Reverse a string: Difference between revisions

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Revision as of 23:34, 13 October 2021

Task

Take a string and reverse it.

For example, "asdf" becomes "fdsa".

Extra credit

Preserve Unicode combining characters.

For example, "as⃝df̅" becomes "f̅ds⃝a", not "̅fd⃝sa".

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

0815

This program reverses each line of its input. <lang 0815>}:r: Start reader loop.

 !~>&   Push a character to the "stack".
 <:a:=- Stop reading on newline.

^:r: @> Rotate the newline to the end and enqueue a sentinel 0. {~ Dequeue and rotate the first character into place. }:p:

 ${~    Print the current character until it's 0.

^:p:

r: Read again.</lang>

Output:

For maximum compatibility, this program uses only the basic instruction set (S/360) and an ASSIST macro (XPRNT) to keep the code as short as possible. <lang 360asm>* Reverse a string 21/05/2016 REVERSE CSECT

        USING  REVERSE,R13        base register
        B      72(R15)            skip savearea
        DC     17F'0'             savearea
        STM    R14,R12,12(R13)    prolog
        ST     R13,4(R15)         "
        ST     R15,8(R13)         "
        LR     R13,R15            "
        MVC    TMP(L'C),C         tmp=c
        LA     R8,C               @c[1]
        LA     R9,TMP+L'C-1       @tmp[n-1]
        LA     R6,1               i=1
        LA     R7,L'C             n=length(c)

LOOPI CR R6,R7 do i=1 to n

        BH     ELOOPI             leave i
        MVC    0(1,R8),0(R9)        substr(c,i,1)=substr(tmp,n-i+1,1)
        LA     R8,1(R8)             @c=@c+1
        BCTR   R9,0                 @tmp=@tmp-1
        LA     R6,1(R6)             i=i+1
        B      LOOPI              next i

ELOOPI XPRNT C,L'C print c

        L      R13,4(0,R13)       epilog
        LM     R14,R12,12(R13)    "
        XR     R15,R15            "
        BR     R14                exit

C DC CL12'edoC attesoR' TMP DS CL12

        REGEQU
        END    REVERSE</lang>

Output:

Rosetta Code

This second example uses MVCIN introduced in S/370 architecture. <lang 360asm>* Reverse a string 25/04/2020 REVERSEI CSECT

        USING  REVERSEI,R13       base register
        B      72(R15)            skip savearea
        DC     17F'0'             savearea
        STM    R14,R12,12(R13)    prolog
        ST     R13,4(R15)         "
        ST     R15,8(R13)         "
        LR     R13,R15            "
        MVCIN  BB,AA+L'AA-1
        XPRNT  BB,L'BB            print bb
        L      R13,4(0,R13)       epilog
        LM     R14,R12,12(R13)    "
        XR     R15,R15            "
        BR     R14                exit

AA DC CL12'edoC attesoR' a BB DS CL(L'AA) b

        REGEQU
        END    REVERSEI</lang>

Output:

Rosetta Code

8080 Assembly

This is a routine that reverses a string with a terminator in place.

Back when the 8080 was commonly used, there wasn't really a set standard about how to store strings. Zero-terminated strings were already in use by the C language (and therefore, programs written in it). CP/M, on the other hand, used $ as a string terminator. (Later versions would even allow the programmer to set it himself with a system call!) Therefore, to allow for some flexibility, this routine also allows you to set it yourself, using the A register.

There were other ways of representing strings, like setting the high bit of the last character to mark the end (saves a byte per string, but halves the character set size), or prepending the length (making it unnecessary to scan through the string to find the end, but capping string size at 255 bytes), or even storing tuples of lengths and pointers (easy for a garbage collector to manage). These are not supported, as they would be completely different routines, though the arrayrev entry point will reverse a byte array if you already have its start and end.

Unicode is not supported either. While it wouldn't be impossible to do, I think writing a full UTF-8 implementation is beyond the scope of the task.

<lang 8080asm> org 100h jmp test

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; Reverse a string under HL in place ;; strrev0: reverse a zero-terminated string ;; strrev: reverse a string terminated by the value in A ;; arrayrev: reverse bytes starting at DE and ending at HL ;; Destroys a, b, d, e, h, l registers.

strrev0: xra a ; Zero A strrev: mov d,h ; Copy string begin to DE mov e,l dcx h strrev_end: inx h ; Find string end in HL cmp m jnz strrev_end dcx h ; Point HL to last character arrayrev: mov a,h ; If HL<DE, we're done cmp d rc mov a,l cmp e rc ldax d ; Get low character in string mov b,m ; Get high character in string mov m,a ; Swap them mov a,b stax d inx d ; Move the low pointer up, dcx h ; and the high pointer down jmp arrayrev

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; Test code (CP/M): ask the user for a string, and reverse it

prompt: db " :gnirts a retne esaelP$" bufdef: db 127, 0 buf: ds 128 ; one extra byte that will remain 0

newline: lxi d,newline_str mvi c,9 jmp 5 newline_str: db 13, 10, "$"

test: ;; Reverse and output the prompt mvi a,'$' ; CP/M string is $-terminated lxi h,prompt ; Reverse the string call strrev lxi d,prompt ; Output the string mvi c,9 call 5

;; Get input and reverse it lxi d,bufdef mvi c,10 call 5 call newline lxi h,buf call strrev0 ; 0-terminated due to buffer definition

;; Output reversed input lxi h,buf loop: mov e,m xra a ora e rz ; Stop when done mvi c,2 push h call 5 pop h inx h jmp loop</lang>

8th

In 8th strings are UTF-8 and the language retains characters per-se: <lang forth> "abc" s:rev </lang>

Output:

"cba"

ACL2

<lang Lisp>(reverse "hello")</lang>

ACL2 does not support unicode.

ActionScript

<lang ActionScript>function reverseString(string:String):String { var reversed:String = new String(); for(var i:int = string.length -1; i >= 0; i--) reversed += string.charAt(i); return reversed; }

function reverseStringCQAlternative(string:String):String { return string.split().reverse().join(); }</lang>

Ada

<lang ada>with Ada.Text_IO; use Ada.Text_IO;

procedure Reverse_String is

  function Reverse_It (Item : String) return String is
     Result : String (Item'Range);
  begin
     for I in Item'range loop
        Result (Result'Last - I + Item'First) := Item (I);
     end loop;
     return Result;
  end Reverse_It;

begin

  Put_Line (Reverse_It (Get_Line));

end Reverse_String;</lang>

Agda

Using the Agda standard library, version 0.6. <lang agda2>module reverse_string where

open import Data.String open import Data.List

reverse_string : String → String reverse_string s = fromList (reverse (toList s))</lang>

Aime

<lang aime>o_(b_reverse("Hello, World!"), "\n");</lang>

ALGOL 68

Works with: ALGOL 68 version Standard - no extensions to language used

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

<lang algol68>PROC reverse = (REF STRING s)VOID:

 FOR i TO UPB s OVER 2 DO
   CHAR c = s[i];
   s[i] := s[UPB s - i + 1];
   s[UPB s - i + 1] := c
 OD;

main: (

 STRING text := "Was it a cat I saw";
 reverse(text);
 print((text, new line))

)</lang>

Output:

was I tac a ti saW

Apex

<lang java> String str = 'Hello World!'; str = str.reverse(); system.debug(str); </lang>

APL

AppleScript

Works with: AppleScript version 2.0 or newer.

<lang AppleScript>reverseString("Hello World!")

on reverseString(str)

   reverse of characters of str as string

end reverseString</lang>

NB. Since coercing lists to string involves the interpolation of the current value of AppleScript's text item delimiters between the list items, it's considered best practice to set the delimiters explicitly to their default value of {""} (or just "") before doing an operation like this, in case they've been set to something else elsewhere in the script:

<lang applescript>reverseString("Hello World!")

on reverseString(str)

   set astid to AppleScript's text item delimiters
   set AppleScript's text item delimiters to ""
   set reversedString to reverse of characters of str as text
   set AppleScript's text item delimiters to astid
   return reversedString

end reverseString</lang>

Or, if we want a polymorphic reverse() for both strings and lists, we can define it either in terms of a generic fold/reduce, or using the built-in method for lists:

<lang AppleScript>-- Using either a generic foldr(f, a, xs)

-- reverse1 :: [a] -> [a] on reverse1(xs)

   script rev
       on |λ|(a, x)
           a & x
       end |λ|
   end script
   
   if class of xs is text then
       foldr(rev, {}, xs) as text
   else
       foldr(rev, {}, xs)
   end if

end reverse1

-- or the built-in reverse method for lists

-- reverse2 :: [a] -> [a] on reverse2(xs)

   if class of xs is text then
       (reverse of characters of xs) as text
   else
       reverse of xs
   end if

end reverse2

-- TESTING reverse1 and reverse2 with same string and list --------------------------------------------------------------------------- on run

   script test
       on |λ|(f)
           map(f, ["Hello there !", {1, 2, 3, 4, 5}])
       end |λ|
   end script
   
   map(test, [reverse1, reverse2])

end run

-- GENERIC FUNCTIONS ---------------------------------------------------------------------------

-- foldr :: (a -> b -> a) -> a -> [b] -> a on foldr(f, startValue, xs)

   tell mReturn(f)
       set v to startValue
       set lng to length of xs
       repeat with i from lng to 1 by -1
           set v to |λ|(v, item i of xs, i, xs)
       end repeat
       return v
   end tell

end foldr

-- map :: (a -> b) -> [a] -> [b] on map(f, xs)

   tell mReturn(f)
       set lng to length of xs
       set lst to {}
       repeat with i from 1 to lng
           set end of lst to |λ|(item i of xs, i, xs)
       end repeat
       return lst
   end tell

end map

-- Lift 2nd class handler function into 1st class script wrapper -- mReturn :: Handler -> Script on mReturn(f)

   if class of f is script then
       f
   else
       script
           property |λ| : f
       end script
   end if

end mReturn</lang>

Output:

<lang AppleScript>{{"! ereht olleH", {5, 4, 3, 2, 1}},

{"! ereht olleH", {5, 4, 3, 2, 1}}}</lang>

Applesoft BASIC

<lang ApplesoftBasic>10 A$ = "THE FIVE BOXING WIZARDS JUMP QUICKLY" 20 GOSUB 100REVERSE 30 PRINT R$ 40 END

100 REMREVERSE A$ 110 R$ = "" 120 FOR I = 1 TO LEN(A$) 130 R$ = MID$(A$, I, 1) + R$ 140 NEXT I 150 RETURN</lang>

Arturo

<lang rebol>str: "Hello World"

print reverse str</lang>

Output:

dlroW olleH

AutoHotkey

"Normal" version:

<lang AutoHotkey>MsgBox % reverse("asdf")

reverse(string) {

 Loop, Parse, string
   reversed := A_LoopField . reversed
 Return reversed

}</lang>

A ''much'' slower version:

<lang AHK>Reverse(String){ ; credit to Rseding91

  If (A_IsUnicode){
     SLen := StrLen(String) * 2
     VarSetCapacity(RString,SLen)
     
     Loop,Parse,String
        NumPut(Asc(A_LoopField),RString,SLen-(A_Index * 2),"UShort")
  } Else {
     SLen := StrLen(String)
     VarSetCapacity(RString,SLen)
     
     Loop,Parse,String
        NumPut(Asc(A_LoopField),RString,SLen-A_Index,"UChar")
  }
  
  VarSetCapacity(RString,-1)
  
  Return RString

}</lang>

AutoIt

<lang AutoIt>#AutoIt Version: 3.2.10.0 $mystring="asdf" $reverse_string = "" $string_length = StringLen($mystring)

For $i = 1 to $string_length

  $last_n_chrs = StringRight($mystring, $i)
  $nth_chr = StringTrimRight($last_n_chrs, $i-1)
  $reverse_string= $reverse_string & $nth_chr

Avail

<lang Avail>"asfd" reversed</lang>

AWK

<lang awk>function reverse(s) {

 p = ""
 for(i=length(s); i > 0; i--) { p = p substr(s, i, 1) }
 return p

}

BEGIN {

 print reverse("edoCattesoR")

}</lang>

Recursive

<lang awk>function reverse(s ,l) {

 l = length(s)
 return l < 2 ? s:( substr(s,l,1) reverse(substr(s,1,l-1)) )

}

BEGIN {

 print reverse("edoCattesoR")

}</lang>

using split, then joining in front

<lang awk># Usage: awk -f reverse.awk -v s=Rosetta

function rev(s, i,len,a,r) {

  len = split(s, a, "")
 #for (i in a) r = a[i] r	# may not work - order is not guaranteed !
  for (i=1; i<=len; i++) r = a[i] r
  return r

} BEGIN {

  if(!s) s = "Hello, world!" 
  print s, "<-->", rev(s)

} </lang>

Output:

 Rosetta <--> attesoR

Babel

This example will handle UTF-8 encoded Unicode but doesn't handle combining characters. <lang babel>strrev: { str2ar ar2ls reverse ls2lf ar2str }</lang>

str2ar - this operator converts a UTF-8 encoded string to an array of Unicode codepoints
ar2ls - this operator converts the array to a linked-list
reverse - this operator reverses a linked-list
ls2lf - this operator undoes the effect of ar2ls
ar2str - this operator undoes the effect of str2ar

BaCon

<lang freebasic>OPTION UTF8 TRUE s$ = "asdf" PRINT REVERSE$(s$)</lang>

Unicode preservation works in BaCon 3.6 and higher.

BASIC

Works with: QuickBasic version 4.5

<lang qbasic>function reverse$(a$)

  b$ = ""
  for i = 1 to len(a$)
     b$ = mid$(a$, i, 1) + b$
  next i
  reverse$ = b$

end function</lang>

IS-BASIC

<lang IS-BASIC>100 INPUT PROMPT "String: ":TX$ 120 LET REV$="" 130 FOR I=LEN(TX$) TO 1 STEP-1 140 LET REV$=REV$&TX$(I) 150 NEXT 160 PRINT REV$</lang>

Sinclair ZX81 BASIC

<lang basic>10 INPUT S$ 20 LET T$="" 30 FOR I=LEN S$ TO 1 STEP -1 40 LET T$=T$+S$(I) 50 NEXT I 60 PRINT T$</lang>

Batch File

<lang dos>@echo off setlocal enabledelayedexpansion call :reverse %1 res echo %res% goto :eof

reverse

set str=%~1 set cnt=0

loop

if "%str%" equ "" ( goto :eof ) set chr=!str:~0,1! set str=%str:~1% set %2=%chr%!%2! goto loop</lang>

BBC BASIC

<lang bbcbasic> PRINT FNreverse("The five boxing wizards jump quickly")

     END
     
     DEF FNreverse(A$)
     LOCAL B$, C%
     FOR C% = LEN(A$) TO 1 STEP -1
       B$ += MID$(A$,C%,1)
     NEXT
     = B$</lang>

Befunge

Reads a line from stdin and write the reverse to stdout. Can be made to repeat indefinitely by removing the final @ command.

