S Truman »»» (17)

jq

Here we can use the Unix-like `uniq` function defined on (JSON) arrays to define `collapse` on (JSON) strings:

# Input: an array
# Output: a stream
def uniq: foreach .[] as $x ({x:nan, y:.[0]}; {x:$x, y:.x}; select(.x != .y) | .x);

def collapse: explode | [uniq] | implode;

The program for the given task can now be written in three lines:

def Guillemets: "«««\(.)»»»";

"Original: \(Guillemets) has length \(length)",
(collapse | "Collapsed \(Guillemets) has length \(length)")

Using the following invocation on the five "raw" (i.e. unquoted) strings yields the results as shown:

   jq -Rrf program.jq raw.txt

Original: «««»»» has length 0
Collapsed «««»»» has length 0
Original: «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» has length 72
Collapsed «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» has length 70
Original: «««..1111111111111111111111111111111111111111111111111111111111111117777888»»» has length 72
Collapsed «««.178»»» has length 4
Original: «««I never give 'em hell, I just tell the truth, and they think it's hell. »»» has length 72
Collapsed «««I never give 'em hel, I just tel the truth, and they think it's hel. »»» has length 69
Original: «««                                                    --- Harry S Truman  »»» has length 72
Collapsed ««« - Hary S Truman »»» has length 17

Julia

const teststrings = [
    "",
    """"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln """,
    "..1111111111111111111111111111111111111111111111111111111111111117777888",
    """I never give 'em hell, I just tell the truth, and they think it's hell. """,
    "                                                    --- Harry S Truman  "]

function collapse(s)
    len = length(s)
    if len < 2
        return s, len, s, len
    end
    result = last = s[1]
    for c in s[2:end]
        if c != last
            last = c
            result *= c
        end
    end
    return s, len, result, length(result)
end

function testcollapse(arr)
    for s in arr
        (s1, len1, s2, len2) = collapse(s)
        println("«««$s1»»» (length $len1)\n    collapses to:\n«««$s2»»» (length $len2).\n")
    end
end

testcollapse(teststrings)

Output:

«««»»» (length 0)
    collapses to:
«««»»» (length 0).

«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» (length 72)
    collapses to:
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» (length 70).

«««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (length 72)
    collapses to:
«««.178»»» (length 4).

«««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (length 72)
    collapses to:
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (length 69).

«««                                                    --- Harry S Truman  »»» (length 72)
    collapses to:
««« - Hary S Truman »»» (length 17).

Condensed version:

const teststrings = [
    "",
    """"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln """,
    "..1111111111111111111111111111111111111111111111111111111111111117777888",
    """I never give 'em hell, I just tell the truth, and they think it's hell. """,
    "                                                    --- Harry S Truman  "]

collapse(s) = (t = isempty(s) ? "" : s[1:1]; for c in s if c != t[end] t *= c end; end; t)

for s in teststrings
    n, t = length(s), collapse(s)
    println("«««$s»»» (length $n)\n    collapses to:\n«««$t»»» (length $(length(t))).\n")
end

Kotlin

Translation of: Go

fun collapse(s: String): String {
    val cs = StringBuilder()
    var last: Char = 0.toChar()
    for (c in s) {
        if (c != last) {
            cs.append(c)
            last = c
        }
    }
    return cs.toString()
}

fun main() {
    val strings = arrayOf(
        "",
        "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark"
    )
    for (s in strings) {
        val c = collapse(s)
        println("original : length = ${s.length}, string = «««$s»»»")
        println("collapsed : length = ${c.length}, string = «««$c»»»")
        println()
    }
}

Output:

original : length = 0, string = «««»»»
collapsed : length = 0, string = «««»»»

original : length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
collapsed : length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

original : length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
collapsed : length = 4, string = «««.178»»»

original : length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
collapsed : length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

original : length = 72, string = «««                                                   ---  Harry S Truman  »»»
collapsed : length = 17, string = ««« - Hary S Truman »»»

original : length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
collapsed : length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

original : length = 16, string = «««headmistressship»»»
collapsed : length = 14, string = «««headmistreship»»»

original : length = 8, string = «««aardvark»»»
collapsed : length = 7, string = «««ardvark»»»

Ksh

#!/bin/ksh

# Determine if a string is collapsible (repeated letters)

#	# Variables:
#
typeset -a strings 
strings[0]="" 
strings[1]='"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln' 
strings[2]="..1111111111111111111111111111111111111111111111111111111111111117777888" 
strings[3]="I never give 'em hell, I just tell the truth, and they think it's hell." 
strings[4]="                                                    --- Harry S Truman"

typeset -a Guillemet=( "«««" "»»»" )

#	# Functions:
#
#	# Function _collapse(str) - return colapsed version of str
#
function _collapse {
	typeset _str ; _str="$1"
	typeset _i _buff ; integer _i

	for ((_i=1; _i<${#_str}; _i++)); do
		if [[ "${_str:$((_i-1)):1}" == "${_str:${_i}:1}" ]]; then
			continue
		else
			_buff+=${_str:$((_i-1)):1}
		fi
	done
	[[ "${_str:$((_i-1)):1}" != "${_str:${_i}:1}" ]] && _buff+=${_str:$((_i-1)):1}
	echo "${_buff}"
}

 ######
# main #
 ######
for ((i=0; i<${#strings[*]}; i++)); do
	str=$(_collapse "${strings[i]}")
	print ${#strings[i]} "${Guillemet[0]}${strings[i]}${Guillemet[1]}"
	print ${#str} "${Guillemet[0]}${str}${Guillemet[1]}\n"
done

Output:

0 «««»»»

71 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln»»» 69 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln»»»

72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»» 4 «««.178»»»

71 «««I never give 'em hell, I just tell the truth, and they think it's hell.»»» 68 «««I never give 'em hel, I just tel the truth, and they think it's hel.»»»

70 ««« --- Harry S Truman»»» 16 ««« - Hary S Truman»»»

Lua

Translation of: C#

function collapse(s)
    local ns = ""
    local last = nil
    for c in s:gmatch"." do
        if last then
            if last ~= c then
                ns = ns .. c
            end
            last = c
        else
            ns = ns .. c
            last = c
        end
    end
    return ns
end

function test(s)
    print("old: " .. s:len() .. " <<<" .. s .. ">>>")
    local a = collapse(s)
    print("new: " .. a:len() .. " <<<" .. a .. ">>>")
end

function main()
    test("")
    test("The better the 4-wheel drive, the further you'll be from help when ya get stuck!")
    test("headmistressship")
    test("\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ")
    test("..1111111111111111111111111111111111111111111111111111111111111117777888")
    test("I never give 'em hell, I just tell the truth, and they think it's hell. ")
    test("                                                    --- Harry S Truman  ")
end

main()

Output:

old: 0 <<<>>>
new: 0 <<<>>>
old: 80 <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
new: 77 <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>
old: 16 <<<headmistressship>>>
new: 14 <<<headmistreship>>>
old: 72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
new: 70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>
old: 72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
new: 4 <<<.178>>>
old: 72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
new: 69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>
old: 72 <<<                                                    --- Harry S Truman  >>>
new: 17 <<< - Hary S Truman >>>

Mathematica / Wolfram Language

ClearAll[StringCollapse]
StringCollapse[s_String] := FixedPoint[StringReplace[y_ ~~ y_ :> y], s]
strings = {"", 
"\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
"..1111111111111111111111111111111111111111111111111111111111111117777888", 
"I never give 'em hell, I just tell the truth, and they think it's hell. ",
"                                                    --- Harry S Truman  "};
Do[
 Print["«««" <> s <> "»»»"];
 Print["Length = ", StringLength[s]];
 Print["«««" <> StringCollapse[s] <> "»»»"];
 Print["Length = ", StringLength[StringCollapse[s]]]
 ,
 {s, strings}
 ]

Output:

«««»»»
Length = 0
«««»»»
Length = 0
«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
Length = 72
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
Length = 70
«««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
Length = 72
«««.178»»»
Length = 4
«««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
Length = 72
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
Length = 69
«««                                                    --- Harry S Truman  »»»
Length = 72
««« - Hary S Truman »»»
Length = 17

MATLAB / Octave

function r = collapse(s)
  ix=find((s(1:end-1)==s(2:end))+1;
  r=s;
  r(ix)=[];     

  fprintf(1,'Input:  <<<%s>>>  length: %d\n',s,length(s));
  fprintf(1,'Output: <<<%s>>>  length: %d\n',r,length(r));
  fprintf(1,'Character to be squeezed: "%s"\n',c);

end 


collapse('', ' ') 
collapse('║╚═══════════════════════════════════════════════════════════════════════╗', '-') 
collapse('║"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ║', '7')
collapse('║..1111111111111111111111111111111111111111111111111111111111111117777888║', '.')
collapse('║I never give ''em hell, I just tell the truth, and they think it''s hell. ║', '.')
collapse('║                                                    --- Harry S Truman  ║', '.')
collapse('║                                                    --- Harry S Truman  ║', '-')
collapse('║                                                    --- Harry S Truman  ║', 'r')

