Re: Processing pairs of values from even-sized array

[ciderpunx]

I'd normally use a slightly simpler version of JohnGG's splice approach. Something like

my @arr = qw/1 2 3 4 5 6 7 8/;
while(my ($x,$y) = splice @arr,0,2) {
  say "$x => $y";
}

Something I've needed in the past was to get two arrays (odds and evens). Grep can do this. You could combine them into a hash thus:

my @arr = qw/1 2 3 4 5 6 7 8/;

my ($j,$i)=(0,0);
my ($x,$y) = ([grep { ++$i % 2} @arr] , [grep {$j++ % 2} @arr]);
my %hash;
@hash{@{$x}}=@{$y};

for(keys %hash) {
  say "$_ => $hash{$_}"
}

Gives us:

1 => 2
3 => 4
7 => 8
5 => 6

Or you could iterate them (which is safer if some x may be undef)

my @arr = qw/1 2 3 4 5 6 7 8/;
push @arr, undef;
push @arr, 9;


my ($j,$i)=(0,0);
my ($x,$y) = ([grep { ++$i % 2} @arr] , [grep {$j++ % 2} @arr]);
$i=0;
for(@{$x}){
  say $_ ." => " . $y->[$i++] if($_);
}

Obviously that doesn't scale to big arrays. I'm sure List::MoreUtils is always the better approach.

[johngg]

Something I've needed in the past was to get two arrays (odds and evens).

You can also push onto two arrays using a ternary.

knoppix@Microknoppix:~$ perl -E '
> push @{ $_ % 2 ? \ @odds : \ @evens }, $_ for 1 .. 8;
> say qq{ Odds : @odds};
> say qq{Evens : @evens};'
 Odds : 1 3 5 7
Evens : 2 4 6 8
knoppix@Microknoppix:~$

I hope this is of interest.

[AnomalousMonk]

List::MoreUtils also has  part to do similar things:

>perl -wMstrict -le
"use List::MoreUtils qw(part);
 use Data::Dumper;
 ;;
 my $i;
 my @parts = part { ++$i % 2 } 1 .. 8;
 ;;
 print Dumper \@parts;
"
$VAR1 = [
          [
            2,
            4,
            6,
            8
          ],
          [
            1,
            3,
            5,
            7
          ]
        ];

[johngg]

Thank you for pointing that out, I rarely come across List::MoreUtils as it is not a core module so I wasn't aware of part. Very useful if you can work with the resulting AoA but it seems a bit awkward if you really do need separate arrays.

knoppix@Microknoppix:~$ perl -MList::MoreUtils=part -E '
> @parts = part {
>    ++ $i;
>    $i % 5
>    ? $i % 2
>      ? 0
>      : 1
>    : 2
>    } 1 .. 20;
> @odds = @{ $parts[ 0 ] };
> @evens = @{ $parts[ 1 ] };
> @fivers = @{ $parts[ 2 ] };
> say qq{@odds};
> say qq{@evens};
> say qq{@fivers};'
1 3 7 9 11 13 17 19
2 4 6 8 12 14 16 18
5 10 15 20
knoppix@Microknoppix:~$

knoppix@Microknoppix:~$ perl -E '
> push @{
>    $_ % 5
>    ? $_ % 2
>      ? \ @odds
>      : \ @evens
>    : \ @fivers
>    }, $_ for 1 .. 20;
> say qq{@odds};
> say qq{@evens};
> say qq{@fivers};'
1 3 7 9 11 13 17 19
2 4 6 8 12 14 16 18
5 10 15 20
knoppix@Microknoppix:~$

Perhaps I'm missing something. I have tried pushing references to @odds etc. onto @parts before using part without success so I can't seem get away from having to do @odds = @{ $parts[ 0 ] }; etc.