Bracmat

<lang bracmat> ( reverse

 = L x
   .     :?L
       & @( !arg
          :   ?
              ( %?x
              & utf$!x
              & !x !L:?L
              & ~`
              )
              ?
          )
     | str$!L
 )

& out$reverse$Ελληνικά</lang>

Output:

άκινηλλΕ

Brainf***

<lang bf>[-]>,[>,]<[.<]</lang> Another solution: <lang bf>,----- ----- [+++++ +++++ > , ----- -----] If a newline is hit counter will be zero and input loop ends <[.<] run all chars backwards and print them

just because it looks good we print CRLF +++++ +++++ +++ . --- .</lang>

Brat

<lang brat>p "olleh".reverse #Prints "hello"</lang>

Burlesque

<lang burlesque> "Hello, world!"<- </lang>

C

<lang c>#include <stdio.h>

include <stdlib.h>
include <locale.h>
include <wchar.h>

const char *sa = "abcdef"; const char *su = "as⃝df̅"; /* Should be in your native locale encoding. Mine is UTF-8 */

int is_comb(wchar_t c) { if (c >= 0x300 && c <= 0x36f) return 1; if (c >= 0x1dc0 && c <= 0x1dff) return 1; if (c >= 0x20d0 && c <= 0x20ff) return 1; if (c >= 0xfe20 && c <= 0xfe2f) return 1; return 0; }

wchar_t* mb_to_wchar(const char *s) { wchar_t *u; size_t len = mbstowcs(0, s, 0) + 1; if (!len) return 0;

u = malloc(sizeof(wchar_t) * len); mbstowcs(u, s, len); return u; }

wchar_t* ws_reverse(const wchar_t* u) { size_t len, i, j; wchar_t *out; for (len = 0; u[len]; len++); out = malloc(sizeof(wchar_t) * (len + 1)); out[len] = 0; j = 0; while (len) { for (i = len - 1; i && is_comb(u[i]); i--); wcsncpy(out + j, u + i, len - i); j += len - i; len = i; } return out; }

char *mb_reverse(const char *in) { size_t len; char *out; wchar_t *u = mb_to_wchar(in); wchar_t *r = ws_reverse(u); len = wcstombs(0, r, 0) + 1; out = malloc(len); wcstombs(out, r, len); free(u); free(r); return out; }

int main(void) { setlocale(LC_CTYPE, "");

printf("%s => %s\n", sa, mb_reverse(sa)); printf("%s => %s\n", su, mb_reverse(su)); return 0; }</lang>

Output:

abcdef => fedcba
as⃝df̅ => f̅ds⃝a

Library: GLib

<lang c>#include <glib.h> gchar *srev (const gchar *s) {

   if (g_utf8_validate(s,-1,NULL)) {
       return g_utf8_strreverse (s,-1);

} } // main int main (void) {

   const gchar *t="asdf";
   const gchar *u="as⃝df̅";
   printf ("%s\n",srev(t));
   printf ("%s\n",srev(u));
   return 0;

}</lang>

C#

C# does not have a built-in Reverse method for strings, and cannot reverse them in place because they are immutable. One way to implement this is to convert the string to an array of characters, reverse that, and return a new string from the reversed array: <lang csharp>static string ReverseString(string input) {

   char[] inputChars = input.ToCharArray();
   Array.Reverse(inputChars);
   return new string(inputChars);

}</lang>

As of .Net 3.5 the LINQ-to-objects allows the Reverse() extension method to be called on a string, since String implements the IEnumerable<char> interface. Because of this, the return type of Reverse is IEnumerable<char>. Fortunately, LINQ also provides the ToArray extension method, which can be used in conjunction with the constructor of string that accepts a char array: <lang csharp>using System.Linq;

// ...

return new string(input.Reverse().ToArray());

// ...</lang>

Version supporting combining characters:

System.Globalization.StringInfo provides a means of separating a string into individual graphemes. <lang csharp>public string ReverseElements(string s) {

   // In .NET, a text element is series of code units that is displayed as one character, and so reversing the text
   // elements of the string correctly handles combining character sequences and surrogate pairs.
   var elements = System.Globalization.StringInfo.GetTextElementEnumerator(s);
   return string.Concat(AsEnumerable(elements).OfType<string>().Reverse());

}

// Wraps an IEnumerator, allowing it to be used as an IEnumerable. public IEnumerable AsEnumerable(IEnumerator enumerator) {

   while (enumerator.MoveNext())
       yield return enumerator.Current;

}</lang>

C++

<lang cpp>#include <iostream>

include <string>
include <algorithm>

int main() {

 std::string s;
 std::getline(std::cin, s);
 std::reverse(s.begin(), s.end()); // modifies s
 std::cout << s << std::endl;
 return 0;

}</lang>

Caché ObjectScript

USER>Write $Reverse("Hello, World")

dlroW ,olleH

Ceylon

<lang Ceylon> shared void run() {

  while(true) {
     process.write("> ");
     String? text = process.readLine();
     if (is String text) {
     print(text.reversed);
     }
     else {
     break;
     }
   }

} </lang>

Clipper

Works with versions since 5, because LOCAL variables and the += operator was not implemented before. <lang Clipper>FUNCTION Reverse(sIn)

  LOCAL sOut := "", i
  FOR i := Len(sIn) TO 1 STEP -1
     sOut += Substr(sIn, i, 1)
  NEXT

RETURN sOut</lang>

Clojure

A Simple implementation with the magic of "conj" function

<lang lisp> (defn reverse-string [s]

 "Returns a string with all characters in reverse"
 (apply str (reduce conj '() s)))

</lang>

Other alternatives (resorting to the "reverse" function in the standard library)

For normal strings, the reverse function can be used to do the bulk of the work. However, it returns a character sequence, which has to be converted back to a string. a) <lang lisp>(defn str-reverse [s] (apply str (reverse s)))</lang>

b) <lang lisp>(apply str (interpose " " (reverse (.split "the quick brown fox" " "))))</lang>

Supporting combining characters

Handling combining characters present a trickier task. We need to protect the relative ordering of the combining character and the character to its left. Thus, before reversing, the characters need to be grouped. <lang lisp>(defn combining? [c]

 (let [type (Character/getType c)]
   ;; currently hardcoded to the types taken from the sample string
   (or (= type 6) (= type 7))))

(defn group

 "Group normal characters with their combining characters"
 [chars]
 (cond (empty? chars) chars

(empty? (next chars)) (list chars) :else (let [dres (group (next chars))] (cond (combining? (second chars)) (cons (cons (first chars) (first dres)) (rest dres)) :else (cons (list (first chars)) dres)))))

(defn str-reverse

 "Unicode-safe string reverse"
 [s]
 (apply str (apply concat (reverse (group s)))))</lang>

Output:

user=> s
"as⃝df̅"
user=> (str-reverse s)
"f̅ds⃝a"[
user=> (str-reverse (str-reverse s))
"as⃝df̅"
user=>

COBOL

<lang cobol>FUNCTION REVERSE('QWERTY')</lang>

CoffeeScript

<lang javascript>"qwerty".split("").reverse().join ""</lang>

ColdFusion

You can reverse anything that can be written to the document in hashmarks (i.e. strings, numbers, now( ), etc.). <lang cfm><cfset myString = "asdf" /> <cfset myString = reverse( myString ) /></lang>

Common Lisp

<lang lisp>(reverse my-string)</lang>

Component Pascal

BlackBox Component Builder <lang oberon2> MODULE BbtReverseString; IMPORT StdLog;

PROCEDURE ReverseStr(str: ARRAY OF CHAR): POINTER TO ARRAY OF CHAR; VAR top,middle,i: INTEGER; c: CHAR; rStr: POINTER TO ARRAY OF CHAR; BEGIN NEW(rStr,LEN(str$) + 1); top := LEN(str$) - 1; middle := (top - 1) DIV 2; FOR i := 0 TO middle DO rStr[i] := str[top - i]; rStr[top - i] := str[i]; END; IF ODD(LEN(str$)) THEN rStr[middle + 1] := str[middle + 1] END; RETURN rStr; END ReverseStr;

PROCEDURE Do*; VAR x: CHAR; BEGIN StdLog.String("'asdf' reversed:> ");StdLog.String(ReverseStr("asdf"));StdLog.Ln END Do; END BbtReverseString. </lang> Execute: ^Q BbtReverseString.Do

Output:

'asdf' reversed:> fdsa

Cowgol

<lang cowgol>include "cowgol.coh"; include "strings.coh";

Reverse a string in place

sub StrRev(s: [uint8]): (r: [uint8]) is

   r := s;
   var e := s;
   while [e] != 0 loop 
       e := @next e; 
   end loop;
   e := @prev e;
   while e > s loop
       var c := [s];
       [s] := [e];
       [e] := c;
       s := @next s;
       e := @prev e;
   end loop;

end sub;

Test

var buf: uint8[32]; var str: [uint8] := "\nesreveR"; CopyString(str, &buf[0]); print(StrRev(&buf[0]));</lang>

Output:

Reverse

Crystal

<lang ruby># version 0.21.1

strings = ["asdf", "as⃝df̅"] strings.each do |s|

 puts "#{s} -> #{s.reverse}"

end</lang>

Output:

asdf -> fdsa
as⃝df̅ -> f̅ds⃝a

D

<lang d>void main() { import std.range, std.conv;

string s1 = "hello"; // UTF-8 assert(s1.retro.text == "olleh");

wstring s2 = "hello"w; // UTF-16 assert(s2.retro.wtext == "olleh"w);

dstring s3 = "hello"d; // UTF-32 assert(s3.retro.dtext == "olleh"d);

// without using std.range: dstring s4 = "hello"d; assert(s4.dup.reverse == "olleh"d); // simple but inefficient (copies first, then reverses) }</lang>

Dart

Since Dart strings are sequences of UTF-16 code units, it would not be sufficient to simply reverse the characters in strings, as this would not work with UTF-16 surrogate pairs (pairs of UTF-16 code units that represent single characters outside the Unicode BMP). However, Dart provides a method to convert strings to sequences of unicode code points (called "runes" in Dart), and these sequences can easily be reversed and used to create new strings, so a string reversal function can be written with a single line of Dart code:

<lang dart>String reverse(String s) => new String.fromCharCodes(s.runes.toList().reversed);</lang>

A more complete example with unit tests would look like this:

<lang dart>import 'package:unittest/unittest.dart';

String reverse(String s) => new String.fromCharCodes(s.runes.toList().reversed);

main() {

 group("Reverse a string -", () {
   test("Strings with ASCII characters are reversed correctly.", () {
     expect(reverse("hello, world"), equals("dlrow ,olleh"));
   });
   test("Strings with non-ASCII BMP characters are reversed correctly.", () {
     expect(reverse("\u4F60\u4EEC\u597D"), equals("\u597D\u4EEC\u4F60"));
   });
   test("Strings with non-BMP characters are reversed correctly.", () {
     expect(reverse("hello, \u{1F310}"), equals("\u{1F310} ,olleh"));
   });
 });

}</lang>

DBL

<lang DBL>K= STR_OUT= FOR J=%TRIM(STR_IN) STEP -1 UNTIL 1 DO BEGIN

     INCR K
     STR_OUT(K:1)=STR_IN(J:1)
  END

</lang>

Dc

Reversing "Hello world!" which is "22405534230753963835153736737" in Dc's numerical string representaion.
Due to using "~" this example needs GNU Dc or OpenBSD Dc. <lang dc>22405534230753963835153736737 [ 256 ~ d SS 0<F LS SR 1+ ] d sF x 1 - [ 1 - d 0<F 256 * LR + ] d sF x P</lang>

!dlrow olleH

Delphi

<lang Delphi>function ReverseString(const InString: string): string; var

 i: integer;

begin

 for i := Length(InString) downto 1 do
   Result := Result + InString[i];

end;</lang> You could also use this RTL function Introduced in Delphi 6: <lang Delphi>StrUtils.ReverseString</lang>

Another alternative. <lang Delphi> function Reverse(const s: string): string; var

 i, aLength, ahalfLength: Integer;
 c: Char;

begin

 Result := s;
 aLength := Length(s);
 ahalfLength := aLength div 2;
 if aLength > 1 then
   for i := 1 to ahalfLength do
   begin
     c := result[i];
     result[i] := result[aLength - i + 1];
     result[aLength - i + 1] := c;
   end;

end;</lang> All versions has the same perfomance, then StrUtils is recomended.

DWScript

See Delphi.

Dyalect

<lang dyalect>let str = "asdf"

func String.reverse() {

   var cs = []
   let len = this.len()
   for n in 1..len {
       cs.add(this[len - n])
   }
   String(values: cs)

}

str.reverse()</lang>

Déjà Vu

<lang dejavu>!print concat chars "Hello"</lang>

Output:

olleH

E

<lang e>pragma.enable("accumulator") def reverse(string) {

 return accum "" for i in (0..!(string.size())).descending() { _ + string[i] }

}</lang>

EchoLisp

<lang lisp> (define (string-reverse string)

   (list->string (reverse (string->list string))))

(string-reverse "ghij")

   → jihg

(string-reverse "un roc lamina l animal cornu")

   → unroc lamina l animal cor nu

</lang>

EGL

<lang EGL>function reverse( str string ) returns( string ) result string; for ( i int from StrLib.characterLen( str ) to 1 decrement by 1 ) result ::= str[i:i]; end return( result ); end</lang>

Eiffel

<lang eiffel>class

   APPLICATION

create

   make

feature

   make
           -- Demonstrate string reversal.
       do
           my_string := "Hello World!"
           my_string.mirror
           print (my_string)
       end
   my_string: STRING
           -- Used for reversal

end</lang>

Output:

!dlroW olleH

Ela

<lang ela>reverse_string str = rev len str

 where len = length str
       rev 0 str = ""
       rev n str = toString (str : nn) +> rev nn str
         where nn = n - 1

reverse_string "Hello"</lang>

Output:

"olleH"

Another approach is to covert a string to a list, reverse a list and then convert it back to a string:

<lang ela>open string fromList <| reverse <| toList "Hello" ::: String</lang>

Elena

ELENA 4.x: <lang elena>import system'routines; import extensions; import extensions'text;

extension extension {

   reversedLiteral()
       = self.toArray().sequenceReverse().summarize(new StringWriter());

}

public program() {

   console.printLine("Hello World".reversedLiteral())

}</lang>

Output:

dlroW olleH

Elixir

Elixir handles Unicode graphemes correctly by default. <lang elixir> IO.puts (String.reverse "asdf") IO.puts (String.reverse "as⃝df̅") </lang>

Output:

fdsa
f̅ds⃝a

Elm

<lang elm>-- The import on the next line provides the reverse string -- functionality satisfying the rosettacode.org task description. import String exposing (reverse)

-- The rest is fairly boilerplate code demonstrating -- interactively that the reverse function works. import Html exposing (Html, Attribute, text, div, input) import Html.Attributes exposing (placeholder, value, style) import Html.Events exposing (on, targetValue) import Html.App exposing (beginnerProgram)

main = beginnerProgram { model = "", view = view, update = update }

update newStr oldStr = newStr

view : String -> Html String view forward =

 div []
   ([ input
       [ placeholder "Enter a string to be reversed."
       , value forward
       , on "input" targetValue 
       , myStyle
       ]
       []
    ] ++ 
    [ let backward = reverse forward 
      in div [ myStyle] [text backward] 
    ])

myStyle : Attribute msg myStyle =

 style
   [ ("width", "100%")
   , ("height", "20px")
   , ("padding", "5px 0 0 5px")
   , ("font-size", "1em")
   , ("text-align", "left")
   ]</lang>

Link to live demo: http://dc25.github.io/reverseStringElm/

Emacs Lisp

<lang lisp>(reverse "Hello World")</lang>

Output:

"dlroW olleH"

Erlang

<lang erlang>1> lists:reverse("reverse!"). "!esrever"</lang> Erlang also supports binary strings, which uses its binary format. There is no standard function to reverse a binary sequence, but the following one does the job well enough. It works by changing the endianness (from little to big or the opposite) of the whole sequence, effectively reversing the string. <lang erlang>reverse(Bin) ->

   Size = size(Bin)*8,
   <<T:Size/integer-little>> = Bin,
   <<T:Size/integer-big>>.</lang>

Output:

1> test:reverse(<<"hello">>).
<<"olleh">>

ERRE

<lang ERRE> PROGRAM REVERSE_STRING

PROCEDURE REVERSE(A$->R$)

  LOCAL I%
  R$=""
  FOR I=1 TO LEN(A$) DO
    R$=MID$(A$,I,1)+R$
  END FOR

END PROCEDURE

BEGIN

  A$="THE FIVE BOXING WIZARDS JUMP QUICKLY"
  REVERSE(A$->R$)
  PRINT(R$)

END PROGRAM </lang>

Euler Math Toolbox

<lang Euler Math Toolbox> >function strrev (s) := chartostr(fliplr(strtochar(s))) >strrev("This is a test!")