Output:

Input:  <<<>>>  length: 0
Output: <<<>>>  length: 0
ans = 
Input:  <<<║╚═══════════════════════════════════════════════════════════════════════╗>>>  length: 222
Output: <<<║╚═══════════════════════════════════════════════════════════════════════╗>>>  length: 222
ans = ║╚═══════════════════════════════════════════════════════════════════════╗
Input:  <<<║"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ║>>>  length: 78
Output: <<<║"If I were two-faced, would I be wearing this one?" - Abraham Lincoln ║>>>  length: 76
ans = ║"If I were two-faced, would I be wearing this one?" - Abraham Lincoln ║
Input:  <<<║..1111111111111111111111111111111111111111111111111111111111111117777888║>>>  length: 78
Output: <<<║.178║>>>  length: 10
ans = ║.178║
Input:  <<<║I never give 'em hell, I just tell the truth, and they think it's hell. ║>>>  length: 78
Output: <<<║I never give 'em hel, I just tel the truth, and they think it's hel. ║>>>  length: 75
ans = ║I never give 'em hel, I just tel the truth, and they think it's hel. ║
Input:  <<<║                                                    --- Harry S Truman  ║>>>  length: 78
Output: <<<║ - Hary S Truman ║>>>  length: 23
ans = ║ - Hary S Truman ║
Input:  <<<║                                                    --- Harry S Truman  ║>>>  length: 78
Output: <<<║ - Hary S Truman ║>>>  length: 23
ans = ║ - Hary S Truman ║
Input:  <<<║                                                    --- Harry S Truman  ║>>>  length: 78
Output: <<<║ - Hary S Truman ║>>>  length: 23
ans = ║ - Hary S Truman ║
PASSES 1 out of 1 test

Miranda

main :: [sys_message]
main = [Stdout (lay (map test tests))]

tests :: [[char]]
tests
= [
  "",
  "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
  "..1111111111111111111111111111111111111111111111111111111111111111111788",
  "I never give 'em hell, I just tell the truth, and they think it's hell. ",
  "                                                     --- Harry S Truman "
  ]

test :: [char]->[char]
test s = lay [before, after]
         where before = disp s
               after  = disp (collapse s)

disp :: [char]->[char]
disp s = show (#s) ++ ": <<<" ++ s ++ ">>>"

collapse :: [*]->[*]
collapse []       = []
collapse [x]      = [x]
collapse (x:y:xs) = collapse (y:xs),   if x=y
                  = x:collapse (y:xs), otherwise

Output:

0: <<<>>>
0: <<<>>>

72: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72: <<<..1111111111111111111111111111111111111111111111111111111111111111111788>>>
4: <<<.178>>>

72: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72: <<<                                                     --- Harry S Truman >>>
17: <<< - Hary S Truman >>>

Modula-2

MODULE StrCollapse;
FROM InOut IMPORT WriteString, WriteCard, WriteLn;
FROM Strings IMPORT Length;

(* Collapse a string *) 
PROCEDURE Collapse(in: ARRAY OF CHAR; VAR out: ARRAY OF CHAR);
    VAR i, o: CARDINAL;        
BEGIN
    i := 0;
    o := 0;
    WHILE (i < HIGH(in)) AND (in[i] # CHR(0)) DO
        IF (o = 0) OR (out[o-1] # in[i]) THEN
            out[o] := in[i];
            INC(o);
        END;
        INC(i);
    END;
    out[o] := CHR(0);
END Collapse;

(* Display a string and collapse it as stated in the task *)
PROCEDURE Task(s: ARRAY OF CHAR);
    VAR buf: ARRAY [0..127] OF CHAR;
    PROCEDURE LengthAndBrackets(s: ARRAY OF CHAR);
    BEGIN
        WriteCard(Length(s), 2);
        WriteString(" <<<");
        WriteString(s);
        WriteString(">>>");
        WriteLn();
    END LengthAndBrackets;
BEGIN
    LengthAndBrackets(s);
    Collapse(s, buf);
    LengthAndBrackets(buf);
    WriteLn();
END Task;

BEGIN
    Task("");
    Task('"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ');
    Task("..1111111111111111111111111111111111111111111111111111111111111117777888");
    Task("I never give 'em hell, I just tell the truth, and they think it's hell. ");
    Task("                                                    --- Harry S Truman  ");
END StrCollapse.

Output:

 0 <<<>>>
 0 <<<>>>

72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
 4 <<<.178>>>

72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72 <<<                                                    --- Harry S Truman  >>>
17 <<< - Hary S Truman >>>

NetLogo

To run the task:

Enter the code below into the Code Pane of NetLogo
On the Interface Pane, in the Command Center, type "demo" and hit enter.

to-report split [ string ] 
  ;; utility reporter to split a string into a list
  report n-values length string [ [ n ] -> item n string ]
end

to-report collapse [ string ]
  ;; reporter that actually does the collapse function
  ifelse ( string = "" )
  [ report "" ]
  [ report reduce [ [ a b ] -> (word a ifelse-value last a != b [ b ] [ "" ] ) ] split string ]
end

to-report format [ string ]
  ;; reporter to format the output as required
  report ( word "<<<" string ">>> " length string )
end

to demo-collapse [ string ]
  ;; procedure to display the required output
  output-print format string
  output-print format collapse string
end

to demo
  ;; procedure to perform the test cases
  foreach 
  [ ""
    "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
    "..1111111111111111111111111111111111111111111111111111111111111117777888"
    "I never give 'em hell, I just tell the truth, and they think it's hell. "
    "                                                    --- Harry S Truman  "
  ]
  demo-collapse
end

Output:

observer> demo
<<<>>> 0
<<<>>> 0
<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> 72
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> 70
<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> 72
<<<.178>>> 4
<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> 72
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> 69
<<<                                                    --- Harry S Truman  >>> 72
<<< - Hary S Truman >>> 17

Nim

import unicode, strformat

proc collapse(s: string) =
  let original = s.toRunes
  echo fmt"Original:  length = {original.len}, string = «««{s}»»»"
  var previous = Rune(0)
  var collapsed: seq[Rune]
  for rune in original:
    if rune != previous:
      collapsed.add(rune)
      previous = rune
  echo fmt"Collapsed: length = {collapsed.len}, string = «««{collapsed}»»»"
  echo ""


const Strings = ["",
        "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌",]


for s in Strings:
  s.collapse()

Output:

Original:  length = 0, string = «««»»»
Collapsed: length = 0, string = «««»»»

Original:  length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
Collapsed: length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

Original:  length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
Collapsed: length = 4, string = «««.178»»»

Original:  length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
Collapsed: length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

Original:  length = 72, string = «««                                                   ---  Harry S Truman  »»»
Collapsed: length = 17, string = ««« - Hary S Truman »»»

Original:  length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
Collapsed: length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

Original:  length = 16, string = «««headmistressship»»»
Collapsed: length = 14, string = «««headmistreship»»»

Original:  length = 8, string = «««aardvark»»»
Collapsed: length = 7, string = «««ardvark»»»

Original:  length = 8, string = «««😍😀🙌💃😍😍😍🙌»»»
Collapsed: length = 6, string = «««😍😀🙌💃😍🙌»»»

Perl

use strict;
use warnings;
use utf8;
binmode STDOUT, ":utf8";

my @lines = split "\n", <<~'STRINGS';

    "If I were two-faced, would I be wearing this one?" --- Abraham Lincoln
    ..1111111111111111111111111111111111111111111111111111111111111117777888
    I never give 'em hell, I just tell the truth, and they think it's hell.
                                                        --- Harry S Truman
    The American people have a right to know if their president is a crook.
                                                        --- Richard Nixon
    AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
    STRINGS

for (@lines) {
    my $squish = s/(.)\1+/$1/gr;
    printf "\nLength: %2d <<<%s>>>\nCollapsible: %s\nLength: %2d <<<%s>>>\n",
      length($_), $_, $_ ne $squish ? 'True' : 'False', length($squish), $squish
}

Output:

Length:  0 <<<>>>
Collapsible: False
Length:  0 <<<>>>

Length: 72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
Collapsible: True
Length: 70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

Length: 72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
Collapsible: True
Length:  4 <<<.178>>>

Length: 72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
Collapsible: True
Length: 69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

Length: 72 <<<                                                    --- Harry S Truman  >>>
Collapsible: True
Length: 17 <<< - Hary S Truman >>>

