Re^3: Switch the odd/even elements of an array

Expanding the solution to allow for reversing groups of multiple elements.

use strict;
use warnings;

use feature qw{ say };
use List::Util qw{ min };

my @arr = ( 1 .. 9 );
say qq{@arr};

my @twos = reverseGroupsOf( 2, @arr );
say qq{@twos};

my @threes = reverseGroupsOf( 3, @arr );
say qq{@threes};

my @fours = reverseGroupsOf( 4, @arr );
say qq{@fours};

sub reverseGroupsOf
{
    my $groupsOf = shift;
    my $rcReverse;
    $rcReverse = sub
       {
           reverse( map shift, 1 .. min scalar( @_, $groupsOf ),
           @_ ? &$rcReverse : ();
       };
    &$rcReverse;
}
[download]

1 2 3 4 5 6 7 8 9
2 1 4 3 6 5 8 7 9
3 2 1 6 5 4 9 8 7
4 3 2 1 8 7 6 5 9
[download]

I hope this is of interest.

Cheers,

JohnGG

Comment on Re^3: Switch the odd/even elements of an array Select or Download Code

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Re^4: Switch the odd/even elements of an array by wind (Priest) on Mar 06, 2011 at 05:00 UTC
I tend to like the mathematical solutions instead. The following would let you reverse any N elements at a time in an array. `my @foo = (1..15); my $n = 5; my @bar = @foo[map {$_ + $n - 1 - 2 * ($_ % $n)} (0..$#foo)]; print "@bar";` [download] Outputs `5 4 3 2 1 10 9 8 7 6 15 14 13 12 11` [download] Obviously not as spiffy as using xor to switch odds and evens, but still pretty concise. Alternatively, if you want to do a rotation of the elements instead of a reversal, you can use the following translation: `my @foo = (1..15); my $n = 5; my $i = 2; my @bar = @foo[map {my $r = $_ % $n; $_ - $r + (($r + $i) % $n)} (0..$ +#foo)]; print "@bar";` [download] Outputs `3 4 5 1 2 8 9 10 6 7 13 14 15 11 12` [download] - Miller	[reply] [d/l] [select]
Re^5: Switch the odd/even elements of an array by johngg (Canon) on Mar 06, 2011 at 17:37 UTC
I've always steered clear of using prototypes but I thought I'd have a go with them to see if I could come up with a general routine a bit like `List::MoreUtils::natatime()` but taking a code block rather than returning an iterator. This is what I came up with, putting a prototype declaration at the top of the script and fleshing out the subroutine later. `... sub groupsOf (&$@); ... sub groupsOf (&$@) { my $rcToRun = shift; my $groupsOf = shift; my $rcDoIt; $rcDoIt = sub { $rcToRun->( map shift, 1 .. min scalar( @_ ), $groupsOf ), @_ ? &$rcDoIt : (); }; &$rcDoIt; } ...` [download] It seems to do the right thing. With regard to rotating the elements of an array, I would have thought the `push @array, shift @array for 1 .. $n;` idiom was easier to understand but others may disagree. Here is `sub groupsOf()` in action. Read more... (1223 Bytes) The output. Read more... (551 Bytes) I hope this is of interest. Cheers, JohnGG	[reply] [d/l] [select]