!tset a si sihT

</lang>

Euphoria

<lang euphoria> include std/sequence.e printf(1, "%s\n", {reverse("abcdef") }) </lang>

Explore

The Scratch solution, which requires making variables named "i" and "inv" first, works, unmodified:
https://i.ibb.co/3c9k641/Reverse-a-string-in-Explore-using-the-Scratch-solution.png

This example uses a special block located in the Strings category:
https://i.ibb.co/4pM9G8b/Reverse-a-string-in-Explore-using-a-special-block.png

Ezhil

1. இந்த நிரல் தரப்படும் சரம் ஒன்றைத் தலைகீழாகத் திருப்பி அச்சிடும்
2. உதாரணமாக "abc" என்ற சரம் தரப்பட்டால் அதனைத் திருப்பி "cba" என அச்சிடும்
3. "எழில்" மொழியின்மூலம் இரண்டு வகைகளில் இதனைச் செய்யலாம். இரண்டு உதாரணங்களும் இங்கே தரப்பட்டுள்ளன

நிரல்பாகம் திருப்புக (சரம்1)

   ## முதல் வகை

   சரம்2 = ""

   @( சரம்1  இல் இ) ஒவ்வொன்றாக
         சரம்2 =  இ + சரம்2
       முடி

   பின்கொடு சரம்2

முடி

நிரல்பாகம் மீண்டும்திருப்புக (சரம்1)

   ## இரண்டாம் வகை
   
   சரநீளம் = len(சரம்1)
   
   சரம்2 = ""

   @(எண் = 0, எண் < சரநீளம், எண் = எண் + 1) ஆக

     சரம்2 = எடு(சரம்1, எண்) + சரம்2

   முடி

   பின்கொடு சரம்2

முடி

அ = உள்ளீடு("ஓர் எழுத்துச் சரத்தைத் தாருங்கள் ")

பதிப்பி "நீங்கள் தந்த எழுத்துச் சரம்" அ

பதிப்பி "அதனை முதல் வகையில் திருப்பியுள்ளோம்: " திருப்புக(அ)

பதிப்பி "வேறொரு வகையில் திருப்பியுள்ளோம்: " மீண்டும்திருப்புக(அ)

</lang>

F#

The function

<lang fsharp> // Reverse a string. Nigel Galloway: August 14th., 2019 let strRev α=let N=System.Globalization.StringInfo.GetTextElementEnumerator(α)

            List.unfold(fun n->if n then Some(N.GetTextElement(),N.MoveNext()) else None)(N.MoveNext())|>List.rev|>String.concat ""

</lang>

The Task

I was a little concerned when entering this task because in the edit window the overline appears above the d, but when previewed it is correctly above the f, using Firefox anyway. Using XTERM the output is correct with the s inside a circle but appears as sO in Firefox. <lang fsharp> printfn "%s" (strRev "as⃝df̅") printfn "%s" (strRev "Nigel") </lang>

Output:

f̅ds⃝a
legiN

Factor

A string is a sequence and there is a default reverse implementation for those. <lang factor>"hello" reverse</lang> string-reverse preserves graphemes: <lang factor>"as⃝df̅" string-reverse "f̅ds⃝a" = .</lang>

FALSE

This solution does not take into account combination characters: <lang false>1_ [^$1_=~][]#% [$1_=~][,]#</lang> This solution does take into account combination characters (except for half-marks): <lang false>1_ [^$1_=~][

 $$767>\879\>&
 1ø$7615>\7620\>&|
 1ø$8399>\8428\>&|
 [\]?

]#% [$1_=~][,]#</lang>

Fancy

<lang fancy>"hello world!" reverse</lang>

FBSL

A slow way <lang qbasic>Function StrRev1(ByVal $p1) dim $b = "" REPEAT len(p1) b = b & RIGHT(p1,1) p1 = LEFT(p1,LEN(p1)-1) END REPEAT return b End Function </lang>

A much faster (twice at least) way <lang qbasic>Function StrRev2(ByVal $p1) dim $b = "", %i for i = len(p1) DOWNTO 1 b = b & MID(p1,i,1) next return b End Function</lang>

An even faster way using PEEK, POKE, double-calls and quantity-in-hand <lang qbasic>Function StrRev3( $s ) FOR DIM x = 1 TO LEN(s) \ 2 PEEK(@s + LEN - x, $1) POKE(@s + LEN - x, s{x})(@s + x - 1, PEEK) NEXT RETURN s end function </lang>

An even faster way using the DynC (Dynamic C) mode <lang c>DynC StringRev($theString) As String

  void rev(char *str)
  {

int len = strlen(str); char *HEAD = str; char *TAIL = str + len - 1; char temp; int i; for ( i = 0; i <= len / 2; i++, HEAD++, TAIL--) { temp = *HEAD; *HEAD = *TAIL; *TAIL = temp; }

  }
  char *main(char* theString)
  {
     rev(theString);
     return theString;
  }

End DynC</lang>

Using DynASM, the Dynamic Assembler mode. <lang asm>DYNASM RevStr(BYVAL s AS STRING) AS STRING

  // get length of string
  // divide by two
  // setup pointers to head and tail
  // iterate from 1 to (length \ 2)
  //   swap head with tail
  //   increment head pointer
  //   decrement tail pointer

  ENTER 0, 0 // = PUSH EBP: MOV EBP, ESP
  PUSH EBX // by Windows convention EBX, EDI, ESI must be saved before modification
  
  MOV EAX, s // get string pointer
  MOV ECX, EAX // duplicate it
  
  .WHILE BYTE PTR [ECX] <> 0

INC ECX // propagate to tail

  .WEND
  
  MOV EDX, ECX // duplicate tail pointer
  DEC EDX // set it to last byte before trailing zero
  
  SUB ECX, EAX // get length in ECX in 1 CPU cycle
  SHR ECX, 1 // get length \ 2 in 1 CPU cycle; that's the beauty of power-of-two division

  .WHILE ECX > 0
     
     MOV BL, [EDX] // no need to XOR; just overwrite BL and BH contents
     MOV BH, [EAX] // DynAsm deduces data size from destination register sizes
     
     MOV [EDX], BH // ditto, source register sizes
     MOV [EAX], BL
     
     INC EAX // propagate pointers
     DEC EDX
     
     DEC ECX // decrement counter
     
  .WEND
  
  // point to start of string again
  MOV EAX, s // MOV = 1 CPU cycle, PUSH + POP = 2 CPU cycles
  
  POP EBX // by Windows convention ESI, EDI, EBX must be restored if modified
  LEAVE // = POP EBP
  RET

END DYNASM </lang>

Forth

Method 1

<lang forth>: exchange ( a1 a2 -- )

 2dup c@ swap c@ rot c! swap c! ;

reverse ( c-addr u -- )

 1- bounds begin 2dup > while
   2dup exchange
   -1 /string
 repeat 2drop ;

s" testing" 2dup reverse type \ gnitset</lang>

Method 2 Using the stack

<lang forth>\ reverse a string using the data stack for temporary storage

mystring ( -- caddr len) S" ABCDEFGHIJKLMNOPQRSTUVWXYZ987654321" ;

pushstr ( caddr len -- c..c[n]) bounds do I c@ loop ;

popstr ( c.. c[n] caddr len -- ) bounds do I c! loop ;

reverse ( caddr len -- ) 2dup 2>r pushstr 2r> popstr ;</lang>

Forth Console Output

mystring type ABCDEFGHIJKLMNOPQRSTUVWXYZ987654321 ok
mystring 2dup reverse type 123456789ZYXWVUTSRQPONMLKJIHGFEDCBA ok

Using the Forth-2012 Xchars wordset to handle multi-byte characters

Characters accessed with C@ C! are usually bytes and can therefore only represent characters in 8-bit encodings (e.g., Latin-1). Forth-2012 added the Xchars wordset for dealing with multi-byte encodings such as UTF-8; actually these words are not needed much, because the magic of UTF-8 means that most byte-oriented code works as intended, but the present task is one of the few examples where that is not good enough.

The xchars wordset offers several ways to skin this cat; this is just one way to do it, not necessarily the best one. Because the xchars wordset currently does not support recognizing combining characters, this code does not get extra credit.

<lang forth>: xreverse {: c-addr u -- c-addr2 u :}

   u allocate throw u + c-addr swap over u + >r begin ( from to r:end)

over r@ u< while over r@ over - x-size dup >r - 2dup r@ cmove swap r> + swap repeat

   r> drop nip u ;

\ example use s" ώщыē" xreverse type \ outputs "ēыщώ"</lang>

Fortran

Works with: Fortran version 90 and later

<lang fortran>PROGRAM Example

 CHARACTER(80) :: str = "This is a string"
 CHARACTER :: temp
 INTEGER :: i, length

 WRITE (*,*) str
 length = LEN_TRIM(str) ! Ignores trailing blanks. Use LEN(str) to reverse those as well
 DO i = 1, length/2
    temp = str(i:i)
    str(i:i) = str(length+1-i:length+1-i)
    str(length+1-i:length+1-i) = temp
 END DO
 WRITE(*,*) str

END PROGRAM Example</lang>

Output:

This is a string
gnirts a si sihT

Another implementation that uses a recursive not-in-place algorithm: <lang fortran>program reverse_string

 implicit none
 character (*), parameter :: string = 'no devil lived on'

 write (*, '(a)') string
 write (*, '(a)') reverse (string)

contains

 recursive function reverse (string) result (res)

   implicit none
   character (*), intent (in) :: string
   character (len (string)) :: res

   if (len (string) == 0) then
     res = 
   else
     res = string (len (string) :) // reverse (string (: len (string) - 1))
   end if

 end function reverse

end program reverse_string</lang>

Output:

no devil lived on
no devil lived on

Another shorter implementation (adapted version from stackoverflow question 10605574 how-to-reverse-a-chain-of-character-fortran-90): <lang fortran> program reverse_string

 implicit none
 character (80) :: cadena
 integer :: k, n
 !
 cadena = "abcdefgh"
 n = len_trim (cadena)
 !
 write (*,*) cadena
 forall (k=1:n) cadena (k:k) = cadena (n-k+1:n-k+1)
 write (*,*) cadena
 !

end program reverse_string</lang>

Output:

abcdefgh
hgfedcba

FreeBASIC

<lang freebasic>' FB 1.05.0 Win64

Function ReverseString(s As Const String) As String

 If s = "" Then Return s
 Dim length As Integer = Len(s)
 Dim r As String = Space(length)
 For i As Integer = 0 To length - 1
    r[i] = s[length - 1 - i]
 Next
 Return r

End Function

Dim s As String = "asdf" Print "'"; s; "' reversed is '"; ReverseString(s); "'"</lang>

Output:

'asdf' reversed is 'fdsa'

Frink

The built-in reverse function reverses a string or the elements of a list.

Frink's built-in reverse[string] is quite smart and uses a grapheme-based algorithm to handle Unicode correctly. That is, it preserves "user-perceived characters" that may consist of characters, combining accents, high-plane Unicode characters (that is, above U+FFFF,) surrogate pairs, etc. correctly.

Many languages will not work correctly with upper-plane Unicode characters because they are represented as Unicode "surrogate pairs" which are represented as two characters in a UTF-16 stream.

For example, the string "g\u0308o" represents a g with combining diaeresis, followed by the letter o. Or, in other words, "g̈o". Note that while there are three Unicode codepoints, only two "graphemes" are displayed. Using Frink's smart "reverse" function preserves these combined graphemes. A naive reverse would move the diaeresis over the o instead of the g. <lang frink>println[reverse["abcdef"]]</lang>

Futhark

Futhark has no real strings beyond a little bit of syntactic sugar, so this is the same as reversing an array.

FutureBasic

<lang futurebasic> include "ConsoleWindow"

dim as Str31 str dim as long i

str = "123456789abcdefghijk"

print str print

for i = str[0] to 1 step -1 print mid$( str, i, 1 ); next i </lang>

Output:

123456789abcdefghijk

kjihgfedcba987654321

Gambas

Click this link to run this code <lang gambas>Public Sub Main() Dim sString As String = "asdf" Dim sOutput As String Dim siCount As Short

For siCount = Len(sString) DownTo 1

 sOutput &= Mid(sString, siCount, 1)

End</lang> Output:

fdsa

GAP

<lang gap>Reversed("abcdef");

"fedcba"</lang>

Gema

Reverse each line in the input stream. Using built in function: <lang gema>\L=@reverse{$1}</lang> Not using built in function (recursively apply substring to same rule): <lang gema>\L<U1>=@{$2}$1</lang>

Genie

Pretty sure the output capture fails the extra credit, but that may be more local setup and font installs rather than the glib functions used.

<lang genie>[indent=4] /*

  Reverse a string, in Genie
  valac reverse.gs

/

init

   utf8:string = "asdf"
   combining:string = "asdf̅"

   print utf8
   print utf8.reverse()

   print combining
   print combining.reverse()</lang>

Output:

prompt$ valac reverse.gs     
prompt$ ./reverse        
asdf
fdsa
as?df?
?fd?sa

GFA Basic

<lang> PRINT @reverse$("asdf") ' FUNCTION reverse$(string$)

 LOCAL result$,i%
 result$=""
 FOR i%=1 TO LEN(string$)
   result$=MID$(string$,i%,1)+result$
 NEXT i%
 RETURN result$

ENDFUNC </lang>

Go

Functions below assume UTF-8 encoding. (The task mentions Unicode but does not specify an encoding.) Strings in Go are not restricted to be UTF-8, but Go has good support for it and works with UTF-8 most natually. As shown below, certain string conversions work in UTF-8 and the range clause over a string works in UTF-8. Go also has a Unicode package in the standard library that makes easy work of recognizing combining characters for this task. <lang go>package main

import (

   "fmt"
   "unicode"
   "unicode/utf8"

)

// no encoding func reverseBytes(s string) string {

   r := make([]byte, len(s))
   for i := 0; i < len(s); i++ {
       r[i] = s[len(s)-1-i]
   }
   return string(r)

}

// reverseCodePoints interprets its argument as UTF-8 and ignores bytes // that do not form valid UTF-8. return value is UTF-8. func reverseCodePoints(s string) string {

   r := make([]rune, len(s))
   start := len(s)
   for _, c := range s {
       // quietly skip invalid UTF-8
       if c != utf8.RuneError {
           start--
           r[start] = c
       }
   }
   return string(r[start:])

}

// reversePreservingCombiningCharacters interprets its argument as UTF-8 // and ignores bytes that do not form valid UTF-8. return value is UTF-8. func reversePreservingCombiningCharacters(s string) string {

   if s == "" {
       return ""
   }
   p := []rune(s)
   r := make([]rune, len(p))
   start := len(r)
   for i := 0; i < len(p); {
       // quietly skip invalid UTF-8
       if p[i] == utf8.RuneError {
           i++
           continue
       }
       j := i + 1
       for j < len(p) && (unicode.Is(unicode.Mn, p[j]) ||
           unicode.Is(unicode.Me, p[j]) || unicode.Is(unicode.Mc, p[j])) {
           j++
       }
       for k := j - 1; k >= i; k-- {
           start--
           r[start] = p[k]
       }
       i = j
   }
   return (string(r[start:]))

}

func main() {

   test("asdf")
   test("as⃝df̅")

}

func test(s string) {

   fmt.Println("\noriginal:      ", []byte(s), s)
   r := reverseBytes(s)
   fmt.Println("reversed bytes:", []byte(r), r)
   fmt.Println("original code points:", []rune(s), s)
   r = reverseCodePoints(s)
   fmt.Println("reversed code points:", []rune(r), r)
   r = reversePreservingCombiningCharacters(s)
   fmt.Println("combining characters:", []rune(r), r)

}</lang>

Output:

original:       [97 115 100 102] asdf
reversed bytes: [102 100 115 97] fdsa
original code points: [97 115 100 102] asdf
reversed code points: [102 100 115 97] fdsa
combining characters: [102 100 115 97] fdsa

original:       [97 115 226 131 157 100 102 204 133] as⃝df̅
reversed bytes: [133 204 102 100 157 131 226 115 97] ��fd���sa
original code points: [97 115 8413 100 102 773] as⃝df̅
reversed code points: [773 102 100 8413 115 97] ̅fd⃝sa
combining characters: [102 773 100 115 8413 97] f̅ds⃝a

Groovy

Solution:

<lang groovy>println "Able was I, 'ere I saw Elba.".reverse()</lang>

Output:

.ablE was I ere' ,I saw elbA

Extra Credit:

<lang groovy>def string = "as⃝df̅"

List combiningBlocks = [

   Character.UnicodeBlock.COMBINING_DIACRITICAL_MARKS,
   Character.UnicodeBlock.COMBINING_DIACRITICAL_MARKS_SUPPLEMENT,
   Character.UnicodeBlock.COMBINING_HALF_MARKS,
   Character.UnicodeBlock.COMBINING_MARKS_FOR_SYMBOLS

] List chars = string as List chars[1..-1].eachWithIndex { ch, i ->

   if (Character.UnicodeBlock.of((char)ch) in combiningBlocks) {
       chars[i..(i+1)] = chars[(i+1)..i]
   }

} println chars.reverse().join()</lang>

Output:

f̅ds⃝a

Harbour

<lang visualfoxpro>FUNCTION Reverse( sIn )

  LOCAL cOut := "", i

  FOR i := Len( sIn ) TO 1 STEP -1
     cOut += Substr( sIn, i, 1 )
  NEXT

  RETURN cOut</lang>

Haskell

<lang haskell>reverse = foldl (flip (:)) []</lang> This function as defined in the Haskell Prelude.