Length: 72 <<<The American people have a right to know if their president is a crook. >>>
Collapsible: True
Length: 71 <<<The American people have a right to know if their president is a crok. >>>

Length: 72 <<<                                                    --- Richard Nixon   >>>
Collapsible: True
Length: 17 <<< - Richard Nixon >>>

Length: 120 <<<AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ>>>
Collapsible: False
Length: 120 <<<AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ>>>

Length: 72 <<<AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA>>>
Collapsible: True
Length:  1 <<<A>>>

Phix

There is already a builtin we can abuse for this. The documentation of unique() [added for 0.8.1] states:
`If "PRESORTED" is specified, but s is not actually sorted, then only adjacent duplicates are removed.`
If you don't have builtins/punique.e to hand, you may want to take a quick look at squeeze() in [[1]], but obviously w/o ch.

with javascript_semantics
constant tests = {"",
                  `"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln `,
                  "..1111111111111111111111111111111111111111111111111111111111111117777888",
                  "I never give 'em hell, I just tell the truth, and they think it's hell. ",
                  "                                                    --- Harry S Truman  "},
         fmt = """		
length %2d  input: <<<%s>>>
length %2d output: <<<%s>>>
"""
for i=1 to length(tests) do
    string ti = tests[i], ci = unique(ti, "PRESORTED")
    printf(1,fmt,{length(ti),ti,length(ci),ci})
end for
 
function collapsible(string t)
--  sequence utf32 = utf8_to_utf32(t) -- maybe
--  for i=2 to length(utf32) do       --  """
--      if utf32[i]=utf32[i-1] then   --  """
    for i=2 to length(t) do
        if t[i]=t[i-1] then
            return true
        end if
    end for
    return false
end function
 
puts(1,"\nAs predicate: ")
for i=1 to length(tests) do
    printf(1,"%t ",collapsible(tests[i]))
end for
puts(1,"\n")

Output:

length  0  input: <<<>>>
length  0 output: <<<>>>

length 72  input: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
length 70 output: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

length 72  input: <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
length  4 output: <<<.178>>>

length 72  input: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
length 69 output: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

length 72  input: <<<                                                    --- Harry S Truman  >>>
length 17 output: <<< - Hary S Truman >>>

As predicate: false true true true true

PHP

<?php

function collapseString($string) {
    $previousChar = null;
    $collapse = '';
    $charArray = preg_split('//u', $string, -1, PREG_SPLIT_NO_EMPTY);
    for ($i = 0 ; $i < count($charArray) ; $i++) {
        $currentChar = $charArray[$i];
        if ($previousChar !== $currentChar) {
            $collapse .= $charArray[$i];
        }
        $previousChar = $currentChar;
    }
    return $collapse;
}

function isCollapsible($string) {
    return ($string !== collapseString($string));
}

$strings = array(
    '',
    'another non colapsing string',
    '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
    '..1111111111111111111111111111111111111111111111111111111111111117777888',
    "I never give 'em hell, I just tell the truth, and they think it's hell. ",
    '                                                    --- Harry S Truman  ',
    '0112223333444445555556666666777777778888888889999999999',
    "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
    'headmistressship',
    "😍😀🙌💃😍😍😍🙌",
);

foreach ($strings as $original) {
    echo 'Original : <<<', $original, '>>> (len=', mb_strlen($original), ')', PHP_EOL;
    if (isCollapsible($original)) {
        $collapse = collapseString($original);
        echo 'Collapse : <<<', $collapse, '>>> (len=', mb_strlen($collapse), ')', PHP_EOL, PHP_EOL;
    } else {
        echo 'Collapse : string is not collapsing...', PHP_EOL, PHP_EOL;
    }
}

Output:

Original : <<<>>> (len=0)
Collapse : string is not collapsing...

Original : <<<another non colapsing string>>> (len=28)
Collapse : string is not collapsing...

Original : <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> (len=72)
Collapse : <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> (len=70)

Original : <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> (len=72)
Collapse : <<<.178>>> (len=4)

Original : <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> (len=72)
Collapse : <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> (len=69)

Original : <<<                                                    --- Harry S Truman  >>> (len=72)
Collapse : <<< - Hary S Truman >>> (len=17)

Original : <<<0112223333444445555556666666777777778888888889999999999>>> (len=55)
Collapse : <<<0123456789>>> (len=10)

Original : <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>> (len=80)
Collapse : <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>> (len=77)

Original : <<<headmistressship>>> (len=16)
Collapse : <<<headmistreship>>> (len=14)

Original : <<<😍😀🙌💃😍😍😍🙌>>> (len=8)
Collapse : <<<😍😀🙌💃😍🙌>>> (len=6)

PL/M

Because of the limited character set supported by the PL/M-80 compiler, uppercase letters have been substituted for lowercase letters, the question mark for the period, and the single quotation mark for the double quotation mark. This has no other effect on the output. While it would be possible to write the ASCII values as numbers, this would make the code less clear. The COLLAPSE procedure compares bytes and would be fine with any 8-bit input.

100H:
BDOS: PROCEDURE (FN, ARG); DECLARE FN BYTE, ARG ADDRESS; GO TO 5; END BDOS;
EXIT: PROCEDURE; CALL BDOS(0,0); END EXIT;
PRINT: PROCEDURE (S); DECLARE S ADDRESS; CALL BDOS(9, S); END PRINT;

/* PRINT NUMBER */
PRINT$NUMBER: PROCEDURE (N);
    DECLARE S (6) BYTE INITIAL ('.....$');
    DECLARE (N, P) ADDRESS, C BASED P BYTE;
    P = .S(5);
DIGIT:
    P = P - 1;
    C = N MOD 10 + '0';
    N = N / 10;
    IF N > 0 THEN GO TO DIGIT;
    CALL PRINT(P);
END PRINT$NUMBER;

/* STRING LENGTH */
STR$LEN: PROCEDURE (STR) ADDRESS;
    DECLARE (STR, I) ADDRESS, S BASED STR BYTE;
    I = 0;
    DO WHILE S(I) <> '$';
        I = I + 1;
    END;
    RETURN I;
END STR$LEN;

/* COLLAPSE */
COLLAPSE: PROCEDURE (IN, OUT);
    DECLARE (IN, OUT) ADDRESS, (I BASED IN, O BASED OUT, C) BYTE;
    C = I;
    DO WHILE C <> '$';
        IN = IN + 1;
        IF I <> C THEN DO;
            O = C;
            OUT = OUT + 1;
            C = I;
        END;
    END;
    O = '$';
END COLLAPSE;

/* PRINT STRING AND LENGTH WITH BRACKETS */
PRINT$BRACKETS: PROCEDURE (S);
    DECLARE S ADDRESS;
    CALL PRINT$NUMBER(STR$LEN(S));
    CALL PRINT(.' <<<$');
    CALL PRINT(S);
    CALL PRINT(.('>>>',13,10,'$'));
END PRINT$BRACKETS;

/* GIVEN A STRING, PRINT IT AND ITS COLLAPSED FORM */
SHOW: PROCEDURE (S);
    DECLARE S ADDRESS, BUFFER (256) BYTE;
    CALL COLLAPSE(S, .BUFFER);
    CALL PRINT$BRACKETS(S);
    CALL PRINT$BRACKETS(.BUFFER);
    CALL PRINT(.(13,10,'$'));
END SHOW;

/* STRINGS FROM THE TASK */
DECLARE X (5) ADDRESS;
X(0)=.'$';
X(1)=.('''IF I WERE TWO-FACED, WOULD I BE WEARING ',
       'THIS ONE.'' --- ABRAHAM LINCOLN $');
X(2)=.('..111111111111111111111111111111111111111',
       '1111111111111111111111117777888$');
X(3)=.('I NEVER GIVE ''EM HELL, I JUST TELL THE TR',
       'UTH, AND THEY THINK IT''S HELL. $');
X(4)=.('                                         ',
       '            --- HARRY S TRUMAN $');
       
DECLARE I BYTE;
DO I=0 TO LAST(X);
    CALL SHOW(X(I));
END;
CALL EXIT;
EOF

Output:

0 <<<>>>
0 <<<>>>

72 <<<'IF I WERE TWO-FACED, WOULD I BE WEARING THIS ONE.' --- ABRAHAM LINCOLN >>>
70 <<<'IF I WERE TWO-FACED, WOULD I BE WEARING THIS ONE.' - ABRAHAM LINCOLN >>>

72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
4 <<<.178>>>

72 <<<I NEVER GIVE 'EM HELL, I JUST TELL THE TRUTH, AND THEY THINK IT'S HELL. >>>
69 <<<I NEVER GIVE 'EM HEL, I JUST TEL THE TRUTH, AND THEY THINK IT'S HEL. >>>

72 <<<                                                     --- HARRY S TRUMAN >>>
17 <<< - HARY S TRUMAN >>>

Prolog

collapse_( [], [] ).
collapse_( [A], [A] ).
collapse_( [A,A|T], R ) :- collapse_( [A|T], R ).
collapse_( [A,B|T], [A|R] ) :- dif( A, B ), collapse_( [B|T], R ).

collapse( Str, Collapsed ) :-
    string_chars( Str, Chars ),
    collapse_( Chars, Result ),
    string_chars( Collapsed, Result ).

