Loops/Wrong ranges: Difference between revisions

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-> $start, $stop, $inc {
-> $start, $stop, $inc {
my $seq = flat ($start, *+$inc … $stop);
my $seq = flat ($start, *+$inc … $stop);
printf "Start: %3s, Stop: %3s, Increment: %3s | ", $start, $stop.Str, $inc;
printf "Start: %3s, Stop: %3s, Increment: %3s | ", $start, $stop.Str, $inc;
# only show up to the first 15 elements of possibly infinite sequences
# only show up to the first 15 elements of possibly infinite sequences
put $seq[15].defined ?? $seq[^15].grep: +*.so !! $seq
put $seq[^15].grep: +*.so
}</lang>
}</lang>
{{out}}
{{out}}

Revision as of 13:35, 16 September 2018

Loops/Wrong ranges is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Some languages have syntax or function(s) to generate a range of numeric values from a start value, a stop value, and an increment.

The purpose of this task is to select the range syntax/function that would generate at least two increasing numbers when given a stop value more than the start value and a positive increment of less than half the difference. You are than to use that same syntax/function but with different parameters; and show, here, what would happen.

Use these values if possible:

start stop increment Comment
-2 2 1 Normal
-2 2 0 Zero increment
-2 2 -1 Increments away from stop value
-2 2 10 First increment is beyond stop value
2 -2 1 Start more than stop: positive increment
2 2 1 Start equal stop: positive increment
2 2 -1 Start equal stop: negative increment
2 2 0 Start equal stop: zero increment
0 0 0 Start equal stop equal zero: zero increment

Perl 6

Works with: Rakudo version 2018.08

It would be odd to call ANY of these sequences "wrong" in Perl 6. Perl 6 specifically has built in capability of working with infinite sequences. Just because a sequence is infinite, doesn't mean you can't define it, work with it or use values from it. Sure, if you try to reify the whole thing you may be waiting a while, but there is nothing preventing you from using a portion of it.

Perl 6 sequence definitions specifically allow "ending points" that may never occur in the sequence. Since that is the case, you don't even really need to specify a stop value. You can just say stop at "whatever". Whatever is spelled "*" in Perl 6.

There is additional syntax you can add to stop at the nearest value, last value previous or first value successor to the "stop value" (Note I didn't say less than or greater than the stop value since the sequence can be ascending, descending or non-monotonic).

Also note: The iterator function for the sequence is literally a function. It is any expression that produces a value. These sequences all use simple arithmatic increments but that is not a limitation of the sequence operator.


<lang perl6># Given sequence definitions

  1. start stop inc. Comment

for -2, 2, 1, # Normal 

     -2,    2,    0, # Zero increment
     -2,    2,   -1, # Increments away from stop value
     -2,    2,   10, # First increment is beyond stop value
      2,   -2,    1, # Start more than stop: positive increment
      2,    2,    1, # Start equal stop: positive increment
      2,    2,    0, # Start equal stop: zero increment
      0,    0,    0, # Start equal stop equal zero: zero increment
  1. Additional "problematic" sequences
      1,  Inf,    3, # Endpoint literally at infinity
      1,    2,    0, # Increment will never reach endpoint
      0,    π,  τ/8, # Floating point numbers
    1.4,    *, -7.1  # Whatever

 -> $start, $stop, $inc {
   my $seq = flat ($start, *+$inc … $stop);
   printf "Start: %3s, Stop: %3s, Increment: %3s | ", $start, $stop.Str, $inc;
   # only show up to the first 15 elements of possibly infinite sequences
   put $seq[^15].grep: +*.so

}</lang>

Output:
Start:  -2, Stop:   2, Increment:   1 | -2 -1 0 1 2
Start:  -2, Stop:   2, Increment:   0 | -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2
Start:  -2, Stop:   2, Increment:  -1 | -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16
Start:  -2, Stop:   2, Increment:  10 | -2 8 18 28 38 48 58 68 78 88 98 108 118 128 138
Start:   2, Stop:  -2, Increment:   1 | 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Start:   2, Stop:   2, Increment:   1 | 2
Start:   2, Stop:   2, Increment:   0 | 2
Start:   0, Stop:   0, Increment:   0 | 0
Start:   1, Stop: Inf, Increment:   3 | 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43
Start:   1, Stop:   2, Increment:   0 | 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Start:   0, Stop: 3.141592653589793, Increment: 0.7853981633974483 | 0 0.7853981633974483 1.5707963267948966 2.356194490192345 3.141592653589793
Start: 1.4, Stop:   *, Increment: -7.1 | 1.4 -5.7 -12.8 -19.9 -27 -34.1 -41.2 -48.3 -55.4 -62.5 -69.6 -76.7 -83.8 -90.9 -98

Python

Python has the range function. <lang python>import re from itertools import islice # To limit execution if it would generate huge values

  1. list(islice('ABCDEFG', 2)) --> ['A', 'B']
  2. list(islice('ABCDEFG', 4)) --> ['A', 'B', 'C', 'D']


data = start stop increment Comment -2 2 1 Normal -2 2 0 Zero increment -2 2 -1 Increments away from stop value -2 2 10 First increment is beyond stop value 2 -2 1 Start more than stop: positive increment 2 2 1 Start equal stop: positive increment 2 2 -1 Start equal stop: negative increment 2 2 0 Start equal stop: zero increment 0 0 0 Start equal stop equal zero: zero increment

table = [re.split(r'\s\s+', line.strip()) for line in data.strip().split('\n')]

  1. %%

for _start, _stop, _increment, comment in table[1:]: 

   start, stop, increment = [int(x) for x in (_start, _stop, _increment)]
   print(f'{comment.upper()}:\n  range({start}, {stop}, {increment})')
   error, values = None, None
   try: 
       values = list(islice(range(start, stop, increment), 999))
   except ValueError as e:
       error = e
       print('  !!ERROR!!', e)
   if values is not None:
       if len(values) < 22:
           print('    =', values)
       else:
           print('    =', str(values[:22])[:-1], '...')