Though variants using a helper function with an additional accumulator argument are more efficient, and are now used by default in GHC.List unless the USE_REPORT_PRELUDE key is set.

Perhaps, for example: <lang haskell>accumulatingReverse :: [a] -> [a] accumulatingReverse lst =

 let rev xs a = foldl (flip (:)) a xs
 in rev lst []</lang>

Supporting combining characters

<lang haskell>import Data.Char (isMark) import Data.List (groupBy) myReverse = concat . reverse . groupBy (const isMark)</lang> groupBy (const isMark) is an unusual way of splitting a string into its combined characters

HicEst

<lang hicest>CHARACTER string = "Hello World", tmp

L = LEN( string ) DO i = 1, L/2

 tmp = string(i)
 string(i) = string(L-i+1)
 string(L-i+1) = tmp

ENDDO

WRITE(Messagebox, Name) string </lang>

Icon and Unicon

<lang Icon>procedure main(arglist) s := \arglist[1] | "asdf" write(s," <-> ", reverse(s)) # reverse is built-in end</lang>

Io

<lang io>"asdf" reverse</lang>

J

Reverse (|.) reverses a list of items (of any shape or type). <lang j> |.'asdf' fdsa</lang> Extra credit: First, a function to determine whether a Unicode character is a combining character: <lang j> ranges=.16b02ff 16b036f, 16b1dbf 16b1dff, 16b20cf 16b20ff, 16bfe1f 16bfe2f

  iscombining=. 2 | ranges&I.</lang>

Then we need to box groups of letters and combining characters, reverse, and unbox. The boxing function can be carried out easily with dyad cut, which uses the indices of the ones on the right as the starting points for groups of characters. For clarity, its inverse will be defined as raze, which simply runs together the items inside boxes of its argument. <lang j> split=. (<;.1~ -.@iscombining) :. ;</lang>

After this, the solution is just to reverse under the split transformation. This also takes place under J code to convert from Unicode to integers. <lang j> |.&.split&.(3 u: 7&u:) 'as⃝df̅' f̅ds⃝a</lang>

Java

<lang java>public static String reverseString(String s) {

   return new StringBuffer(s).reverse().toString();

}</lang>

Works with: Java version 1.5+

<lang java5>public static String reverseString(String s) {

   return new StringBuilder(s).reverse().toString();

}</lang>

JavaScript

ES5

<lang javascript>//using chained methods function reverseStr(s) {

 return s.split().reverse().join();

}

//fast method using for loop function reverseStr(s) {

 for (var i = s.length - 1, o = ; i >= 0; o += s[i--]) { }
 return o;

}

//fast method using while loop (faster with long strings in some browsers when compared with for loop) function reverseStr(s) {

 var i = s.length, o = ;
 while (i--) o += s[i];
 return o;

}</lang>

ES6

<lang JavaScript>(() => {

   // .reduceRight() can be useful when reversals
   // are composed with some other process

   let reverse1 = s => Array.from(s)
       .reduceRight((a, x) => a + (x !== ' ' ? x : ' <- '), ),

       // but ( join . reverse . split ) is faster for
       // simple string reversals in isolation

       reverse2 = s => s.split().reverse().join();

   return [reverse1, reverse2]
       .map(f => f("Some string to be reversed"));

})();</lang>

Output:

<lang JavaScript>["desrever <- eb <- ot <- gnirts <- emoS", "desrever eb ot gnirts emoS"]</lang>

jq

jq's explode/implode filters are based on codepoints, and therefore "reverse_string" as defined here will reverse the sequence of codepoints. The topic of Unicode combining characters is a large one that is not touched on here. <lang jq>def reverse_string: explode | reverse | implode;</lang> Examples:

"nöel" | reverse_string  # => "leön"

"as⃝df̅" | reverse_string  # => "̅fd⃝sa"

Jsish

ECMAScript has no builtin string reversal, so split the characters into an array, reverse the array and join it back together.

Jsi only supports UTF-8 literals so far (in release 2.8), character by character manipulation routines of multibyte UTF-8 data will not be correct. No extra credit, yet. <lang javascript>var str = "Never odd or even"; puts(str); puts(str.split().reverse().join());</lang>

Output:

Never odd or even
neve ro ddo reveN

Julia

<lang julia>julia> reverse("hey") "yeh"</lang> The reverse function reverses codepoints (because this is the right behavior for the main application of string reversal: reversed string processing by external C libraries). However, starting in Julia 0.4, you can also reverse the graphemes if you want (i.e. to reverse "visual order" including combining characters etc.) by: <lang julia>julia> join(reverse(collect(graphemes("as⃝df̅")))) "f̅ds⃝a"</lang>

K

Monadic reverse (| ) verb reverses a string or list of any shape <lang K>

    |"asdf"

"fdsa"

    | 23 4 5 1

1 5 4 23 </lang>

Kotlin

<lang kotlin>fun main(args: Array<String>) {

   println("asdf".reversed())

}</lang>

L++

<lang lisp>(include "string" "algorithm") (main

 (decl std::string s)
 (std::getline std::cin s)
 (std::reverse (s.begin) (s.end))
 (prn s))</lang>

LabVIEW

This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.

Lambdatalk

<lang Scheme> {S.reverse hello brave new world} -> world new brave hello </lang>

Lang5

<lang lang5>: flip "" split reverse "" join ; "qwer asdf" flip .</lang>

langur

This accounts for code points, but not for graphemes. <lang langur>writeln cp2s reverse s2cp q(don't you know)</lang>

Output:

wonk uoy t'nod

Lasso

<lang Lasso>local(input) = 'asdf'

input->reverse</lang>

Using Query Expression & Array

More verbose than the string->reverse method, but this example illustrates different techniques to achieve the same result: using string->values to iterate over a string in order, inserting at position 1, and joining the resulting array as a string. <lang Lasso>local(input = 'asdf', output = array) with i in #input->values do #output->insertFirst(#i)

output->join</lang>

LC3 Assembly

A string is stored as a zero-terminated array of character codes. To reverse it, we first scan forwards until we find the end; we then move backwards again, copying each code into a block of memory we have reserved for the purpose; and finally, when we have got back to the beginning, we append a terminal zero to the new string we have created. We can then call PUTS to print it. <lang lc3asm> .ORIG 0x3000

       LEA        R1,STRING
       LEA        R2,GNIRTS
       LD         R3,MINUS1
       NOT        R5,R1
       ADD        R5,R5,1

SCAN LDR R4,R1,0

       BRZ        COPY
       ADD        R1,R1,1
       BRNZP      SCAN

COPY ADD R1,R1,R3

       ADD        R4,R1,R5
       BRN        COPIED
       LDR        R4,R1,0
       STR        R4,R2,0
       ADD        R2,R2,1
       BRNZP      COPY

COPIED AND R4,R4,0

       STR        R4,R2,0

       LEA        R0,GNIRTS
       PUTS

       HALT

MINUS1 .FILL 0xFFFF

STRING .STRINGZ "If thou beest he -- but O how fall'n! how chang'd" GNIRTS .BLKW 128

       .END</lang>

Output:

d'gnahc woh !n'llaf woh O tub -- eh tseeb uoht fI

LFE

Ordinary string: <lang lisp> > (lists:reverse "asdf") "fdsa" </lang>

Create a UTF-8 encoded string: <lang lisp> > (set encoded (binary ("åäö ð" utf8)))

B(195 165 195 164 195 182 32 195 176)

</lang>

Display it, to be sure: <lang lisp> > (io:format "~tp~n" (list encoded)) <<"åäö ð"/utf8>> </lang>

Reverse it: <lang lisp> > (lists:reverse (unicode:characters_to_list encoded)) "ð öäå" </lang>

Liberty BASIC

<lang lb>input$ ="abcdefgABCDEFG012345" print input$ print ReversedStr$( input$)

end

function ReversedStr$(in$)

   for i =len(in$) to 1 step -1
   ReversedStr$ =ReversedStr$ +mid$( in$, i, 1)
   next i

end function</lang>

Lingo

Lingo strings are always UTF-8 encoded and string operations are based on Unicode code points, so the "extra credit" is built-in: <lang lingo>on reverse (str)

 res = ""
 repeat with i = str.length down to 1
   put str.char[i] after res
 end repeat
 return res

end</lang> To reverse a string byte-wise, the ByteArray data type has to be used: <lang lingo>on reverseBytes (str)

 ba = byteArray(str)
 res = byteArray()
 repeat with i = ba.length down to 1
   res[res.length+1] = ba[i]
 end repeat
 return res

end</lang>

LiveCode

<lang livecode>function reverseString S

  repeat with i = length(S) down to 1
     put char i of S after R
  end repeat
  return R

end reverseString</lang>

LLVM

<lang llvm>; This is not strictly LLVM, as it uses the C library function "printf".

LLVM does not provide a way to print values, so the alternative would be
to just load the string into memory, and that would be boring.

Additional comments have been inserted, as well as changes made from the output produced by clang such as putting more meaningful labels for the jumps

$"main.printf" = comdat any

@main.str = private unnamed_addr constant [12 x i8] c"Hello world\00", align 1 @"main.printf" = linkonce_odr unnamed_addr constant [4 x i8] c"%s\0A\00", comdat, align 1

define void @reverse(i64, i8*) {

 %3 = alloca i8*, align 8          ; allocate str (local)
 %4 = alloca i64, align 8          ; allocate len (local)
 %5 = alloca i64, align 8          ; allocate i
 %6 = alloca i64, align 8          ; allocate j
 %7 = alloca i8, align 1           ; allocate t
 store i8* %1, i8** %3, align 8    ; set str (local) to the parameter str
 store i64 %0, i64* %4, align 8    ; set len (local) to the paremeter len
 store i64 0, i64* %5, align 8     ; i = 0
 %8 = load i64, i64* %4, align 8   ; load len
 %9 = sub i64 %8, 1                ; decrement len
 store i64 %9, i64* %6, align 8    ; j =
 br label %loop

loop:

 %10 = load i64, i64* %5, align 8  ; load i
 %11 = load i64, i64* %6, align 8  ; load j
 %12 = icmp ult i64 %10, %11       ; i < j
 br i1 %12, label %loop_body, label %exit

loop_body:

 %13 = load i8*, i8** %3, align 8                  ; load str
 %14 = load i64, i64* %5, align 8                  ; load i
 %15 = getelementptr inbounds i8, i8* %13, i64 %14 ; address of str[i]
 %16 = load i8, i8* %15, align 1                   ; load str[i]
 store i8 %16, i8* %7, align 1                     ; t = str[i]
 %17 = load i64, i64* %6, align 8                  ; load j
 %18 = getelementptr inbounds i8, i8* %13, i64 %17 ; address of str[j]
 %19 = load i8, i8* %18, align 1                   ; load str[j]
 %20 = getelementptr inbounds i8, i8* %13, i64 %14 ; address of str[i]
 store i8 %19, i8* %20, align 1                    ; str[i] = str[j]
 %21 = load i8, i8* %7, align 1                    ; load t
 %22 = getelementptr inbounds i8, i8* %13, i64 %17 ; address of str[j]
 store i8 %21, i8* %22, align 1                    ; str[j] = t

-- loop increment

 %23 = load i64, i64* %5, align 8  ; load i
 %24 = add i64 %23, 1              ; increment i
 store i64 %24, i64* %5, align 8   ; store i
 %25 = load i64, i64* %6, align 8  ; load j
 %26 = add i64 %25, -1             ; decrement j
 store i64 %26, i64* %6, align 8   ; store j
 br label %loop

exit:

 ret void

}

define i32 @main() {

-- char str[]

 %1 = alloca [12 x i8], align 1

-- memcpy(str, "Hello world")

 %2 = bitcast [12 x i8]* %1 to i8*
 call void @llvm.memcpy.p0i8.p0i8.i64(i8* %2, i8* getelementptr inbounds ([12 x i8], [12 x i8]* @main.str, i32 0, i32 0), i64 12, i32 1, i1 false)

-- printf("%s\n", str)

 %3 = getelementptr inbounds [12 x i8], [12 x i8]* %1, i32 0, i32 0
 %4 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @"main.printf", i32 0, i32 0), i8* %3)

-- %7 = strlen(str)

 %5 = getelementptr inbounds [12 x i8], [12 x i8]* %1, i32 0, i32 0
 %6 = call i64 @strlen(i8* %5)

-- reverse(%6, str)

 call void @reverse(i64 %6, i8* %5)

-- printf("%s\n", str)

 %7 = getelementptr inbounds [12 x i8], [12 x i8]* %1, i32 0, i32 0
 %8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @"main.printf", i32 0, i32 0), i8* %7)

-- end of main

 ret i32 0

}

--- The declaration for the external C printf function.

declare i32 @printf(i8*, ...)