Output:

?- collapse("122333444455555666666777777788888888999999999", New).
New = "123456789" .

?- collapse("", New).
New = "" .

?- collapse("\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ", New).
New = "\"If I were two-faced, would I be wearing this one?\" - Abraham Lincoln " .

?- collapse("..1111111111111111111111111111111111111111111111111111111111111117777888", New).
New = ".178" .

?- collapse("I never give 'em hell, I just tell the truth, and they think it's hell. ", New).
New = "I never give 'em hel, I just tel the truth, and they think it's hel. " .

?- collapse("                                                    --- Harry S Truman  ", New).
New = " - Hary S Truman " .

?- collapse("The better the 4-wheel drive, the further you'll be from help when ya get stuck!", New).
New = "The beter the 4-whel drive, the further you'l be from help when ya get stuck!" .

33 ?- collapse("headmistressship", New).
New = "headmistreship" .

To determine if a string is collapsed or not, query if the result string is the same as the starting string. eg:

35 ?- S = "122333444455555666666777777788888888999999999", collapse(S,S).
false.

37 ?- S = "123456789", collapse(S,S).
S = "123456789" .

PureBasic

EnableExplicit
DataSection
  STR1:
  Data.s ""
  STR2:
  Data.s "'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln "
  STR3:
  Data.s "..1111111111111111111111111111111111111111111111111111111111111117777888"
  STR4:
  Data.s "I never give 'em hell, I just tell the truth, and they think it's hell. "
  STR5:
  Data.s "                                                    --- Harry S Truman  "
EndDataSection

Procedure.s collapse(txt$)
  Define *c.Character=@txt$, last.c=0, result$
  While *c\c
    If *c\c<>last
      result$+Chr(*c\c)
      last=*c\c
    EndIf
    *c+SizeOf(Character)
  Wend
  ProcedureReturn result$
EndProcedure

OpenConsole("")
Define *p_data=?STR1, buf$
While *p_data<=?STR4+StringByteLength(PeekS(?STR4))+2
  buf$=PeekS(*p_data)
  PrintN("Before collapse: «««"+buf$+"»»» (length: "+Str(Len(buf$))+")")
  buf$=collapse(PeekS(*p_data))
  PrintN("After  collapse: «««"+buf$+~"»»» (length: "+Str(Len(buf$))+~")\n")
  *p_data+StringByteLength(PeekS(*p_data))+2
Wend
Input()

Output:

Before collapse: «««»»» (length: 0)
After  collapse: «««»»» (length: 0)

Before collapse: «««'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln »»» (length: 72)
After  collapse: «««'If I were two-faced, would I be wearing this one?' - Abraham Lincoln »»» (length: 70)

Before collapse: «««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (length: 72)
After  collapse: «««.178»»» (length: 4)

Before collapse: «««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (length: 72)
After  collapse: «««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (length: 69)

Before collapse: «««                                                    --- Harry S Truman  »»» (length: 72)
After  collapse: ««« - Hary S Truman »»» (length: 17)

Python

from itertools import groupby

def collapser(txt):
    return ''.join(item for item, grp in groupby(txt))

if __name__ == '__main__':
    strings = [
            "",
            '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
            "..1111111111111111111111111111111111111111111111111111111111111117777888",
            "I never give 'em hell, I just tell the truth, and they think it's hell. ",
            "                                                   ---  Harry S Truman  ",
            "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
            "headmistressship",
            "aardvark",
            "😍😀🙌💃😍😍😍🙌",
            ]
    for txt in strings:
        this = "Original"
        print(f"\n{this:14} Size: {len(txt)} «««{txt}»»»" )
        this = "Collapsed"
        sqz = collapser(txt)
        print(f"{this:>14} Size: {len(sqz)} «««{sqz}»»»" )

Output:

Original       Size: 0 «««»»»
     Collapsed Size: 0 «««»»»

Original       Size: 72 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
     Collapsed Size: 70 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

Original       Size: 72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
     Collapsed Size: 4 «««.178»»»

Original       Size: 72 «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
     Collapsed Size: 69 «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

Original       Size: 72 «««                                                   ---  Harry S Truman  »»»
     Collapsed Size: 17 ««« - Hary S Truman »»»

Original       Size: 80 «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
     Collapsed Size: 77 «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

Original       Size: 16 «««headmistressship»»»
     Collapsed Size: 14 «««headmistreship»»»

Original       Size: 8 «««aardvark»»»
     Collapsed Size: 7 «««ardvark»»»

Original       Size: 8 «««😍😀🙌💃😍😍😍🙌»»»
     Collapsed Size: 6 «««😍😀🙌💃😍🙌»»»

and for the missing predicate, foregrounded in the task title, and already forgotten in the listing of tests:

'''Determining if a string is collapsible'''

from operator import eq


# isCollapsible :: String -> Bool
def isCollapsible(s):
    '''True if s contains any consecutively
       repeated characters.
    '''
    return False if 2 > len(s) else (
        any(map(eq, s, s[1:]))
    )


# ------------------------- TEST --------------------------
# main :: IO ()
def main():
    '''Determining whether each string is collapsible'''
    xs = [
        "",
        '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌",
        "abcdefghijklmnopqrstuvwxyz"
    ]
    print([
        isCollapsible(x) for x in xs
    ])


# MAIN ---
if __name__ == '__main__':
    main()

Output:

[False, True, True, True, True, True, True, True, True, False]

Quackery

  [ false -1 rot
    witheach
      [ 2dup = iff
          [ drop dip not 
            conclude ]
        else nip ]
    drop ]                       is collapsible ( $ --> b )
 
  [ [] -1 rot
    witheach 
      [ 2dup = iff drop
        else
          [ nip dup dip join ] ]
    drop ]                       is collapse    ( $ --> $ )

  [ dup collapsible iff 
      [ dup collapse 
        swap 2 ] 
    else 
      [ say "(Not collapsible.)" cr
        1 ]
    times 
      [ say "<<<" 
        dup echo$ 
        say ">>>" cr 
        say "   Length: " 
        size echo
        say " characters" cr cr ]
    cr ]                         is task     ( $ -->   )

$ "" task
$ '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ' task
$ "..1111111111111111111111111111111111111111111111111111111111111117777888" task
$ "I never give 'em hell, I just tell the truth, and they think it's hell. " task
$ "                                                    --- Harry S Truman  " task

Output:

(Not collapsible.)
<<<>>>
   Length: 0 characters


<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
   Length: 72 characters

<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>
   Length: 70 characters


<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
   Length: 72 characters

<<<.178>>>
   Length: 4 characters


<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
   Length: 72 characters

<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>
   Length: 69 characters


<<<                                                    --- Harry S Truman  >>>
   Length: 72 characters

<<< - Hary S Truman >>>
   Length: 17 characters

R

collapse_string <- function(string){
    
    str_iterable <- strsplit(string, "")[[1]]
    
    message(paste0("Original String: ", "<<<", string, ">>>\n", 
                   "Length: ", length(str_iterable)))
    
    detect <- rep(TRUE, length(str_iterable))
    
    
    for(i in 2:length(str_iterable)){
      
      if(length(str_iterable)==0) break
      
      if(str_iterable[i] == str_iterable[i-1])
        
        detect[i] <- FALSE
    }
    
    collapsed_string <- paste(str_iterable[detect],collapse = "")
    
    message(paste0("Collapsed string: ", "<<<",collapsed_string, ">>>\n",
                   "Length: ", length(str_iterable[detect])), "\n")
    
}

test_strings <- c(
  "",
  "'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln ",
  "..1111111111111111111111111111111111111111111111111111111111111117777888",
  "I never give 'em hell, I just tell the truth, and they think it's hell. ",
  "                                                    --- Harry S Truman  ",
  "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
  "headmistressship",
  "aardvark",
  "Ciao Mamma, guarda come mi diverto!!"
)

for(test in test_strings){
  collapse_string(test)
}

Output:

  Original String: <<<>>>
  Length: 0
Collapsed string: <<<>>>
  Length: 0

Original String: <<<'If I were two-faced, would I be wearing this one?' --- Abraham Lincoln >>>
  Length: 72
Collapsed string: <<<'If I were two-faced, would I be wearing this one?' - Abraham Lincoln >>>
  Length: 70

