In Perl, how can I iterate over multiple elements of an array?

Question

I have a CSV file that I use split to parse into an array of N items, where N is a multiple of 3.

Is there a way i can do this

foreach my ( $a, $b, $c ) ( @d ) {}

similar to Python?

Answer 1

@z=(1,2,3,4,5,6,7,8,9,0);

for( @tuple=splice(@z,0,3); @tuple; @tuple=splice(@z,0,3) ) 
{ 
  print "$tuple[0] $tuple[1] $tuple[2]\n"; 
}

produces:

1 2 3
4 5 6
7 8 9
0

Answer 2

Not easily. You'd be better off making @d an array of three-element tuples, by pushing the elements onto the array as an array reference:

foreach my $line (<>)
    push @d, [ split /,/, $line ];

(Except that you really ought to use one of the CSV modules from CPAN.

Answer 3

You can use List::MoreUtils::natatime. From the docs:

my @x = ('a' .. 'g');
my $it = natatime 3, @x;
while (my @vals = $it->()) {
    print "@vals\n";
}

natatime is implemented in XS so you should prefer it for efficiency. Just for illustration purposes, here is how one might implement a three element iterator generator in Perl:

#!/usr/bin/perl

use strict; use warnings;

my @v = ('a' .. 'z' );

my $it = make_3it(\@v);

while ( my @tuple = $it->() ) {
    print "@tuple\n";
}

sub make_3it {
    my ($arr) = @_;
    {
        my $lower = 0;
        return sub {
            return unless $lower < @$arr;
            my $upper = $lower + 2;
            @$arr > $upper or $upper = $#$arr;
            my @ret = @$arr[$lower .. $upper];
            $lower = $upper + 1;
            return @ret;
        }
    }
}

Answer 4

I addressed this issue in my module List::Gen on CPAN.

use List::Gen qw/by/;

for my $items (by 3 => @list) {

    # do something with @$items which will contain 3 element slices of @list

    # unlike natatime or other common solutions, the elements in @$items are
    # aliased to @list, just like in a normal foreach loop

}

You could also import the mapn function, which is used by List::Gen to implement by:

use List::Gen qw/mapn/;

mapn {

   # do something with the slices in @_

} 3 => @list;

Answer 5

my @list = (qw(one two three four five six seven eight nine));

while (my ($m, $n, $o) = splice (@list,0,3)) {
  print "$m $n $o\n";
}

this outputs:

one two three
four five six
seven eight nine