Function Attrs: argmemonly nounwind

declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1)

declare i64 @strlen(i8*)</lang>

Output:

Hello world
dlrow olleH

Logo

REVERSE works on both words and lists. <lang logo>print reverse "cat ; tac</lang>

Lua

Built-in string.reverse(s) or s:reverse(). <lang lua>theString = theString:reverse()</lang>

M2000 Interpreter

Using Custom Function

Version 2, using insert to string (with no copies of strings) <lang M2000 Interpreter> Module ReverseString {

     a$="as⃝df̅"
     Print Len(a$), len.disp(a$)
     Let i=1, j=Len(a$)
     z$=String$(" ",j)
     j++
     do {
           k$=mid$(a$, i, 1)
           if i<len(a$) then {
                 while len.disp(k$+mid$(a$, i+1,1)) =len.disp(k$)  {
                       k$+=mid$(a$, i+1,1)
                       i++ 
                       if i>len(a$) then exit
                       j--
                 }
                 j--
                 insert j, len(k$) Z$=K$
           } else j-- :Insert j,1 z$=k$ 
           if i>=len(a$) then exit
           i++
     } Always
     Print len(z$), len.disp(z$)
     Print z$="f̅ds⃝a"
     Print z$

} ReverseString </lang>

using StrRev$()

this function (new to 9.5 version) use StrReverse from Vb6 <lang M2000 Interpreter> a$="as⃝df̅" b$=strrev$(a$) clipboard b$ Print b$="̅fd⃝sa" </lang>

M4

<lang m4>define(`invert',`ifelse(len(`$1'),0,,`invert(substr(`$1',1))'`'substr(`$1',0,1))')</lang>

Maclisp

<lang lisp>(readlist (reverse (explode "my-string")))</lang> Output:

"gnirts-ym"

Maple

<lang Maple>> StringTools:-Reverse( "foo" );

                                "oof"</lang>

Mathematica/Wolfram Language

<lang mathematica>StringReverse["asdf"]</lang>

MATLAB

A built-in function, "fliplr(string)" handles reversing a string of ASCII characters. Unicode is a whole other beast, if you need this functionality test to see if "fliplr()" properly handles the unicode characters you use. If it doesn't then you will need to code a function that is specific to your application.

Sample Usage: <lang MATLAB>>> fliplr(['She told me that she spoke English and I said great. '... 'Grabbed her hand out the club and I said lets skate.'])

ans =

.etaks s'tel dias I dna bulc eht tuo dnah reh debbarG .taerg dias I dna hsilgnE ekops ehs taht em dlot ehS</lang>

Maxima

<lang maxima>sreverse("abcdef"); /* "fedcba" */

sreverse("rats live on no evil star"); /* not a bug :o) */</lang>

MAXScript

<lang maxscript>fn reverseString s = (

   local reversed = ""
   for i in s.count to 1 by -1 do reversed += s[i]
   reversed

)</lang>

min

Works with: min version 0.19.3

<lang min>("" split reverse "" join) :reverse-str</lang>

MiniScript

<lang MiniScript>str = "This is a string" print "Forward: " + str newStr = "" for i in range(str.len-1, 0)

   newStr = newStr + str[i]

end for print "Reversed: " + newStr</lang>

Output:

Forward: This is a string
Reversed: gnirts a si sihT

MIPS Assembly

First, it gets the length of the original string
Then, it allocates memory from the copy
Then it copies the pointer to the original string, and adds the strlen
subtract 1, then that new pointer is at the last char.
while(strlen)
copy char
decrement strlen
decrement source pointer
increment target pointer

.text

strcpy:

 addi $sp, $sp, -4
 sw $s0, 0($sp)
 add $s0, $zero, $zero

L1:

 add $t1, $s0, $a1
 lb $t2, 0($t1)
 add $t3, $s0, $a0
 sb $t2, 0($t3)
 beq $t2, $zero, L2
 addi $s0, $s0, 1
 j L1

L2:

 lw $s0, 0($sp)
 addi $sp, $sp, 4
 jr $ra

.data ex_msg_og: .asciiz "Original string:\n" ex_msg_cpy: .asciiz "\nCopied string:\n" string: .asciiz "Nice string you got there!\n" </lang>

Mirah

<lang mirah>def reverse(s:string)

   StringBuilder.new(s).reverse

end

puts reverse('reversed')</lang>

Modula-2

<lang modula2>MODULE ReverseStr; FROM FormatString IMPORT FormatString; FROM Terminal IMPORT Write,WriteString,WriteLn,ReadChar;

PROCEDURE WriteInt(n : INTEGER); VAR buf : ARRAY[0..15] OF CHAR; BEGIN

   FormatString("%i", buf, n);
   WriteString(buf)

END WriteInt;

PROCEDURE ReverseStr(in : ARRAY OF CHAR; VAR out : ARRAY OF CHAR); VAR ip,op : INTEGER; BEGIN

   ip := 0;
   op := 0;
   WHILE in[ip] # 0C DO
       INC(ip)
   END;
   DEC(ip);
   WHILE ip>=0 DO
       out[op] := in[ip];
       INC(op);
       DEC(ip)
   END

END ReverseStr;

TYPE A = ARRAY[0..63] OF CHAR; VAR is,os : A; BEGIN

   is := "Hello World";
   ReverseStr(is, os);

   WriteString(is);
   WriteLn;
   WriteString(os);
   WriteLn;

   ReadChar

END ReverseStr.</lang>

Modula-3

<lang modula3>MODULE Reverse EXPORTS Main;

IMPORT IO, Text;

PROCEDURE String(item: TEXT): TEXT =

 VAR result: TEXT := "";
 BEGIN
   FOR i := Text.Length(item) - 1 TO 0 BY - 1 DO
     result := Text.Cat(result, Text.FromChar(Text.GetChar(item, i)));
   END;
   RETURN result;
 END String;

BEGIN

 IO.Put(String("Foobarbaz") & "\n");

END Reverse.</lang>

Output:

zabrabooF

MUMPS

<lang MUMPS>REVERSE

;Take in a string and reverse it using the built in function $REVERSE
NEW S
READ:30 "Enter a string: ",S
WRITE !,$REVERSE(S)
QUIT</lang>

Output:

USER>D REVERSE^ROSETTA
Enter a string: Hello, World!
!dlroW ,olleH

Nanoquery

<lang Nanoquery>def reverse(string)

       l = ""

       for char in list(str(string)).reverse()
               l += char
       end

       return l

end</lang>

Neko

No extra credit for UTF in this example. <lang actionscript>/* Reverse a string, in Neko */

var reverse = function(s) {

 var len = $ssize(s)
 if len < 2 return s

 var reverse = $smake(len)
 var pos = 0
 while len > 0 $sset(reverse, pos ++= 1, $sget(s, len -= 1))
 return reverse

}

var str = "never odd or even" $print(str, "\n") $print(reverse(str), "\n\n")

str = "abcdefghijklmnopqrstuvwxyz" $print(str, "\n") $print(reverse(str), "\n\n")

$print("single test\n") str = "a" $print(str, "\n") $print(reverse(str), "\n\n")

$print("empty test\n") str = "" $print(str, "\n") $print(reverse(str), "\n")</lang>

Output:

prompt$ nekoc reverse.neko
prompt$ neko reverse.n
never odd or even
neve ro ddo reven

abcdefghijklmnopqrstuvwxyz
zyxwvutsrqponmlkjihgfedcba

single test
a
a

empty test

Nemerle

Supporting Combining Characters

Compile with:

ncc -reference:System.Windows.Forms reverse.n

<lang Nemerle>using System; using System.Globalization; using System.Windows.Forms; using System.Console; using Nemerle.Utility.NString;

module StrReverse {

   UReverse(text : string) : string
   {
       mutable output = [];
       def elements = StringInfo.GetTextElementEnumerator(text);
       while (elements.MoveNext())
           output ::= elements.GetTextElement().ToString();
       Concat("", output.Reverse());
   }
   
   Main() : void
   {
       def test = "as⃝df̅";
       MessageBox.Show($"$test --> $(UReverse(test))");  //for whatever reason my console didn't display Unicode properly, but a MessageBox worked
   }

}</lang>

Basic Reverse

Doesn't require the System.Globalization namespace, probably a little less overhead. <lang Nemerle>Reverse(text : string) : string {

   mutable output = [];
   foreach (c in text.ToCharArray())
       output ::= c.ToString();
   Concat("", output)

}</lang>

NetRexx

<lang NetRexx>/* NetRexx */

options replace format comments java crossref savelog symbols nobinary

reverseThis = 'asdf' sihTesrever = reverseThis.reverse

say reverseThis say sihTesrever

return</lang>

Output:

asdf
fdsa

NewLISP

<lang NewLISP>(reverse "!dlroW olleH")</lang>

Nial

<lang nial>reverse 'asdf' =fdsa</lang>

Nim

<lang nim>import unicode

proc reverse(s: var string) =

 for i in 0 .. s.high div 2:
   swap(s[i], s[s.high - i])

proc reversed(s: string): string =

 result = newString(s.len)
 for i,c in s:
   result[s.high - i] = c

proc uniReversed(s: string): string =

 result = newStringOfCap(s.len)
 var tmp: seq[Rune] = @[]
 for r in runes(s):
   tmp.add(r)
 for i in countdown(tmp.high, 0):
   result.add(toUtf8(tmp[i]))

proc isComb(r: Rune): bool =

 (r >=% Rune(0x300) and r <=% Rune(0x36f)) or
   (r >=% Rune(0x1dc0) and r <=% Rune(0x1dff)) or
   (r >=% Rune(0x20d0) and r <=% Rune(0x20ff)) or
   (r >=% Rune(0xfe20) and r <=% Rune(0xfe2f))

proc uniReversedPreserving(s: string): string =

 result = newStringOfCap(s.len)
 var tmp: seq[Rune] = @[]
 for r in runes(s):
   if isComb(r): tmp.insert(r, tmp.high)
   else: tmp.add(r)
 for i in countdown(tmp.high, 0):
   result.add(toUtf8(tmp[i]))

for str in ["Reverse This!", "as⃝df̅"]:

 echo "Original string:       ", str
 echo "Reversed:              ", reversed(str)
 echo "UniReversed:           ", uniReversed(str)
 echo "UniReversedPreserving: ", uniReversedPreserving(str)</lang>

Output:

Original string:       Reverse This!
Reversed:              !sihT esreveR
UniReversed:           !sihT esreveR
UniReversedPreserving: !sihT esreveR
Original string:       as⃝df̅
Reversed:              Ìfdâsa
UniReversed:          ‾fd⃝sa
UniReversedPreserving: f̅ds⃝a

Since Nim 0.11.0, the unicode module provides a reversed proc... Hence:

<lang nim>import unicode

doAssert "foobar".reversed == "raboof" doAssert "先秦兩漢".reversed == "漢兩秦先"</lang>

NS-HUBASIC

<lang NS-HUBASIC>10 STRING$="THIS TEXT IS REVERSED." 20 REVERSED$="" 30 FOR I=1 TO LEN(STRING$) 40 REVERSED$=MID$(STRING$,I,1)+REVERSED$ 50 NEXT 60 PRINT REVERSED$</lang>

Oberon

Tested with OBNC. <lang Oberon>MODULE reverse;

  IMPORT Out, Strings;
  
  VAR s: ARRAY 12 + 1 OF CHAR;   
  
  PROCEDURE Swap(VAR c, d: CHAR);
     VAR oldC: CHAR;
  BEGIN
     oldC := c; c := d; d := oldC         
  END Swap;

  PROCEDURE Reverse(VAR s: ARRAY OF CHAR);
     VAR len, i: INTEGER;
  BEGIN
     len := Strings.Length(s);
     FOR i := 0 TO len DIV 2 DO
        Swap(s[i], s[len - 1 - i])
     END
  END Reverse;

BEGIN

  s := "hello, world";
  Reverse(s);
  Out.String(s);
  Out.Ln

END reverse.</lang>

Objeck

<lang objeck> result := "asdf"->Reverse(); </lang>

Objective-C

This extends the NSString object adding a reverseString class method. <lang objc>#import <Foundation/Foundation.h>

@interface NSString (Extended) -(NSString *)reverseString; @end

@implementation NSString (Extended) -(NSString *) reverseString {

   NSUInteger len = [self length];
   NSMutableString *rtr=[NSMutableString stringWithCapacity:len];
   //        unichar buf[1];
   
   while (len > (NSUInteger)0) { 
       unichar uch = [self characterAtIndex:--len]; 
       [rtr appendString:[NSString stringWithCharacters:&uch length:1]];
   }
   return rtr;

} @end</lang> Usage example: <lang objc>int main() {

   @autoreleasepool {
       
       NSString *test = [@"!A string to be reverted!" reverseString];
   
       NSLog(@"%@", test);
   
   }
   return 0;

}</lang>

Supporting combining characters

Extra credit <lang objc>#import <Foundation/Foundation.h>

@interface NSString (Extended) -(NSString *)reverseString; @end

@implementation NSString (Extended) -(NSString *)reverseString { NSInteger l = [self length] - 1; NSMutableString *ostr = [NSMutableString stringWithCapacity:[self length]]; while (l >= 0) { NSRange range = [self rangeOfComposedCharacterSequenceAtIndex:l]; [ostr appendString:[self substringWithRange:range]]; l -= range.length; } return ostr; } @end</lang> Usage example: <lang objc>int main() {

   @autoreleasepool {
       
       NSString *test = [@"as⃝df̅" reverseString];
   
       NSLog(@"%@", test);
   
   }
   return 0;

}</lang>

OCaml

Since OCaml 4.02 we can use the handy String.init function.

Here a version that returns a new allocated string (preserving the original one):

Works with: OCaml version 4.02+

<lang ocaml>let string_rev s =

 let len = String.length s in
 String.init len (fun i -> s.[len - 1 - i])

let () =

 print_endline (string_rev "Hello world!")</lang>

for in place modification we can't use strings anymore because strings became immutable in ocaml 4.02, so the type bytes has to be used instead:

<lang ocaml>let rev_bytes bs =

 let last = Bytes.length bs - 1 in
 for i = 0 to last / 2 do
   let j = last - i in
   let c = Bytes.get bs i in
   Bytes.set bs i (Bytes.get bs j);
   Bytes.set bs j c;
 done

let () =

 let s = Bytes.of_string "Hello World" in
 rev_bytes s;
 print_bytes s;
 print_newline ();

</lang>

Here is a 100% functionnal string reversing function: <lang ocaml>let rec revs_aux strin list index =

 if List.length list = String.length strin
 then String.concat "" list
 else revs_aux strin ((String.sub strin index 1)::list) (index+1)

let revs s = revs_aux s [] 0

let () =

 print_endline (revs "Hello  World!")</lang>

will return "!dlroW olleH"

Octave

<lang octave>s = "a string"; rev = s(length(s):-1:1)</lang>

Oforth

<lang Oforth>reverse</lang>

Ol

<lang scheme> (define (rev s)

  (runes->string (reverse (string->runes s))))

testing

(print (rev "Hello, λ!"))

==> !λ ,olleH

</lang>

OOC

<lang ooc> main: func {

 "asdf" reverse() println() // prints "fdsa"

} </lang>

OpenEdge/Progress

<lang OpenEdge/Progress>FUNCTION reverseString RETURNS CHARACTER (

  INPUT i_c AS CHARACTER

):

  DEFINE VARIABLE cresult AS CHARACTER   NO-UNDO.
  DEFINE VARIABLE ii      AS INTEGER     NO-UNDO.

  DO ii = LENGTH( i_c ) TO 1 BY -1:
     cresult = cresult + SUBSTRING( i_c, ii, 1 ).
  END.
  RETURN cresult.

END FUNCTION.

MESSAGE reverseString( "asdf" ) VIEW-AS ALERT-BOX.</lang>

OxygenBasic

'8 BIT CHARACTERS

string s="qwertyuiop" sys a,b,i,j,le=len s ' for i=1 to le

 j=le-i+1
 if j<=i then exit for
 a=asc s,i
 b=asc s,j
 mid s,j,chr a
 mid s,i,chr b

next '

print s

'16 BIT CHARACTERS

wstring s="qwertyuiop" sys a,b,i,j,le=len s ' for i=1 to le

 j=le-i+1
 if j<=i then exit for
 a=unic s,i
 b=unic s,j
 mid s,j,wchr a
 mid s,i,wchr b

next ' print s </lang>

OxygenBasic x86 Assembler

32 bit code, 8-bit characters: <lang oxygenbasic>

string s="qwertyuiop" sys p=strptr s, le=len s mov esi,p mov edi,esi add edi,le dec edi (

cmp esi,edi
jge exit
mov al,[esi]
mov ah,[edi]
mov [esi],ah
mov [edi],al
inc esi
dec edi
repeat

)

print s </lang>

Oz

Strings are lists. A function "Reverse" defined on lists is part of the implementation. <lang oz>{System.showInfo {Reverse "!dlroW olleH"}}</lang> An efficient (tail-recursive) implementation could look like this: <lang oz>local

  fun {DoReverse Xs Ys}
     case Xs of nil then Ys
     [] X|Xr then {DoReverse Xr X|Ys}
     end
  end

in

  fun {Reverse Xs} {DoReverse Xs nil} end

end</lang> Oz uses a single-byte encoding by default. If you decide to use a multi-byte encoding, Reverse will not work correctly.