Original String: <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
  Length: 72
Collapsed string: <<<.178>>>
  Length: 4

Original String: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
  Length: 72
Collapsed string: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>
  Length: 69

Original String: <<<                                                    --- Harry S Truman  >>>
  Length: 72
Collapsed string: <<< - Hary S Truman >>>
  Length: 17

Original String: <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
Length: 80
Collapsed string: <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>
  Length: 77

Original String: <<<headmistressship>>>
  Length: 16
Collapsed string: <<<headmistreship>>>
  Length: 14

Original String: <<<aardvark>>>
  Length: 8
Collapsed string: <<<ardvark>>>
  Length: 7

Original String: <<<Ciao Mamma, guarda come mi diverto!!>>>
  Length: 36
Collapsed string: <<<Ciao Mama, guarda come mi diverto!>>>
  Length: 34

Racket

#lang racket

(define (collapse text)
  (if (< (string-length text) 2)
      text
      (string-append
       (if (equal? (substring text 0 1) (substring text 1 2))
           "" (substring text 0 1))
       (collapse (substring text 1)))))

; Test cases
(define tcs
  '(""
    "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "
    "..1111111111111111111111111111111111111111111111111111111111111117777888"
    "I never give 'em hell, I just tell the truth, and they think it's hell. "
    "                                                   ---  Harry S Truman  "
    "The better the 4-wheel drive, the further you'll be from help when ya get stuck!"
    "headmistressship"
    "aardvark"
    "😍😀🙌💃😍😍😍🙌"))

(for ([text tcs])
  (let ([collapsed (collapse text)])
    (display (format "Original  (size ~a): «««~a»»»\nCollapsed (size ~a): «««~a»»»\n\n"
                     (string-length text) text
                     (string-length collapsed) collapsed))))

Output:

Original  (size 0): «««»»»
Collapsed (size 0): «««»»»

Original  (size 72): «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
Collapsed (size 70): «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

Original  (size 72): «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
Collapsed (size 4): «««.178»»»

Original  (size 72): «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
Collapsed (size 69): «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

Original  (size 72): «««                                                   ---  Harry S Truman  »»»
Collapsed (size 17): ««« - Hary S Truman »»»

Original  (size 80): «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
Collapsed (size 77): «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

Original  (size 16): «««headmistressship»»»
Collapsed (size 14): «««headmistreship»»»

Original  (size 8): «««aardvark»»»
Collapsed (size 7): «««ardvark»»»

Original  (size 8): «««😍😀🙌💃😍😍😍🙌»»»
Collapsed (size 6): «««😍😀🙌💃😍🙌»»»

Raku

(formerly Perl 6)

Works with: Rakudo version 2019.07.1

Technically, the task is asking for a boolean. "Determine if a string is collapsible" is answerable with True/False, so return a boolean as well.

map {
    my $squish = .comb.squish.join;
    printf "\nLength: %2d <<<%s>>>\nCollapsible: %s\nLength: %2d <<<%s>>>\n",
      .chars, $_, .chars != $squish.chars, $squish.chars, $squish
}, lines
 
q:to/STRINGS/;
    
    "If I were two-faced, would I be wearing this one?" --- Abraham Lincoln 
    ..1111111111111111111111111111111111111111111111111111111111111117777888
    I never give 'em hell, I just tell the truth, and they think it's hell. 
                                                        --- Harry S Truman  
    The American people have a right to know if their president is a crook. 
                                                        --- Richard Nixon   
    AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
    STRINGS

Output:

Length:  0 <<<>>>
Collapsible: False
Length:  0 <<<>>>

Length: 72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
Collapsible: True
Length: 70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

Length: 72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
Collapsible: True
Length:  4 <<<.178>>>

Length: 72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
Collapsible: True
Length: 69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

Length: 72 <<<                                                    --- Harry S Truman  >>>
Collapsible: True
Length: 17 <<< - Hary S Truman >>>

Length: 72 <<<The American people have a right to know if their president is a crook. >>>
Collapsible: True
Length: 71 <<<The American people have a right to know if their president is a crok. >>>

Length: 72 <<<                                                    --- Richard Nixon   >>>
Collapsible: True
Length: 17 <<< - Richard Nixon >>>

Length: 72 <<<AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ>>>
Collapsible: False
Length: 72 <<<AАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑAАΑ>>>

Length: 72 <<<AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA>>>
Collapsible: True
Length:  1 <<<A>>>

Refal

$ENTRY Go {
    , ('')
      ('"If I were two-faced, would I be wearing this '
       'one?" --- Abraham Lincoln ')
      ('..11111111111111111111111111111111111111111111'
       '11111111111111111117777888')
      ('I never give \'em hell, I just tell the truth, '
       'and they think it\'s hell. ')
      ('                                              '
       '      --- Harry S Truman  '): e.Strings
    = <Each Show e.Strings>;
};

Each {
    s.F = ;
    s.F t.I e.X = <Mu s.F t.I> <Each s.F e.X>;
};

Brackets {
    e.X, <Lenw e.X>: s.L e.X = 
        <Prout <Symb s.L> ': <<<' e.X '>>>'>;
};

Show {
    (e.X) = <Brackets e.X>
            <Brackets <Collapse e.X>>
            <Prout>;
};

Collapse {
    = ;
    s.C s.C e.S = <Collapse s.C e.S>;
    s.C e.S = s.C <Collapse e.S>;
};

Output:

0: <<<>>>
0: <<<>>>

72: <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
70: <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

72: <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
4: <<<.178>>>

72: <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
69: <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

72: <<<                                                    --- Harry S Truman  >>>
17: <<< - Hary S Truman >>>

REXX

/*REXX program "collapses" all immediately repeated characters in a string (or strings).*/
@.=                                              /*define a default for the  @.  array. */
parse arg x                                      /*obtain optional argument from the CL.*/
if x\=''  then      @.1= x                       /*if user specified an arg, use that.  */
          else do;  @.1=
                    @.2= '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln '
                    @.3=  ..1111111111111111111111111111111111111111111111111111111111111117777888
                    @.4= "I never give 'em hell, I just tell the truth, and they think it's hell. "
                    @.5= '                                                   ---  Harry S Truman  '
               end

     do j=1;    L= length(@.j)                   /*obtain the length of an array element*/
     say copies('═', 105)                        /*show a separator line between outputs*/
     if j>1  &  L==0     then leave              /*if arg is null and  J>1, then leave. */
     new= collapse(@.j)
     say 'string' word("isn't is",1+collapsible) 'collapsible' /*display semaphore value*/
     say '    length='right(L, 3)     "   input=«««"  ||  @.j  ||  '»»»'
       w= length(new)
     say '    length='right(w, 3)     "  output=«««"  ||  new  ||  '»»»'
     end   /*j*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
collapse: procedure expose collapsible; parse arg y 1 $ 2  /*get the arg; get 1st char. */
            do k=2  to length(y)                 /*traipse through almost all the chars.*/
            _= substr(y, k, 1)                   /*pick a character from  Y  (1st arg). */
            if _==right($, 1)  then iterate      /*Is this the same character?  Skip it.*/
            $= $ || _                            /*append the character, it's different.*/
            end   /*j*/
          collapsible= y\==$;       return $     /*set boolean to  true  if collapsible.*/

output when using the internal default inputs:

═════════════════════════════════════════════════════════════════════════════════════════════════════════
string isn't collapsible
    length=  0    input=«««»»»
    length=  0   output=«««»»»
═════════════════════════════════════════════════════════════════════════════════════════════════════════
string is collapsible
    length= 72    input=«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
    length= 70   output=«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
═════════════════════════════════════════════════════════════════════════════════════════════════════════
string is collapsible
    length= 72    input=«««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
    length=  4   output=«««.178»»»
═════════════════════════════════════════════════════════════════════════════════════════════════════════
string is collapsible
    length= 72    input=«««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
    length= 69   output=«««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
═════════════════════════════════════════════════════════════════════════════════════════════════════════
string is collapsible
    length= 72    input=«««                                                   ---  Harry S Truman  »»»
    length= 17   output=««« - Hary S Truman »»»
═════════════════════════════════════════════════════════════════════════════════════════════════════════

Ring

load "stdlib.ring"

see "working..." + nl + nl
str = ["The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
       "I never give 'em hell, I just tell the truth, and they think it's hell.",
       "..1111111111111111111111111111111111111111111111111111111111111117777888"]
strsave = str
for n = 1 to len(str)
    for m = 1 to len(str[n])-1
        if substr(str[n],m,1) = substr(str[n],m+1,1)
           str[n] = left(str[n],m) + right(str[n],len(str[n])-m-1)
           for p = len(str[n]) to 2 step -1
               if substr(str[n],p,1) = substr(str[n],p-1,1)
                  str[n] = left(str[n],p-1) + right(str[n],len(str[n])-p)
               ok
           next
        ok
     next
next

for n = 1 to len(str)
    see "" + len(strsave[n]) + "«««" + strsave[n] + "»»»" + nl
    see "" + len(str[n]) + "«««" + str[n] + "»»»" + nl + nl
next

see "done..." + nl

Output:

working...

