(CLPM) reorder via swap

Geez. This isn't supposed to be obfuscation, but it ended up that way, in the sense that the algorithm is a bit difficult to extract. The swapping is the easy part... those two parallel arrays are not.

# reorder an array using only swap(ARRAY, X, Y),
# given a new set of indices

my @array = qw( put these in a different order );
my @order = qw( 1 3 5 4 2 0 );
reorder_swap(\@array, \@order);

# expected outcome:
# qw( these a order different in put )

sub reorder_swap {
  my ($a, $o) = @_;
  my @P = my @R = 0 .. $#$o;

  for (0 .. $#$o) {
    next if $P[$$o[$_]] == $_;
    swap($a, $P[$$o[$_]], $_);
    (@P[$R[$_],$$o[$_]], @R[$P[$$o[$_]],$_]) =
    (@P[$$o[$_],$R[$_]], @R[$_,$P[$$o[$_]]]);
  }
}

sub swap {
  my ($a, $x, $y) = @_;
  @$a[$x,$y] = @$a[$y,$x];
}
[download]

For simplicity's sake, here's the for loop with a temporary variable instead:

  for (0 .. $#$o) {
    my $idx = $$o[$_];
    next if $P[$idx] == $_;
    swap($a, $P[$idx], $_);
    (@P[$R[$_],$idx], @R[$P[$idx],$_]) =
    (@P[$idx,$R[$_]], @R[$_,$P[$idx]]);
  }
[download]

_____________________________________________________
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

Comment on (CLPM) reorder via swap Select or Download Code

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Re: (CLPM) reorder via swap by o0lit3 (Friar) on Aug 17, 2004 at 18:53 UTC
Without using swap, we have a slightly less obfuscated and (uninteresting) way... `@array = qw(put these in a different order); @order = qw(1 3 5 4 2 0); print reorder(\@array, \@order); sub reorder() { #23456789012345678901234 map${@_[0]}[$_],@{$_[1]} }` [download] Uninteresting as it may be, it might make for a decent golf... Can this be done in fewer strokes? What if the input were two strings on <STDIN>, separated by a line feed? We then might have something like: `#!/perl -ln0a map$F[$_],@F[@F/2..$#F]` [download]	[reply] [d/l] [select]
Re^2: (CLPM) reorder via swap by japhy (Canon) on Aug 17, 2004 at 21:09 UTC
Assuming we want to return a list in the ordered fashion: `sub reorder { #234567890123456 @{+pop}[@{+pop}] }` [download] I'm taking an array slice instead of using `map()`. And we can get even shorter if we call it as `reorder(@elements, \@neworder)`: `sub reorder { #2345678901 @_[@{+pop}] }` [download] _____________________________________________________ Jeff `japhy` Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and `perl` hacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart	[reply] [d/l] [select]
Re^3: (CLPM) reorder via swap by o0lit3 (Friar) on Aug 18, 2004 at 14:54 UTC
Beautiful and elegant... it's simplicity like this that always reminds me why I love this language. But please help me out a little. What is the `+` operator doing here? I know that `@{+pop}` is somehow saying the same as `@{pop()}`, but I'm not sure why. ...And while I'm on the subject, why am I not allowed to simply say `@{pop}` (besides the fact that I've used a split infinitve)?	[reply] [d/l] [select]
Re^4: (CLPM) reorder via swap by japhy (Canon) on Aug 18, 2004 at 14:58 UTC


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