PARI/GP

Version #1.

<lang parigp>reverse(s)=concat(Vecrev(s))</lang>

Version #2.

Works with: PARI/GP version 2.7.4 and above

<lang parigp> \\ Return reversed string str. \\ 3/3/2016 aev sreverse(str)={return(Strchr(Vecrev(Vecsmall(str))))}

{ \\ TEST1 print(" *** Testing sreverse from Version #2:"); print(sreverse("ABCDEF")); my(s,sr,n=10000000); s="ABCDEFGHIJKL"; for(i=1,n, sr=sreverse(s)); } </lang>

Output:

 *** Testing sreverse from Version #2:
FEDCBA
(17:28) gp > ##
  ***   last result computed in 8,642 ms.

<lang parigp> \\ Version #1 upgraded to complete function. Practically the same. reverse(str)={return(concat(Vecrev(str)))}

{ \\ TEST2 print(" *** Testing reverse from Version #1:"); print(reverse("ABCDEF")); my(s,sr,n=10000000); s="ABCDEFGHIJKL"; for(i=1,n, sr=reverse(s)); } </lang>

Output:

 *** Testing reverse from Version #1:
FEDCBA
(17:31) gp > ##
  ***   last result computed in 11,814 ms.

Pascal

The following examples handle correctly only single-byte encodings.

Standard Pascal

The following only works on implementations which implement Level 1 of standard Pascal (many popular compilers don't).

Standard Pascal doesn't have a separate string type, but uses arrays of char for strings. Note that Standard Pascal doesn't allow a return type of char array, therefore the destination array is passed through a var parameter (which is more efficient anyway). <lang pascal>{ the result array must be at least as large as the original array } procedure reverse(s: array[min .. max: integer] of char, var result: array[min1 .. max1: integer] of char);

var
 i, len: integer;
begin
 len := max-min+1;
 for i := 0 to len-1 do
  result[min1 + len-1 - i] := s[min + i]
end;</lang>

<lang pascal>{Copy and paste it in your program} function revstr(my_s:string):string;

   var out_s:string;
   ls,i:integer;
   begin
   ls:=length(my_s);
   for i:=1 to ls do
   out_s:=out_s+my_s[ls-i+1];
   revstr:=out_s;
   end;</lang>

Extended Pascal, Turbo Pascal, Delphi and compatible compilers

<lang pascal>function reverse(s:string):string; var i:integer; var tmp:char; begin

   for i:=1 to length(s) div 2 do
     begin
      tmp:=s[i];
      s[i]:=s[length(s)+1-i];
      s[length(s)+1-i]:=tmp;
      reverse:=s;
     end;

end;</lang> alternative as procedure which changes the original <lang pascal>procedure revString(var s:string); var

 i,j:integer;
 tmp:char;

begin

 i := 1;
 j := length(s);
 while i<j do
 begin
    tmp:=s[i];
    s[i]:=s[j];
    s[j]:=tmp;
    inc(i);
    dec(j)
 end;

end;</lang>

Peloton

Padded out, variable length Chinese dialect <lang sgml><# 显示指定变量反转顺序字串>集装箱|猫坐在垫子</#></lang>

This assigns the reverse of 'the cat sat on the mat' to the variable 'container' and displays the result which is

子垫在坐猫

which Google Translate renders as

Sub-pad sitting cat

.

The same again but with everything in Korean. <lang sgml><# 보이십 할당하 변물건 열거꾸 문자그>컨테이너|고양이가 매트 위에 앉아</#></lang>

Reversing the Korean makes an untranslatable-by-Google mess of the sentence, viz

아앉 에위 트매 가이양고

.

The short-opcode version in English dialect is <lang sgml><@ SAYLETVARREVLIT>集装箱|猫坐在垫子</@></lang> Peloton works in Unicode.

Perl

<lang perl>use utf8; binmode STDOUT, ":utf8";

to reverse characters (code points):

print reverse('visor'), "\n";

to reverse graphemes:

print join("", reverse "José" =~ /\X/g), "\n";

$string = 'ℵΑΩ 駱駝道 🤔 🇸🇧 🇺🇸 🇬🇧‍ 👨‍👩‍👧‍👦🆗🗺'; print join("", reverse $string =~ /\X/g), "\n";</lang>

Output:

rosiv
ésoJ
🗺🆗👨‍👩‍👧‍👦 🇬🇧‍ 🇺🇸 🇸🇧 🤔 道駝駱 ΩΑℵ

Pharo

<lang Pharo>'123' reversed</lang>

Phix

<lang Phix>?reverse("asdf")</lang> However that would go horribly wrong on utf8 strings, even without combining characters, so... this seems ok on "as\u203Ddf\u0305", as long as it is displayed in a message box rather than a Windows Console (even with chcp 65001 and Lucida Console, the characters do not combine).
Note that XXXX_redacted#Phix adds some ZERO-WIDTH-JOINER handling to the inner code. <lang Phix>function unicode_reverse(string utf8) sequence utf32 = utf8_to_utf32(utf8) integer ch

   -- The assumption is made that <char><comb1><comb2>
   -- and <char><comb2><comb1> etc would work the same.
   -- The following loop converts <char><comb1><comb2>
   -- to <comb1><comb2><char>, as a pre-reverse() step.
   for i=1 to length(utf32) do
       ch = utf32[i]
       if (ch>=0x300 and ch<=0x36f)
       or (ch>=0x1dc0 and ch<=0x1dff)
       or (ch>=0x20d0 and ch<=0x20ff)
       or (ch>=0xfe20 and ch<=0xfe2f) then
           utf32[i] = utf32[i-1]
           utf32[i-1] = ch
       end if
   end for
   utf32 = reverse(utf32)
   utf8 = utf32_to_utf8(utf32)
   return utf8

end function</lang>

PHP

<lang php>strrev($string);</lang> If you want Unicode support, you have to use some multibyte function. Sadly, PHP doesn't contain mb_strrev(). One of functions which support Unicode and is useful in this case is preg_split(). <lang php>// Will split every Unicode character to array, reverse array and will convert it to string. join(, array_reverse(preg_split('""u', $string, -1, PREG_SPLIT_NO_EMPTY)));</lang>

PicoLisp

<lang PicoLisp>(pack (flip (chop "äöüÄÖÜß")))</lang>

Output:

-> "ßÜÖÄüöä"

Pike

For simple ASCII: <lang Pike>reverse("foo");</lang>

Output:

"oof"

When dealing with Unicode (or any other supported encoding) care must be taken primarily in the output step to serialize the Unicode string into something the sink can handle. IO functions will throw an error if sent raw wide strings.

<lang Pike>#charset utf8 void main() {

   string s = "ßÜÖÄüöää ἀρχῇ";    
   write("%s\n", string_to_utf8( reverse(s) ));

}</lang>

Output:

ῇχρἀ ääöüÄÖÜß

PL/I

<lang PL/I>s = reverse(s);</lang>

Plain English

<lang plainenglish>To run: Start up. Put "asdf" into a string. Reverse the string. Shut down.</lang>

Plain $T e X$

Works well if the string has no space (spaces are gobbled).

<lang tex>\def\gobtoA#1\revA{}\def\gobtoB#1\revB{} \def\reverse#1{\reversei{}#1\revA\revB\revB\revB\revB\revB\revB\revB\revB\revA} \def\reversei#1#2#3#4#5#6#7#8#9{\gobtoB#9\revend\revB\reversei{#9#8#7#6#5#4#3#2#1}} \def\revend\revB\reversei#1#2\revA{\gobtoA#1} \reverse{Rosetta} \bye</lang>

Output:

attesoR

Pop11

<lang pop11>define reverse_string(s);

   lvars i, l = length(s);
   for i from l by -1 to 1 do
       s(i);
   endfor;
   consstring(l);

enddefine;</lang>

PostScript

The following implementation works on arrays of numerics as well as characters ( string ). <lang PostScript>/reverse{ /str exch def /temp str 0 get def /i 0 def str length 2 idiv{ /temp str i get def str i str str length i sub 1 sub get put str str length i sub 1 sub temp put /i i 1 add def }repeat str pstack }def</lang>

Output:

<lang PostScript>[1 2 3] reverse % input [3 2 1]

(Hello World) reverse % input (dlroW olleH)</lang>

PowerBASIC

<lang powerbasic>#DIM ALL

COMPILER PBCC 6

FUNCTION PBMAIN () AS LONG CON.PRINT STRREVERSE$("PowerBASIC") END FUNCTION</lang>

PowerShell

Test string <lang powershell>$s = "asdf"</lang>

Array indexing

Creating a character array from the end to the string's start and join it together into a string again.

Works with: PowerShell version 1

<lang powershell>[string]::Join(, $s[$s.Length..0])</lang>

Works with: PowerShell version 2

<lang powershell>-join ($s[$s.Length..0])</lang>

Works with: PowerShell version 2

<lang powershell>[array]::Reverse($s)</lang>

Regular expressions

Creating a regular expression substitution which captures every character of the string in a capture group and uses a reverse-ordered string of references to those to construct the reversed string.

Works with: PowerShell version 1

<lang powershell>$s -replace

     ('(.)' * $s.Length),
     [string]::Join(, ($s.Length..1 | ForEach-Object { "`$$_" }))</lang>

Works with: PowerShell version 2

<lang powershell>$s -replace

     ('(.)' * $s.Length),
     -join ($s.Length..1 | ForEach-Object { "`$$_" } )</lang>

Works with: PowerShell version 3

<lang PowerShell> [Regex]::Matches('abc','.','RightToLeft').Value -join </lang>

Output:

cba

Prolog

Works with: SWI Prolog

<lang prolog>reverse("abcd", L), string_to_list(S,L).</lang>

Output:

L = [100,99,98,97],
S = "dcba".

The main workings are hidden inside the reverse/2 predicate, so lets write one to see how it works: <lang prolog>accRev([H|T], A, R) :- accRev(T, [H|A], R). accRev([], A, A).

rev(L,R) :- accRev(L,[],R).</lang>

PureBasic

<lang PureBasic>Debug ReverseString("!dekrow tI")</lang>

Python

Optimized for user input

<lang python>input()[::-1]</lang>

Already known string

<lang python>string[::-1]</lang> or <lang python>.join(reversed(string))</lang>

Python: Unicode reversal

(See this article for more information from which this is improved)

Note: How this looks may be subject to how the tool you are using to view this page can render Unicode. <lang python>import unicodedata

def ureverse(ustring):

   'Reverse a string including unicode combining characters'
   groupedchars = []
   uchar = list(ustring)
   while uchar:
       if unicodedata.combining(uchar[0]) != 0:
           groupedchars[-1] += uchar.pop(0)
       else:
           groupedchars.append(uchar.pop(0))
   # Grouped reversal
   groupedchars = groupedchars[::-1]

   return .join(groupedchars)

def say_string(s):

   return ' '.join([s, '=', ' | '.join(unicodedata.name(ch, ) for ch in s)])

def say_rev(s):

   print(f"Input:              {say_string(s)}")
   print(f"Character reversed: {say_string(s[::-1])}")
   print(f"Unicode reversed:   {say_string(ureverse(s))}")
   print(f"Unicode reverse²:   {say_string(ureverse(ureverse(s)))}")

if __name__ == '__main__':

   ucode = .join(chr(int(n[2:], 16)) for n in 
                    'U+0041 U+030A U+0073 U+0074 U+0072 U+006F U+0308 U+006D'.split())
   say_rev(ucode)</lang>

Output:

Input:              Åström = LATIN CAPITAL LETTER A | COMBINING RING ABOVE | LATIN SMALL LETTER S | LATIN SMALL LETTER T | LATIN SMALL LETTER R | LATIN SMALL LETTER O | COMBINING DIAERESIS | LATIN SMALL LETTER M
Character reversed: m̈orts̊A = LATIN SMALL LETTER M | COMBINING DIAERESIS | LATIN SMALL LETTER O | LATIN SMALL LETTER R | LATIN SMALL LETTER T | LATIN SMALL LETTER S | COMBINING RING ABOVE | LATIN CAPITAL LETTER A
Unicode reversed:   mörtsÅ = LATIN SMALL LETTER M | LATIN SMALL LETTER O | COMBINING DIAERESIS | LATIN SMALL LETTER R | LATIN SMALL LETTER T | LATIN SMALL LETTER S | LATIN CAPITAL LETTER A | COMBINING RING ABOVE
Unicode reverse²:   Åström = LATIN CAPITAL LETTER A | COMBINING RING ABOVE | LATIN SMALL LETTER S | LATIN SMALL LETTER T | LATIN SMALL LETTER R | LATIN SMALL LETTER O | COMBINING DIAERESIS | LATIN SMALL LETTER M

If this code is then used: <lang python>ucode = .join(chr(int(n[2:], 16)) for n in

                'U+006B U+0301 U+0075 U+032D U+006F U+0304 U+0301 U+006E'.split())

say_rev(ucode)</lang> It produces this output

Output:

Input:              ḱṷṓn = LATIN SMALL LETTER K | COMBINING ACUTE ACCENT | LATIN SMALL LETTER U | COMBINING CIRCUMFLEX ACCENT BELOW | LATIN SMALL LETTER O | COMBINING MACRON | COMBINING ACUTE ACCENT | LATIN SMALL LETTER N
Character reversed: ń̄o̭úk = LATIN SMALL LETTER N | COMBINING ACUTE ACCENT | COMBINING MACRON | LATIN SMALL LETTER O | COMBINING CIRCUMFLEX ACCENT BELOW | LATIN SMALL LETTER U | COMBINING ACUTE ACCENT | LATIN SMALL LETTER K
Unicode reversed:   nṓṷḱ = LATIN SMALL LETTER N | LATIN SMALL LETTER O | COMBINING MACRON | COMBINING ACUTE ACCENT | LATIN SMALL LETTER U | COMBINING CIRCUMFLEX ACCENT BELOW | LATIN SMALL LETTER K | COMBINING ACUTE ACCENT
Unicode reverse²:   ḱṷṓn = LATIN SMALL LETTER K | COMBINING ACUTE ACCENT | LATIN SMALL LETTER U | COMBINING CIRCUMFLEX ACCENT BELOW | LATIN SMALL LETTER O | COMBINING MACRON | COMBINING ACUTE ACCENT | LATIN SMALL LETTER N

This uses the unicode string mentioned in the task: <lang python>ucode = .join(chr(int(n, 16))

                for n in ['61', '73', '20dd', '64', '66', '305'])

say_rev(ucode)</lang> It produces this output

Output:

Input:              as⃝df̅ = LATIN SMALL LETTER A | LATIN SMALL LETTER S | COMBINING ENCLOSING CIRCLE | LATIN SMALL LETTER D | LATIN SMALL LETTER F | COMBINING OVERLINE
Character reversed: ̅fd⃝sa = COMBINING OVERLINE | LATIN SMALL LETTER F | LATIN SMALL LETTER D | COMBINING ENCLOSING CIRCLE | LATIN SMALL LETTER S | LATIN SMALL LETTER A
Unicode reversed:   f̅d⃝sa = LATIN SMALL LETTER F | COMBINING OVERLINE | LATIN SMALL LETTER D | COMBINING ENCLOSING CIRCLE | LATIN SMALL LETTER S | LATIN SMALL LETTER A
Unicode reverse²:   as⃝df̅ = LATIN SMALL LETTER A | LATIN SMALL LETTER S | COMBINING ENCLOSING CIRCLE | LATIN SMALL LETTER D | LATIN SMALL LETTER F | COMBINING OVERLINE

Quackery

reverse is predefined (and applies to nests in general, including strings) as: <lang Quackery> [ dup nest? if

     [ [] swap witheach
         [ nested
           swap join ] ] ]     is reverse      (       x --> x       )</lang>

Qi

It's simplest just to use the common lisp REVERSE function. <lang Qi>(REVERSE "ABCD")</lang>

R

Works with: R version 2.8.1

The following code works with UTF-8 encoded strings too. <lang R>revstring <- function(stringtorev) {

  return(
     paste(
          strsplit(stringtorev,"")1[nchar(stringtorev):1]
          ,collapse="")
          )

}</lang>

Alternatively (using rev() function):

<lang R> revstring <- function(s) paste(rev(strsplit(s,"")1),collapse="")</lang>

<lang R>revstring("asdf") revstring("m\u00f8\u00f8se") Encoding("m\u00f8\u00f8se") # just to check if on your system it's something

                             # different!</lang>

Output:

[1] "fdsa"
[1] "esøøm"
[1] "UTF-8"

R can encode strings in Latin1 and UTF-8 (the default may depend on the locale); the Encoding(string) can be used to know if the string is encoded in Latin1 or UTF-8; the encoding can be forced (Encoding(x) <- "latin1"), or we can use iconv to properly translate between encodings whenever possible.