80«««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
77«««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

71«««I never give 'em hell, I just tell the truth, and they think it's hell.»»»
68«««I never give 'em hel, I just tel the truth, and they think it's hel.»»»

72«««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
4«««.178»»»

done...

RPL

Works with: Halcyon Calc version 4.2.7

Translation of: Kotlin

RPL code

Comment

≪ → string 
   ≪  "" DUP
      1 string SIZE FOR j          
         string j DUP SUB
         IF DUP2 ≠ THEN 
            ROT OVER +
            SWAP ROT
         END DROP 
     NEXT DROP
≫ ≫ ‘CLAPS’ STO

CLAPS ( "strrinng" → "string" )
output string = last = ""
scan the input string
  c = jth character
  if c ≠ last
     output string += c
     last = c
 forget c or previous last      
 clean stack

Works with: HP version 49

« → string
  « { "" }
    string SIZE 1 FOR j
       string j DUP SUB SWAP
       IF DUP2 HEAD == THEN NIP ELSE + END
    -1 STEP
    ∑LIST
 » » ‘CLAPS’ STO

When displaying strings, RPL always adds double quotes. To fulfill the artistic touch requirement, we have used guillemets to bracket Lincoln's statement.

≪ { "" 
     "≪ If I were two-faced, would I be wearing this one? ≫ --- Abraham Lincoln "
     "..1111111111111111111111111111111111111111111111111111111111111117777888"
     "I never give 'em hell, I just tell the truth, and they think it's hell. "
     "                                                    --- Harry S Truman  " }
   1 5 FOR j DUP j GET CLAPS SWAP NEXT DROP
≫ EVAL

Output:

5: ""
4: "≪ If I were two-faced, would I be wearing this one? ≫ - Abraham Lincoln "
3: ".178"
2: "I never give 'em hel, I just tel the truth, and they think it's hel. "
1: " - Hary S Truman "

Ruby

This is built in since at least a decade under the method name 'squeeze'. squeeze(" ") would only squeeze spaces.

strings = ["",
        '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌",]

strings.each do |str|
  puts "«««#{str}»»» (size #{str.size})"
  ssq = str.squeeze
  puts "«««#{ssq}»»» (size #{ssq.size})"
  puts
end

Output:

«««»»» (size 0)
«««»»» (size 0)

«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» (size 72)
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» (size 70)

«««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (size 72)
«««.178»»» (size 4)

«««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (size 72)
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (size 69)

«««                                                   ---  Harry S Truman  »»» (size 72)
««« - Hary S Truman »»» (size 17)

«««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»» (size 80)
«««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»» (size 77)

«««headmistressship»»» (size 16)
«««headmistreship»»» (size 14)

«««aardvark»»» (size 8)
«««ardvark»»» (size 7)

«««😍😀🙌💃😍😍😍🙌»»» (size 8)
«««😍😀🙌💃😍🙌»»» (size 6)

Rust

fn collapse_string(val: &str) -> String {
    let mut output = String::new();
    let mut chars = val.chars().peekable();

    while let Some(c) = chars.next() {
        while let Some(&b) = chars.peek() {
            if b == c {
                chars.next();
            } else {
                break;
            }
        }

        output.push(c);
    }

    output
}

fn main() {
    let tests = [
        "122333444455555666666777777788888888999999999",
        "",
        "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                    --- Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
    ];

    for s in &tests {
        println!("Old: {:>3} <<<{}>>>", s.len(), s);
        let collapsed = collapse_string(s);
        println!("New: {:>3} <<<{}>>>", collapsed.len(), collapsed);

        println!();
    }
}

Output:

Old:  45 <<<122333444455555666666777777788888888999999999>>>
New:   9 <<<123456789>>>

Old:   0 <<<>>>
New:   0 <<<>>>

Old:  72 <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
New:  70 <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

Old:  72 <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
New:   4 <<<.178>>>

Old:  72 <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
New:  69 <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

Old:  72 <<<                                                    --- Harry S Truman  >>>
New:  17 <<< - Hary S Truman >>>

Old:  80 <<<The better the 4-wheel drive, the further you'll be from help when ya get stuck!>>>
New:  77 <<<The beter the 4-whel drive, the further you'l be from help when ya get stuck!>>>

Old:  16 <<<headmistressship>>>
New:  14 <<<headmistreship>>>

Scala

Pure imperative programming 😃

object CollapsibleString {

  /**Collapse a string (if possible)*/
  def collapseString (s : String) : String = {
    var res = s
    var isOver = false
    var i = 0
    if(res.size == 0) res
    else while(!isOver){
      if(res(i) == res(i+1)){
        res = res.take(i) ++ res.drop(i+1)
        i-=1
      }
      i+=1
      if(i==res.size-1) isOver = true
    }
    res
  }

  /**Check if a string is collapsible*/
  def isCollapsible (s : String) : Boolean = collapseString(s).length != s.length

  /**Display results as asked in the task*/
  def reportResults (s : String) : String = {
    val sCollapsed = collapseString(s)
    val originalRes = "ORIGINAL  : length = " + s.length() + ", string = «««" + s + "»»»"
    val collapsedRes = "COLLAPSED : length = " + sCollapsed.length() + ", string = «««" + sCollapsed + "»»»"
    //In order to avoid useless computations, the function isCollapsible isn't called
    if(s.length != sCollapsed.length) originalRes + "\n" + collapsedRes + "\n" + "This string IS collapsible !"
    else originalRes + "\n" + collapsedRes + "\n" + "This string is NOT collapsible !"
  }



  def main(args: Array[String]): Unit = {
    println(reportResults(""))
    println("------------") 
    println(reportResults("\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln "))
    println("------------")
    println(reportResults("..1111111111111111111111111111111111111111111111111111111111111117777888"))
    println("------------")
    println(reportResults("I never give 'em hell, I just tell the truth, and they think it's hell. "))
    println("------------")
    println(reportResults("                                                    --- Harry S Truman  "))
    println("------------")
    println(reportResults("The better the 4-wheel drive, the further you'll be from help when ya get stuck!"))
    println("------------")
    println(reportResults("headmistressship"))
    println("------------")
    println(reportResults("aardvark"))
  }


}

Output:

ORIGINAL  : length = 0, string = «««»»»
COLLAPSED : length = 0, string = «««»»»
This string is NOT collapsible !
------------
ORIGINAL  : length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
COLLAPSED : length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
This string IS collapsible !
------------
ORIGINAL  : length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
COLLAPSED : length = 4, string = «««.178»»»
This string IS collapsible !
------------
ORIGINAL  : length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
COLLAPSED : length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
This string IS collapsible !
------------
ORIGINAL  : length = 72, string = «««                                                    --- Harry S Truman  »»»
COLLAPSED : length = 17, string = ««« - Hary S Truman »»»
This string IS collapsible !
------------
ORIGINAL  : length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
COLLAPSED : length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»
This string IS collapsible !
------------
ORIGINAL  : length = 16, string = «««headmistressship»»»
COLLAPSED : length = 14, string = «««headmistreship»»»
This string IS collapsible !
------------
ORIGINAL  : length = 8, string = «««aardvark»»»
COLLAPSED : length = 7, string = «««ardvark»»»
This string IS collapsible !

sed

Since sed has no native support for arithmetic, line length counting is omitted in this solution for simplicity:

h
s/.*/<<<&>>>/
x
s/\(.\)\1*/\1/g
s/.*/<<<&>>>/
H
s/.*//
G

Test:

printf '%s\n' \
    '' \
    '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ' \
    '..1111111111111111111111111111111111111111111111111111111111111117777888' \
    "I never give 'em hell, I just tell the truth, and they think it's hell. " \
    '                                                    --- Harry S Truman  ' \
    | sed -f collapse.sed

Output:


<<<>>>
<<<>>>

<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>

<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>
<<<.178>>>

<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>

<<<                                                    --- Harry S Truman  >>>
<<< - Hary S Truman >>>

Sidef

func squeeze(str) {
    str.gsub(/(.)\1+/, {|s1| s1 })
}

var strings = ["",
        '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌"]

strings.each {|str|
    var ssq = squeeze(str)
    say "«««#{str}»»» (length: #{str.len})"
    say "«««#{ssq}»»» (length: #{ssq.len})\n"
}

Output:

«««»»» (length: 0)
«««»»» (length: 0)

«««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»» (length: 72)
«««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»» (length: 70)