</lang>

Output:
NORMAL:
  range(-2, 2, 1)
    = [-2, -1, 0, 1]
ZERO INCREMENT:
  range(-2, 2, 0)
  !!ERROR!! range() arg 3 must not be zero
INCREMENTS AWAY FROM STOP VALUE:
  range(-2, 2, -1)
    = []
FIRST INCREMENT IS BEYOND STOP VALUE:
  range(-2, 2, 10)
    = [-2]
START MORE THAN STOP: POSITIVE INCREMENT:
  range(2, -2, 1)
    = []
START EQUAL STOP: POSITIVE INCREMENT:
  range(2, 2, 1)
    = []
START EQUAL STOP: NEGATIVE INCREMENT:
  range(2, 2, -1)
    = []
START EQUAL STOP: ZERO INCREMENT:
  range(2, 2, 0)
  !!ERROR!! range() arg 3 must not be zero
START EQUAL STOP EQUAL ZERO: ZERO INCREMENT:
  range(0, 0, 0)
  !!ERROR!! range() arg 3 must not be zero

REXX

Note that a do loop with zero by value, or a do loop that goes in the "wrong" direction is not considered an error in REXX as there are other methods of limiting the range (or stopping condition) within the loop body.   A special check was made in this REXX version to check for a runaway (race) condition. <lang rexx>/*REXX program demonstrates several versions of DO loops with "unusual" interations. */ @.=; @.1= ' -2 2 1 ' /*"normal". */ 

         @.2=  '  -2      2       0  '      /*"normal",                zero  increment.*/
         @.3=  '  -2      2      -1  '      /*increases away from stop, neg  increment.*/
         @.4=  '  -2      2      10  '      /*1st increment > stop, positive increment.*/
         @.5=  '   2     -2       1  '      /*start > stop,         positive increment.*/
         @.6=  '   2      2       1  '      /*start equals stop,    positive increment.*/
         @.7=  '   2      2      -1  '      /*start equals stop,    negative increment.*/
         @.8=  '   2      2       0  '      /*start equals stop,       zero  increment.*/
         @.9=  '   0      0       0  '      /*start equals stop,       zero  increment.*/

zLim= 10 /*a limit to check for runaway (race) loop.*/ 

                                            /*a zero increment is not an error in REXX.*/
 do k=1  while  @.k\==                    /*perform a  DO  loop with several ranges. */
 parse var   @.k    x  y  z  .              /*obtain the three values for a DO loop.   */
 say
 say center('start of performing DO loop number '   k   " with range: "  x y z,  79, '═')
 zz= 0
       do  j=x   to y   by z   until zz>=zLim           /* ◄───  perform the  DO  loop.*/
       say '   j ───►'  right(j, max(3, length(j) ) )   /*right justify J for alignment*/
       if z==0  then zz= zz + 1                         /*if zero inc, count happenings*/
       end   /*j*/

 if zz>=zLim  then say 'the DO loop for the '    k    " entry was terminated (runaway)."
 say center(' end  of performing DO loop number '   k   " with range: "  x y z,  79, '─')
 say
 end         /*k*/                              /*stick a fork in it,  we're all done. */</lang>
output:
══════════start of performing DO loop number  1  with range:  -2 2 1═══════════
   j ───►  -2
   j ───►  -1
   j ───►   0
   j ───►   1
   j ───►   2
────────── end  of performing DO loop number  1  with range:  -2 2 1───────────


══════════start of performing DO loop number  2  with range:  -2 2 0═══════════
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
   j ───►  -2
the DO loop for the  2  entry was terminated (runaway).
────────── end  of performing DO loop number  2  with range:  -2 2 0───────────


══════════start of performing DO loop number  3  with range:  -2 2 -1══════════
────────── end  of performing DO loop number  3  with range:  -2 2 -1──────────


══════════start of performing DO loop number  4  with range:  -2 2 10══════════
   j ───►  -2
────────── end  of performing DO loop number  4  with range:  -2 2 10──────────


══════════start of performing DO loop number  5  with range:  2 -2 1═══════════
────────── end  of performing DO loop number  5  with range:  2 -2 1───────────


═══════════start of performing DO loop number  6  with range:  2 2 1═══════════
   j ───►   2
─────────── end  of performing DO loop number  6  with range:  2 2 1───────────


══════════start of performing DO loop number  7  with range:  2 2 -1═══════════
   j ───►   2
────────── end  of performing DO loop number  7  with range:  2 2 -1───────────


═══════════start of performing DO loop number  8  with range:  2 2 0═══════════
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
   j ───►   2
the DO loop for the  8  entry was terminated (runaway).
─────────── end  of performing DO loop number  8  with range:  2 2 0───────────


═══════════start of performing DO loop number  9  with range:  0 0 0═══════════
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
   j ───►   0
the DO loop for the  9  entry was terminated (runaway).
─────────── end  of performing DO loop number  9  with range:  0 0 0───────────
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