Racket

As in Scheme:

<lang Racket>#lang racket

(define (string-reverse s)

 (list->string (reverse (string->list s))))

(string-reverse "aoeu")</lang>

Output:

Welcome to DrRacket, version 5.3.3.5--2013-02-20(5eddac74/d) [3m].
Language: racket; memory limit: 512 MB.
"ueoa"
>

Raku

(formerly Perl 6)

Works with: rakudo version 2018.08

Raku handles graphemes, multi-byte characters and emoji correctly by default. <lang perl6>say "hello world".flip; say "as⃝df̅".flip; say 'ℵΑΩ 駱駝道 🤔 🇸🇧 🇺🇸 🇬🇧‍ 👨‍👩‍👧‍👦🆗🗺'.flip;</lang>

Output:

dlrow olleh
f̅ds⃝a
🗺🆗👨‍👩‍👧‍👦 🇬🇧‍ 🇺🇸 🇸🇧 🤔 道駝駱 ΩΑℵ

RapidQ

<lang vb> print reverse$("This is a test") </lang>

Rascal

<lang rascal>import String; reverse("string")</lang>

Raven

<lang Raven>"asdf" reverse</lang>

Output:

fdsa

REBOL

<lang REBOL>print reverse "asdf"</lang> Note the string is reversed in place. If you were using it anywhere else, you would find it reversed: <lang REBOL>x: "asdf" print reverse x print x ; Now reversed.</lang> REBOL/View 2.7.6.3.1 14-Mar-2008 does not handle Unicode strings. This is planned for REBOL 3.

Red

<lang Red>>> reverse "asdf" == "fdsa"</lang>

Retro

<lang Retro>'asdf s:reverse s:put</lang>

REXX

All methods shown below also work with NULL values (strings with a zero length).

using REVERSE BIF

<lang rexx>/*REXX program to reverse a string (and show before and after strings).*/

string1 = 'A man, a plan, a canal, Panama!' string2 = reverse(string1)

say ' original string: ' string1 say ' reversed string: ' string2

                                      /*stick a fork in it, we're done.*/</lang>

output

 original string:  A man, a plan, a canal, Panama!
 reversed string:  !amanaP ,lanac a ,nalp a ,nam A

using SUBSTR BIF, left to right

<lang rexx>/*REXX program to reverse a string (and show before and after strings).*/

string1 = 'A man, a plan, a canal, Panama!' string2 =

                                  do j=1  for length(string1)
                                  string2 = substr(string1,j,1)string2
                                  end   /*j*/

say ' original string: ' string1 say ' reversed string: ' string2

                                      /*stick a fork in it, we're done.*/</lang>

output is identical to the 1^st REXX version.

(Regarding the previous example) Another method of coding an abutment (an implied concatenation) is: <lang rexx> string2 = substr(string1,j,1) || string2

                                               /*───── or ─────*/
                                  string2= substr(string1,j,1)string2</lang>

using SUBSTR BIF, right to left

<lang rexx>/*REXX program to reverse a string (and show before and after strings).*/

string1 = 'A man, a plan, a canal, Panama!' string2 =

                                 do j=length(string1)  to 1  by -1
                                 string2 = string2 || substr(string1,j,1)
                                 end   /*j*/

say ' original string: ' string1 say ' reversed string: ' string2

                                      /*stick a fork in it, we're done.*/</lang>

output is identical to the 1^st version.

Ring

RLaB

<lang RLaB>>> x = "rosettacode" rosettacode

// script rx = ""; for (i in strlen(x):1:-1) {

 rx = rx + substr(x, i);

}

>> rx edocattesor</lang>

Robotic

<lang robotic> . "local1 = Main string" . "local2 = Temporary string storage" . "local3 = String length" set "$local1" to "" set "$local2 " to "" set "local3" to 0

input string "String to reverse:" set "$local1" to "&INPUT&" set "$local2" to "$local1" set "local3" to "$local2.length" loop start set "$local1.(('local3' - 1) - 'loopcount')" to "$local2.('loopcount')" loop for "('local3' - 1)"

"Reversed string: &$local1& (Length: &$local1.length&)"

end </lang>

Ruby

<lang ruby>str = "asdf" reversed = str.reverse</lang> or <lang ruby>#encoding: utf-8 "résumé niño".reverse #=> "oñin émusér"</lang> for extra credit <lang ruby> graphemes = 'as⃝df̅'.scan(/\X/) reversed = graphemes.reverse graphemes.join #=> "f̅ds⃝a" </lang>

Run BASIC

<lang runbasic>string$ = "123456789abcdefghijk" for i = len(string$) to 1 step -1

print mid$(string$,i,1);

next i</lang>

Rust

Reversing ASCII byte-slice (in-place):

<lang rust>let mut buffer = b"abcdef".to_vec(); buffer.reverse(); assert_eq!(buffer, b"fedcba");</lang>

Reversing Unicode scalar values:

<lang rust>let output: String = "一二三四五六七八九十".chars().rev().collect(); assert_eq!(output, "十九八七六五四三二一");</lang>

Reversing a Chars iterator doesn't solve the complete problem, because it iterates unicode scalar values, which doesn't account for combining marks:

<lang rust>let output: String = "as⃝df̅".chars().rev().collect(); assert_ne!(output, "f̅ds⃝a"); // should be this assert_eq!(output, "̅fd⃝sa");</lang>

Reversing graphemes clusters, which is provided by the unicode-segmentation crate, solves the problem:

<lang rust>use unicode_segmentation::UnicodeSegmentation;

let output: String = "as⃝df̅".graphemes(true).rev().collect(); assert_eq!(output, "f̅ds⃝a");</lang>

S-lang

Here is an 8-bit version: <lang S-lang>variable sa = "Hello, World", aa = Char_Type[strlen(sa)+1]; init_char_array(aa, sa); array_reverse(aa); % print(aa);

% Unfortunately, strjoin() only joins strings, so we map char() % [sadly named: actually converts char into single-length string] % onto the array:

print( strjoin(array_map(String_Type, &char, aa), "") );</lang>

Output: "dlroW ,olleH"

For a Unicode version, we'll create a variant of init_char_array(). Side note: If needed, strbytelen() would give total length of string.

<lang S-lang>define init_unicode_array(a, buf) {

 variable len = strbytelen(buf), ch, p0 = 0, p1 = 0;
 while (p1 < len) {
   (p1, ch) = strskipchar(buf, p1, 1);
   if (ch < 0) print("oops.");
   a[p0] = ch;
   p0++;
 }

}

variable su = "Σὲ γνωρίζω ἀπὸ τὴν κόψη"; variable au = Int_Type[strlen(su)+1]; init_unicode_array(au, su); array_reverse(au); % print(au); print(strjoin(array_map(String_Type, &char, au), "") );</lang>

Output: "ηψόκ νὴτ ὸπἀ ωζίρωνγ ὲΣ"

Note: The init...array() functions include the terminating '\0' chars, but we don't have to filter them out as char(0) produces a zero-length string.

SAS

<lang sas>data _null_; length a b $11; a="I am Legend"; b=reverse(a); put a; put b; run;</lang>

Sather

<lang sather>class MAIN is

 main is
   s ::= "asdf";
   reversed ::= s.reverse; 
   -- current implementation does not handle multibyte encodings correctly
 end;

end;</lang>

Scala

Easy way: <lang scala>"asdf".reverse</lang>

Slightly less easy way: <lang scala>"asdf".foldRight("")((a,b) => b+a)</lang>

Unicode-aware, method 1:

<lang scala>def reverse(s: String) = {

 import java.text.{Normalizer,BreakIterator}
 val norm = Normalizer.normalize(s, Normalizer.Form.NFKC) // waﬄe -> waffle (optional)
 val it = BreakIterator.getCharacterInstance
 it setText norm
 def break(it: BreakIterator, prev: Int, result: List[String] = Nil): List[String] = it.next match {
   case BreakIterator.DONE => result
   case cur => break(it, cur, norm.substring(prev, cur) :: result)
 }
 break(it, it.first).mkString

}</lang>

Output:

scala> reverse("as⃝df̅")
res0: String = f̅ds⃝a

Unicode-aware, method 2: I can't guarantee it get all the cases, but it does work with combining characters as well as supplementary characters. I did not bother to preserve the order of newline characters, and I didn't even consider directionality beyond just ruling it out. <lang scala>def reverseString(s: String) = {

 import java.lang.Character._
 
 val combiningTypes = List(NON_SPACING_MARK, ENCLOSING_MARK, COMBINING_SPACING_MARK)
 def isCombiningCharacter(c: Char) = combiningTypes contains c.getType
 def isCombiningSurrogate(high: Char, low: Char) = combiningTypes contains getType(toCodePoint(high, low))
 def isCombining(l: List[Char]) = l match {
   case List(a, b) => isCombiningSurrogate(a, b)
   case List(a) => isCombiningCharacter(a)
   case Nil => true
   case _ => throw new IllegalArgumentException("isCombining expects a list of up to two characters")
 }
 
 def cleanSurrogate(l: List[Char]) = l match {
   case List(a, b) if a.isHighSurrogate && b.isLowSurrogate => l
   case List(a, b) if a.isLowSurrogate => Nil
   case List(a, b) => List(a)
   case _ => throw new IllegalArgumentException("cleanSurrogate expects lists of two characters, exactly")
 }
 
 def splitString(string: String) = (string+" ").iterator sliding 2 map (_.toList) map cleanSurrogate toList

 def recurse(fwd: List[List[Char]], rev: List[Char]): String = fwd match {
   case Nil => rev.mkString
   case c :: rest =>
     val (combining, remaining) = rest span isCombining
     recurse(remaining, c ::: combining.foldLeft(List[Char]())(_ ::: _) ::: rev)
 }
 recurse(splitString(s), Nil)

}</lang> REPL on Windows doesn't handle Unicode, so I'll show the bytes instead:

scala> res71 map ("\\u%04x" format _.toInt)
res80: scala.collection.immutable.IndexedSeq[String] = IndexedSeq(\u0061, \u0073, \u20dd, \u0064, \u0066, \u0305)

scala> reverseString(res71) map ("\\u%04x" format _.toInt)
res81: scala.collection.immutable.IndexedSeq[String] = IndexedSeq(\u0066, \u0305, \u0064, \u0073, \u20dd, \u0061)

Scheme

<lang scheme>(define (string-reverse s)

 (list->string (reverse (string->list s))))</lang>

> (string-reverse "asdf")
"fdsa"

Scratch

Sed

<lang sed>#!/bin/sed -f

/../! b

Reverse a line. Begin embedding the line between two newlines

s/^.*$/\ &\ /

Move first character at the end. The regexp matches until
there are zero or one characters between the markers

tx

x

s/$\n.$$.*$$.\n$/\3\2\1/ tx

Remove the newline markers

s/\n//g</lang>

Seed7

Seed7 strings are encoded with UTF-32 therefore no special Unicode solution is necessary <lang seed7>$ include "seed7_05.s7i";

const func string: reverse (in string: stri) is func

 result
   var string: result is "";
 local
   var integer: index is 0;
 begin
   for index range length(stri) downto 1 do
     result &:= stri[index];
   end for;
 end func;

const proc: main is func

 begin
   writeln(reverse("Was it a cat I saw"));
 end func;</lang>

Output:

was I tac a ti saW

Self

In-place reversal: <lang self>'asdf' copyMutable reverse</lang>

SenseTalk

<lang sensetalk>put "asdf" reversed -- reverse on the fly

set imp to "rumpelstiltskin" reverse imp -- reverse in place put imp </lang>

Output:

fdsa
nikstlitslepmur

SequenceL

Using Library Function:

There is a library function to reverse any Sequence. This works for strings since strings are Sequences of characters. <lang sequencel>import <Utilities/Sequence.sl>;

main(args(2)) := Sequence::reverse(args[1]);</lang>

The Library Function:

The following is the library implementation of the reverse function. <lang sequencel>reverse<T> : T(1) -> T(1); reverse(list(1))[i] := let range := - ((1 ... size(list)) - (size(list) + 1)); in list[i] foreach i within range;</lang>

Sidef

<lang ruby>"asdf".reverse; # fdsa "résumé niño".reverse; # oñin émusér</lang>

Simula

<lang simula> BEGIN

  TEXT PROCEDURE REV(S); TEXT S;
  BEGIN
      TEXT T;
      INTEGER L,R;
      T :- COPY(S);
      L := 1; R := T.LENGTH;
      WHILE L < R DO
      BEGIN
          CHARACTER CL,CR;
          T.SETPOS(L); CL := T.GETCHAR;
          T.SETPOS(R); CR := T.GETCHAR;
          T.SETPOS(L); T.PUTCHAR(CR);
          T.SETPOS(R); T.PUTCHAR(CL);
          L := L+1;
          R := R-1;
       END;
       REV :- T;
  END REV;

  TEXT INP;
  INP :- "asdf";

  OUTTEXT(INP);      OUTIMAGE;
  OUTTEXT(REV(INP)); OUTIMAGE;

END </lang>

Output:

asdf
fdsa

Slate

In-place reversal: <lang slate>'asdf' reverse</lang> Non-destructive reversal: <lang slate>'asdf' reversed</lang>

Smalltalk

<lang smalltalk>'asdf' reverse</lang>

Works with: Smalltalk/X

the above does inplace, destructive reverse. It is usually better to use <lang smalltalk>'asdf' reversed</lang> which returns a new string.

SNOBOL4

ASCII-only <lang snobol> output = reverse(reverse("reverse")) end</lang>

Output:

reverse

Standard ML

<lang sml>val str_reverse = implode o rev o explode; val string = "asdf"; val reversed = str_reverse string;</lang>

Stata

Use strreverse if there are only ASCII characters, and ustrreverse if there are Unicode characters in the string.