«««..1111111111111111111111111111111111111111111111111111111111111117777888»»» (length: 72)
«««.178»»» (length: 4)

«««I never give 'em hell, I just tell the truth, and they think it's hell. »»» (length: 72)
«««I never give 'em hel, I just tel the truth, and they think it's hel. »»» (length: 69)

«««                                                   ---  Harry S Truman  »»» (length: 72)
««« - Hary S Truman »»» (length: 17)

«««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»» (length: 80)
«««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»» (length: 77)

«««headmistressship»»» (length: 16)
«««headmistreship»»» (length: 14)

«««aardvark»»» (length: 8)
«««ardvark»»» (length: 7)

«««😍😀🙌💃😍😍😍🙌»»» (length: 8)
«««😍😀🙌💃😍🙌»»» (length: 6)

Smalltalk

Works with: Smalltalk/X

#(
  'The better the 4-wheel drive, the further you''ll be from help when ya get stuck!'
  'headmistressship'
  'aardvark'
  '😍😀🙌💃😍😍😍🙌'
) do:[:eachWord |
  |shortened|

  shortened := 
    String streamContents:[:out |
        eachWord inject:nil into:[:prev :this |
            prev ~= this ifTrue:[out nextPut:this].
            this
        ]
    ].
  Transcript 
    showCR:( eachWord,'(length:',eachWord size,')' );
    showCR:( shortened,'(length:',shortened size,')' ). 
]

Output:

The better the 4-wheel drive, the further you'll be from help when ya get stuck!(length:80)
The beter the 4-whel drive, the further you'l be from help when ya get stuck!(length:77)
headmistressship(length:16)
headmistreship(length:14)
aardvark(length:8)
ardvark(length:7)
😍😀🙌💃😍😍😍🙌(length:8)
😍😀🙌💃😍🙌(length:6)

Swift

let strings = [
	"",
	#""If I were two-faced, would I be wearing this one?" --- Abraham Lincoln "#,
	"..1111111111111111111111111111111111111111111111111111111111111117777888",
	"I never give 'em hell, I just tell the truth, and they think it's hell. ",
	"                                                   ---  Harry S Truman  ",
	"The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
	"headmistressship",
	"aardvark",
	"😍😀🙌💃😍😍😍🙌"
]

let collapsedStrings = strings.map { $0.replacingOccurrences( of: #"(.)\1*"#, with: "$1", options: .regularExpression)}

for (original, collapsed) in zip(strings, collapsedStrings) {
	print (String(format: "%03d «%@»\n%03d «%@»\n", original.count, original, collapsed.count, collapsed))
}

Output:

000 «»
000 «»

072 «"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »
070 «"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »

072 «..1111111111111111111111111111111111111111111111111111111111111117777888»
004 «.178»

072 «I never give 'em hell, I just tell the truth, and they think it's hell. »
069 «I never give 'em hel, I just tel the truth, and they think it's hel. »

072 «                                                   ---  Harry S Truman  »
017 « - Hary S Truman »

080 «The better the 4-wheel drive, the further you'll be from help when ya get stuck!»
077 «The beter the 4-whel drive, the further you'l be from help when ya get stuck!»

016 «headmistressship»
014 «headmistreship»

008 «aardvark»
007 «ardvark»

008 «😍😀🙌💃😍😍😍🙌»
006 «😍😀🙌💃😍🙌»

Tcl

Please note the ;# ' comments, there appears to be a syntax hightlighting bug with RC

set test {
    {}
    {"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln }
    {..1111111111111111111111111111111111111111111111111111111111111117777888}
    {I never give 'em hell, I just tell the truth, and they think it's hell. } ;# '
    {                                                    --- Harry S Truman  }
    {The better the 4-wheel drive, the further you'll be from help when ya get stuck!} ;# '
    {headmistressship}
}

foreach {str} $test {
    # Uses regexp lookbehind to detect repeated characters
    set sub [regsub -all {(.)(\1+)} $str {\1}]

    # Output
    puts [format "Original (length %3d): %s" [string length $str] $str]
    puts [format "Subbed   (length %3d): %s" [string length $sub] $sub]
    puts ----------------------
}

Output:

Original (length   0):
Subbed   (length   0):
----------------------
Original (length  72): "If I were two-faced, would I be wearing this one?" --- Abraham Lincoln
Subbed   (length  70): "If I were two-faced, would I be wearing this one?" - Abraham Lincoln
----------------------
Original (length  72): ..1111111111111111111111111111111111111111111111111111111111111117777888
Subbed   (length   4): .178
----------------------
Original (length  72): I never give 'em hell, I just tell the truth, and they think it's hell.
Subbed   (length  69): I never give 'em hel, I just tel the truth, and they think it's hel.
----------------------
Original (length  72):                                                     --- Harry S Truman
Subbed   (length  17):  - Hary S Truman
----------------------
Original (length  80): The better the 4-wheel drive, the further you'll be from help when ya get stuck!
Subbed   (length  77): The beter the 4-whel drive, the further you'l be from help when ya get stuck!
----------------------
Original (length  16): headmistressship
Subbed   (length  14): headmistreship
----------------------

Visual Basic .NET

Translation of: C#

Module Module1

    Function Collapse(s As String) As String
        If String.IsNullOrEmpty(s) Then
            Return ""
        End If
        Return s(0) + New String(Enumerable.Range(1, s.Length - 1).Where(Function(i) s(i) <> s(i - 1)).Select(Function(i) s(i)).ToArray)
    End Function

    Sub Main()
        Dim input() = {
             "",
            "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
            "headmistressship",
            ControlChars.Quote + "If I were two-faced, would I be wearing this one?" + ControlChars.Quote + " --- Abraham Lincoln ",
            "..1111111111111111111111111111111111111111111111111111111111111117777888",
            "I never give 'em hell, I just tell the truth, and they think it's hell. ",
            "                                                    --- Harry S Truman  "
           }
        For Each s In input
            Console.WriteLine($"old: {s.Length} «««{s}»»»")
            Dim c = Collapse(s)
            Console.WriteLine($"new: {c.Length} «««{c}»»»")
        Next
    End Sub

End Module

Output:

old: 0 «««»»»
new: 0 «««»»»
old: 80 «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
new: 77 «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»
old: 16 «««headmistressship»»»
new: 14 «««headmistreship»»»
old: 72 «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
new: 70 «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»
old: 72 «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
new: 4 «««.178»»»
old: 72 «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
new: 69 «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»
old: 72 «««                                                    --- Harry S Truman  »»»
new: 17 ««« - Hary S Truman »»»

VBA

Function :

Function Collapse(strIn As String) As String
Dim i As Long, strOut As String
    If Len(strIn) > 0 Then
        strOut = Mid$(strIn, 1, 1)
        For i = 2 To Len(strIn)
            If Mid$(strIn, i, 1) <> Mid$(strIn, i - 1, 1) Then
                strOut = strOut & Mid$(strIn, i, 1)
            End If
        Next i
    End If
    Collapse = strOut
End Function

To Call :

Sub CallCollapse()
Dim S As String
    S = vbNullString
    Debug.Print "String : <<<" & S & ">>>", "Lenght : " & Len(S)
    Debug.Print "Collapsed : <<<" & Collapse(S) & ">>>", "Lenght : " & Len(Collapse(S))
    S = """If I were two-faced, would I be wearing this one?"" --- Abraham Lincoln "
    Debug.Print "String : <<<" & S & ">>>", "Lenght : " & Len(S)
    Debug.Print "Collapsed : <<<" & Collapse(S) & ">>>", "Lenght : " & Len(Collapse(S))
    S = "..1111111111111111111111111111111111111111111111111111111111111117777888"
    Debug.Print "String : <<<" & S & ">>>", "Lenght : " & Len(S)
    Debug.Print "Collapsed : <<<" & Collapse(S) & ">>>", "Lenght : " & Len(Collapse(S))
    S = "I never give 'em hell, I just tell the truth, and they think it's hell. "
    Debug.Print "String : <<<" & S & ">>>", "Lenght : " & Len(S)
    Debug.Print "Collapsed : <<<" & Collapse(S) & ">>>", "Lenght : " & Len(Collapse(S))
    S = "                                                    --- Harry S Truman  "
    Debug.Print "String : <<<" & S & ">>>", "Lenght : " & Len(S)
    Debug.Print "Collapsed : <<<" & Collapse(S) & ">>>", "Lenght : " & Len(Collapse(S))
End Sub

Output:

String : <<<>>>             Lenght : 0
Collapsed : <<<>>>          Lenght : 0
String : <<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>>           Lenght : 72
Collapsed : <<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>>          Lenght : 70
String : <<<..1111111111111111111111111111111111111111111111111111111111111117777888>>>           Lenght : 72
Collapsed : <<<.178>>>      Lenght : 4
String : <<<I never give 'em hell, I just tell the truth, and they think it's hell. >>>           Lenght : 72
Collapsed : <<<I never give 'em hel, I just tel the truth, and they think it's hel. >>>           Lenght : 69
String : <<<                                                    --- Harry S Truman  >>>           Lenght : 72
Collapsed : <<< - Hary S Truman >>>       Lenght : 17

