Re: Perl program to generate a descending order of 9-unique digit numbers

[Anonymous Monk]

This is, of course, doable with regex (what isn't?)

#!/usr/bin/perl
use strict; use warnings;
$\ = "\n";
$_ = 9876543210;
my $n = 0; # to limit output

$n++ < 12 or exit while print,
  s/.*\K (.) (?=(.)(??{$1 < $2 and 'x'})
  (.*) (.)(??{$1 < $4 and 'x'}) (.*)
  / $4 . reverse $3.$1.$5 /xe

[Anonymous Monk]

Cleaned up and annotated. It's a perl version of https://en.m.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order

#!/usr/bin/perl -l
use strict; use warnings;
$_ = 321; # for testing, change to 987654321 for final run

1 while print,
  s/.*\K # find the last
  (.) # digit such that
  (.*) # there is a later (latest)
  (.)(??{$1 < $3 and 'x'}) # digit less than it
  (.*) # and get rest
  # swap those two digits ( $1 & $3 )
  # then reverse everything after the first swapped digit
  / $3 . reverse $2.$1.$4 /xe

[Anonymous Monk]

$_ = 987654321; # oops, 9 unique digits