<lang stata>. scalar s="ARS LONGA VITA BREVIS" . di strreverse(s) SIVERB ATIV AGNOL SRA . scalar s="Ἐν ἀρχῇ ἐποίησεν ὁ θεὸς τὸν οὐρανὸν καὶ τὴν γῆν" . di ustrreverse(s) νῆγ νὴτ ὶακ νὸναρὐο νὸτ ςὸεθ ὁ νεσηίοπἐ ῇχρἀ νἘ</lang>

Swift

Swift's strings are iterated by Characters, which represent "Unicode grapheme clusters", so reversing it reverses it with combining characters too:

Works with: Swift version 2.x+

<lang swift>func reverseString(s: String) -> String {

 return String(s.characters.reverse())

} print(reverseString("asdf")) print(reverseString("as⃝df̅"))</lang>

Works with: Swift version 1.x

<lang swift>func reverseString(s: String) -> String {

 return String(reverse(s))

} println(reverseString("asdf")) println(reverseString("as⃝df̅"))</lang>

Output:

fdsa
f̅ds⃝a

Symsyn

<lang Symsyn> | reverse string

c : 'abcdefghijklmnopqrstuvwxyz' d : ' '

c []
i
#c j
- j
if i < j
   c.i d 1
   c.j c.i 1
   d c.j
   - j
   + i
   goif
endif
c []

</lang>

OR

<lang Symsyn> c : 'abcdefghijklmnopqrstuvwxyz'

c []
$s
#c j
if j > 0
   - j
   + c.j $s 1
   goif
endif
$s []

</lang>

Output:

abcdefghijklmnopqrstuvwxyz
zyxwvutsrqponmlkjihgfedcba

Tailspin

<lang tailspin> templates reverse

 '$:[ $... ] -> $(last..first:-1)...;' !

end reverse

'asdf' -> reverse -> !OUT::write

' ' -> !OUT::write

'as⃝df̅' -> reverse -> !OUT::write </lang>

Output:

fdsa
f̅ds⃝a

Tcl

<lang tcl>package require Tcl 8.5 string reverse asdf</lang>

TI-83 BASIC

Note: length( and sub( can be found in the catalog. <lang ti83b>:Str1

For(I,1,length(Ans)-1

sub(Ans,2I,1)+Ans

End

sub(Ans,1,I→Str1</lang>

TMG

Unix TMG: <lang UnixTMG>prog: parse(str); str: smark any(!<<>>) scopy str/done = { 1 2 }; done: ;</lang>

Tosh

<lang Tosh>when flag clicked ask "Say something..." and wait set i to (length of answer) set inv to "" repeat until i = 0 set inv to (join (inv) (letter (i) of answer)) change i by -1 end say inv</lang>

Turing

Iterative solution, for character shovelers:

<lang Turing>function reverse (s : string) : string

   var rs := ""
   for i : 0 .. length (s) - 1
       rs := rs + s (length (s) - i)
   end for
   result rs

end reverse

put reverse ("iterative example") put reverse (reverse ("iterative example"))</lang>

Output:

elpmaxe evitareti
iterative example

Recursive solution, more natural in Turing:

<lang Turing>function reverse (s : string) : string

   if s = "" then
       result s
   else
       result reverse (s (2 .. *)) + s (1)
   end if

end reverse

put reverse ("recursive example") put reverse (reverse ("recursive example"))</lang>

Output:

elpmaxe evisrucer
recursive example

TUSCRIPT

<lang tuscript>$$ MODE TUSCRIPT SET input="was it really a big fat cat i saw" SET reversetext=TURN (input) PRINT "before: ",input PRINT "after: ",reversetext</lang>

Output:

before: was it really a big fat cat i saw
after:  was i tac taf gib a yllaer ti saw

UNIX Shell

!/bin/bash

str=abcde

for((i=${#str}-1;i>=0;i--)); do rev="$rev${str:$i:1}"; done

echo $rev </lang>

or

<lang bash> str='i43go1342iu 23iu4o 23iu14i324y 2i13' rev <<< "$str"

rev is not built-in function, though is in /usr/bin/rev

</lang>

Unlambda

Reverse the whole input: <lang Unlambda>``@c`d``s`|k`@c</lang>

Ursala

<lang Ursala>#import std

cast %s

example = ~&x 'asdf'

verbose_example = reverse 'asdf'</lang>

Output:

'fdsa'

Vala

<lang vala>int main (string[] args) { if (args.length < 2) { stdout.printf ("Please, input a string.\n"); return 0; } var str = new StringBuilder (); for (var i = 1; i < args.length; i++) { str.append (args[i] + " "); } stdout.printf ("%s\n", str.str.strip ().reverse ()); return 0; }</lang>

VBA

Non-recursive version

<lang VBA>Public Function Reverse(aString as String) as String ' returns the reversed string dim L as integer 'length of string dim newString as string

newString = "" L = len(aString) for i = L to 1 step -1

newString = newString & mid$(aString, i, 1)

next Reverse = newString End Function</lang>

Recursive version

<lang VBA>Public Function RReverse(aString As String) As String 'returns the reversed string 'do it recursively: cut the string in two, reverse these fragments and put them back together in reverse order Dim L As Integer 'length of string Dim M As Integer 'cut point

L = Len(aString) If L <= 1 Then 'no need to reverse

 RReverse = aString

Else

 M = Int(L / 2)
 RReverse = RReverse(Right$(aString, L - M)) & RReverse(Left$(aString, M))

End If End Function</lang>

Example dialogue

print Reverse("Public Function Reverse(aString As String) As String")
gnirtS sA )gnirtS sA gnirtSa(esreveR noitcnuF cilbuP

print RReverse("Sunday Monday Tuesday Wednesday Thursday Friday Saturday Love")
evoL yadrutaS yadirF yadsruhT yadsendeW yadseuT yadnoM yadnuS

print RReverse(Reverse("I know what you did last summer"))
I know what you did last summer

VBScript

Works with: Windows Script Host version *

<lang VBScript> WScript.Echo StrReverse("asdf") </lang>

Vedit macro language

This routine reads the text from current line, reverses it and stores the reversed string in text register 10: <lang vedit>Reg_Empty(10) for (BOL; !At_EOL; Char) {

   Reg_Copy_Block(10, CP, CP+1, INSERT)

}</lang> This routine reverses the current line in-place: <lang vedit>BOL while (!At_EOL) {

   Block_Copy(EOL_pos-1, EOL_pos, DELETE)

}</lang>

Visual Basic

Works with: Visual Basic version 6

<lang vb>Debug.Print VBA.StrReverse("Visual Basic")</lang>

Visual Basic .NET

Compiler: >= Visual Basic 2012

Includes both a simple version and a version that uses .NET's built-in ability to enumerate strings by grapheme to support combining characters.

Since the windows console may not support Unicode, the program can optionally redirect its output to a file.

<lang vbnet>#Const REDIRECTOUT = True

Module Program

   Const OUTPATH = "out.txt"

   ReadOnly TestCases As String() = {"asdf", "as⃝df̅", "Les Misérables"}

   ' SIMPLE VERSION
   Function Reverse(s As String) As String
       Dim t = s.ToCharArray()
       Array.Reverse(t)
       Return New String(t)
   End Function

   ' EXTRA CREDIT VERSION
   Function ReverseElements(s As String) As String
       ' In .NET, a text element is series of code units that is displayed as one character, and so reversing the text
       ' elements of the string correctly handles combining character sequences and surrogate pairs.
       Dim elements = Globalization.StringInfo.GetTextElementEnumerator(s)
       Return String.Concat(AsEnumerable(elements).OfType(Of String).Reverse())
   End Function

   ' Wraps an IEnumerator, allowing it to be used as an IEnumerable.
   Iterator Function AsEnumerable(enumerator As IEnumerator) As IEnumerable
       Do While enumerator.MoveNext()
           Yield enumerator.Current
       Loop
   End Function

   Sub Main()
       Const INDENT = "    "

If REDIRECTOUT Then

       Const OUTPATH = "out.txt"
       Using s = IO.File.Open(OUTPATH, IO.FileMode.Create),
             sw As New IO.StreamWriter(s)
           Console.SetOut(sw)

Else

       Try
           Console.OutputEncoding = Text.Encoding.ASCII
           Console.OutputEncoding = Text.Encoding.UTF8
           Console.OutputEncoding = Text.Encoding.Unicode
       Catch ex As Exception
           Console.WriteLine("Failed to set console encoding to Unicode." & vbLf)
       End Try

End If

           For Each c In TestCases
               Console.WriteLine(c)
               Console.WriteLine(INDENT & "SIMPLE:   " & Reverse(c))
               Console.WriteLine(INDENT & "ELEMENTS: " & ReverseElements(c))
               Console.WriteLine()
           Next

If REDIRECTOUT Then

       End Using

End If

   End Sub

End Module</lang>

Output (copied from Notepad):

Output is presented using non-fixed-width typeface to properly display combining characters.

asdf
    SIMPLE:   fdsa
    ELEMENTS: fdsa

as⃝df̅
    SIMPLE:   ̅fd⃝sa
    ELEMENTS: f̅ds⃝a

Les Misérables
    SIMPLE:   selbaŕesiM seL
    ELEMENTS: selbarésiM seL

Wart

Wart doesn't support Unicode yet.

Wren

Library: Wren-str

Library: Wren-upc

<lang ecmascript>import "/str" for Str import "/upc" for Graphemes

for (word in ["asdf", "josé", "møøse", "was it a car or a cat I saw", "😀🚂🦊"]) {

   System.print(Str.reverse(word))

}

for (word in ["as⃝df̅", "ℵΑΩ 駱駝道 🤔 🇸🇧 🇺🇸 🇬🇧‍ 👨‍👩‍👧‍👦🆗🗺"]) {

   System.print(Graphemes.new(word).toList[-1..0].join())

}</lang>

Output:

fdsa
ésoj
esøøm
was I tac a ro rac a ti saw
🦊🚂😀
f̅ds⃝a
🗺🆗👨‍👩‍👧‍👦 🇬🇧‍ 🇺🇸 🇸🇧 🤔 道駝駱 ΩΑℵ

Wortel

<lang wortel>; the @rev operator reverses strings and arrays @rev "abc" ; returns "cba"

or the same thing using a pointer expression

!~r "abc"</lang>

XBS

Using the standard library <lang XBS>log(string.reverse("Hello"))</lang>

Output:

olleH

Using JavaScript methods <lang XBS>log("Hello"->split("")->reverse()->join(""));</lang>

Output:

olleH

Using a custom method <lang XBS>func ReverseString(String){ set Final = ""; for(i=?String-1;0;-1){ Final+=String[i]; } send Final; } log(ReverseString("Hello"));</lang>

Output:

elloH

XPL0

<lang XPL0>include c:\cxpl\codes; \intrinsic 'code' declarations string 0; \use zero-terminated strings, instead of MSb terminated

func StrLen(Str); \Return the number of characters in an ASCIIZ string char Str; int I; for I:= 0 to -1>>1-1 do

       if Str(I) = 0 then return I;

func RevStr(S); \Reverse the order of the bytes in a string char S; int L, I, T; [L:= StrLen(S); for I:= 0 to L/2-1 do

       [T:= S(I);  S(I):= S(L-I-1);  S(L-I-1):= T];

return S; ];

[Text(0, RevStr("a")); CrLf(0);

Text(0, RevStr("ab")); CrLf(0);
Text(0, RevStr("abc")); CrLf(0);
Text(0, RevStr("Able was I ere I saw Elba.")); CrLf(0);

]</lang>

Output:

a
ba
cba
.ablE was I ere I saw elbA

Yorick

This only handles ASCII characters. It works by converting a string to an array of char; dropping the last character (which is the null byte); reversing the order of the characters; then converting back to a string. <lang yorick>strchar(strchar("asdf")(:-1)(::-1))</lang>

Z80 Assembly

This method uses the stack as a temporary store to reverse the order of characters in a string, and self-modifying code to restore the loop counter.

<lang z80>PrintChar equ $BB5A ;Amstrad CPC bios call Terminator equ 0 ;null terminator for strings

org $8000 ld hl, StringA call ReverseString

ld hl, StringA call PrintString

ret ;return to basic

StringA: byte "12345678",0

SUBROUTINES

GetStringLength:

HL = STRING. RETURNS LENGTH IN B. LENGTH IS ONE-INDEXED AND DOES NOT INCLUDE TERMINATOR.

ld b,0 ;clear B loop_getStringLength: ld a,(hl) ;read the next char cp Terminator ;is it the terminator? ret z ;if so, exit inc hl ;point HL to next character inc b ;increase tally jr loop_getStringLength ;repeat

ReverseString: ;reverse the order of letters in a text string. ;e.g. "ABCD" -> "DCBA" ;the terminator stays put. ;INPUT: HL = SOURCE ADDRESS OF STRING push de push hl push hl call GetStringLength pop hl pop de ;LD DE,HL LD a,b ;LOAD B INTO A LD (SMC_ReverseString+1),a ;STORE IT LATER IN THE CODE SO WE CAN RETRIEVE IT.

TO RECAP, BOTH HL AND DE POINT TO THE BEGINNING OF THE STRING WE WANT TO REVERSE. B EQUALS THE LENGTH OF THE STRING.
B HAS BEEN BACKED UP WITHOUT USING THE STACK BY STORING IT AS THE OPERAND OF A LATER INSTRUCTION THAT LOADS B WITH A NUMERIC VALUE.
PUSH BC WOULD NOT HAVE WORKED SINCE THE PROGRAM NEEDS TO PUSH EACH LETTER OF THE STRING DURING THE LOOP.

LOOP_REVERSESTRING_PUSH:

start at the beginning of the string and push each letter in it, except the terminator.

ld a,(de) push af inc de djnz LOOP_REVERSESTRING_PUSH

SMC_ReverseString: ld b,$42 ;LETS US PRESERVE B WITHOUT PUSHING IT. THE $42 IS OVERWRITTEN WITH THE STRING'S LENGTH.

LOOP_REVERSESTRING_POP:

Starting at the beginning of the string, pop A off the stack and store it into the string. This puts the letters back in the reverse
order.

pop af ld (hl),a inc hl djnz LOOP_REVERSESTRING_POP pop de ret</lang>

Output:

87654321

zkl

These only handle ASCII characters, no extra credit.

"this is a test".reverse()

Old school ways to do it:

Build by prepending characters, creates n strings:

"this is a test".reduce(fcn(p,c){c+p})

Convert to list, reverse, convert back to string:

"this is a test".split("").reverse().concat()

Write to a byte buffer and convert to string:

"this is a test".pump(Void,Data().insert.fp(0)).text

The ".fp(0)" creates a closure so each character is fed to data.insert(0,c). pump is a method that sends each character to a function to a sink (in this case /dev/null). The output is the result of the last call, which is data.insert which is self/data. https://en.wikipedia.org/wiki/Comparison_of_programming_languages_%28string_functions%29#reverseProperty "Wikipedia" (as page type) with input value "https://en.wikipedia.org/wiki/Comparison_of_programming_languages_%28string_functions%29#reverse" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process.

Zoea

<lang Zoea> program: reverse_string

 input: xyzzy
 output: yzzyx

</lang>

Zoea Visual

Reverse string

@@ Line 4,300: / Line 4,300: @@
 		LD (SMC_ReverseString+1),a  ;STORE IT LATER IN THE CODE SO WE CAN RETRIEVE IT.
+; TO RECAP, BOTH HL AND DE POINT TO THE BEGINNING OF THE STRING WE WANT TO REVERSE. B EQUALS THE LENGTH OF THE STRING.
+; B HAS BEEN BACKED UP WITHOUT USING THE STACK BY STORING IT AS THE OPERAND OF A LATER INSTRUCTION THAT LOADS B WITH A NUMERIC VALUE.
+; PUSH BC WOULD NOT HAVE WORKED SINCE THE PROGRAM NEEDS TO PUSH EACH LETTER OF THE STRING DURING THE LOOP.
 LOOP_REVERSESTRING_PUSH:
 ;start at the beginning of the string and push each letter in it, except the terminator.
@@ Line 4,324: / Line 4,328: @@
 87654321
 </pre>
 =={{header|zkl}}==