V (Vlang)

Translation of: Go

// Returns collapsed string, original and new lengths in
// unicode code points (not normalized).
fn collapse(s string) (string, int, int) {
    mut r := s.runes()
    le, mut del := r.len, 0
    for i := le - 2; i >= 0; i-- {
        if r[i] == r[i+1] {
            r.delete(i)
            del++
        }
    }
    if del == 0 {
        return s, le, le
    }
    r = r[..le-del]
    return r.string(), le, r.len
}
 
fn main() {
    strings:= [
        "",
        '"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln ',
        "..1111111111111111111111111111111111111111111111111111111111111117777888",
        "I never give 'em hell, I just tell the truth, and they think it's hell. ",
        "                                                   ---  Harry S Truman  ",
        "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
        "headmistressship",
        "aardvark",
        "😍😀🙌💃😍😍😍🙌",
	]
    for s in strings {
        cs, olen, clen := collapse(s)
        println("original : length = ${olen:2}, string = «««$s»»»")
        println("collapsed: length = ${clen:2}, string = «««$cs»»»\n")
    }
}

Output:

original : length =  0, string = «««»»»
collapsed: length =  0, string = «««»»»

original : length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
collapsed: length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

original : length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
collapsed: length =  4, string = «««.178»»»

original : length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
collapsed: length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

original : length = 72, string = «««                                                   ---  Harry S Truman  »»»
collapsed: length = 17, string = ««« - Hary S Truman »»»

original : length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
collapsed: length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

original : length = 16, string = «««headmistressship»»»
collapsed: length = 14, string = «««headmistreship»»»

original : length =  8, string = «««aardvark»»»
collapsed: length =  7, string = «««ardvark»»»

original : length =  8, string = «««😍😀🙌💃😍😍😍🙌»»»
collapsed: length =  6, string = «««😍😀🙌💃😍🙌»»»

Wren

Translation of: Go

Library: Wren-fmt

import "./fmt" for Fmt

// Returns collapsed string, original and new lengths in
// unicode code points (not normalized).
var collapse = Fn.new { |s|
    var c = s.codePoints.toList
    var le = c.count
    if (le < 2) return [s, le, le]
    for (i in le-2..0) {
        if (c[i] == c[i+1]) c.removeAt(i)
    }
    var cc = c.reduce("") { |acc, cp| acc + String.fromCodePoint(cp) }
    return [cc, le, cc.count]
}

var strings = [
    "",
    "\"If I were two-faced, would I be wearing this one?\" --- Abraham Lincoln ",
    "..1111111111111111111111111111111111111111111111111111111111111117777888",
    "I never give 'em hell, I just tell the truth, and they think it's hell. ",
    "                                                   ---  Harry S Truman  ",
    "The better the 4-wheel drive, the further you'll be from help when ya get stuck!",
    "headmistressship",
    "aardvark",
    "😍😀🙌💃😍😍😍🙌"
]

for (s in strings) {
    var r = collapse.call(s)
    System.print("original : length = %(Fmt.d(2, r[1])), string = «««%(s)»»»")
    System.print("collapsed: length = %(Fmt.d(2, r[2])), string = «««%(r[0])»»»\n")
}

Output:

original : length =  0, string = «««»»»
collapsed: length =  0, string = «««»»»

original : length = 72, string = «««"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln »»»
collapsed: length = 70, string = «««"If I were two-faced, would I be wearing this one?" - Abraham Lincoln »»»

original : length = 72, string = «««..1111111111111111111111111111111111111111111111111111111111111117777888»»»
collapsed: length =  4, string = «««.178»»»

original : length = 72, string = «««I never give 'em hell, I just tell the truth, and they think it's hell. »»»
collapsed: length = 69, string = «««I never give 'em hel, I just tel the truth, and they think it's hel. »»»

original : length = 72, string = «««                                                   ---  Harry S Truman  »»»
collapsed: length = 17, string = ««« - Hary S Truman »»»

original : length = 80, string = «««The better the 4-wheel drive, the further you'll be from help when ya get stuck!»»»
collapsed: length = 77, string = «««The beter the 4-whel drive, the further you'l be from help when ya get stuck!»»»

original : length = 16, string = «««headmistressship»»»
collapsed: length = 14, string = «««headmistreship»»»

original : length =  8, string = «««aardvark»»»
collapsed: length =  7, string = «««ardvark»»»

original : length =  8, string = «««😍😀🙌💃😍😍😍🙌»»»
collapsed: length =  6, string = «««😍😀🙌💃😍🙌»»»

XPL0

string 0;
char C, I, J, Last;

proc Collapse(S);       \Eliminate immediately repeated characters from string
char S;
[I:= 0;  J:= 0;  Last:= -1;
loop [if S(I) # Last then
        [C(J):= S(I);
        if S(I) = 0 then quit;
        J:= J+1;
        ];
     Last:= S(I);
     I:= I+1;
     ];
];

int String, K;
[String:= [
    "",
    "^"If I were two-faced, would I be wearing this one?^" --- Abraham Lincoln ",
    "..1111111111111111111111111111111111111111111111111111111111111117777888",
    "I never give 'em hell, I just tell the truth, and they think it's hell. ",
    "                                                    --- Harry S Truman  "];
C:= Reserve(79+1);      \space for collapsed string
for K:= 0 to 4 do
    [Collapse(String(K));
    Text(0, "<<<");  Text(0, String(K));  Text(0, ">>> ");  IntOut(0, I);  CrLf(0);
    Text(0, "<<<");  Text(0, C);  Text(0, ">>> ");  IntOut(0, J);  CrLf(0);
    ];
]

Output:

<<<>>> 0
<<<>>> 0
<<<"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln >>> 72
<<<"If I were two-faced, would I be wearing this one?" - Abraham Lincoln >>> 70
<<<..1111111111111111111111111111111111111111111111111111111111111117777888>>> 72
<<<.178>>> 4
<<<I never give 'em hell, I just tell the truth, and they think it's hell. >>> 72
<<<I never give 'em hel, I just tel the truth, and they think it's hel. >>> 69
<<<                                                    --- Harry S Truman  >>> 72
<<< - Hary S Truman >>> 17

zkl

fcn collapsible(str){			// no Unicode
   sink:=Sink(String);
   str.reduce('wrap(c1,c2){ if(c1!=c2) sink.write(c2); c2 },"");  // prime with \0
   cstr:=sink.close();
   return(str.len()!=cstr.len(), cstr);
}

strings:=
0'^
"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln 
..1111111111111111111111111111111111111111111111111111111111111117777888
I never give 'em hell, I just tell the truth, and they think it's hell. 
                                                    --- Harry S Truman  
The American people have a right to know if their president is a crook. 
                                                    --- Richard Nixon   
The better the 4-wheel drive, the further you'll be from help when ya get stuck!
headmistressship
aardvark^
.split("\n");
#<<<

foreach s in (strings){
   println("Before: %3d >>>%s<<<".fmt(s.len(),s));
   _,cstr:=collapsible(s);
   println("After:  %3d >>>%s<<<\n".fmt(cstr.len(),cstr));
}

Output:

Before:   0 >>><<<
After:    0 >>><<<

Before:  72 >>>"If I were two-faced, would I be wearing this one?" --- Abraham Lincoln <<<
After:   70 >>>"If I were two-faced, would I be wearing this one?" - Abraham Lincoln <<<

Before:  72 >>>..1111111111111111111111111111111111111111111111111111111111111117777888<<<
After:    4 >>>.178<<<

Before:  72 >>>I never give 'em hell, I just tell the truth, and they think it's hell. <<<
After:   69 >>>I never give 'em hel, I just tel the truth, and they think it's hel. <<<

Before:  72 >>>                                                    --- Harry S Truman  <<<
After:   17 >>> - Hary S Truman <<<

Before:  72 >>>The American people have a right to know if their president is a crook. <<<
After:   71 >>>The American people have a right to know if their president is a crok. <<<

Before:  72 >>>                                                    --- Richard Nixon   <<<
After:   17 >>> - Richard Nixon <<<

Before:  80 >>>The better the 4-wheel drive, the further you'll be from help when ya get stuck!<<<
After:   77 >>>The beter the 4-whel drive, the further you'l be from help when ya get stuck!<<<

Before:  16 >>>headmistressship<<<
After:   14 >>>headmistreship<<<

Before:   8 >>>aardvark<<<
After:    7 >>>ardvark<<<