How to push an array into another array?-CodePudding

How do we correctly push array into array and then retrieve each of the outer array (explain the both asked each separated clearly of other)?

illustrated

my @f; my @e;
for $i (0..40) {
    @e=($i =2, $i 1);
    push(@f,@e); # just it right ? 
}
    
# how go on get it under multi array control

CodePudding user response：

Data::Dumper is a good tool to explore what is going on here. It makes it easy to visualise your data structures.

If we run your code and then display @f using Data::Dumper, we get this:

$VAR1 = [
          2,
          3,
          3,
          4,
          4,
          5,
          5,
          6,
          6,
[ ... snip ... ]
          40,
          40,
          41,
          41,
          42,
          42,
          43
        ];

So that's not doing what you want. If you push an array onto another array then Perl just adds each element from the second array to the end of the first array. It becomes impossible to tell which element belongs to which array. This is known as "array flattening".

It happens because an element in an array can only hold a single scalar value. And another array is not a scalar value. But we can take a reference to an array and that is then a scalar value. You get a reference to an array using \.

The change to your code is simple:

my @e=($i =2, $i 1); # declare @e inside the loop
push(@f,\@e); # Take a reference

And the output we now get is:

$VAR1 = [
          [
            2,
            3
          ],
          [
            3,
            4
          ],
          [
            4,
            5
          ],
[ ... snip ... ]
          [
            39,
            40
          ],
          [
            40,
            41
          ],
          [
            41,
            42
          ],
          [
            42,
            43
          ]
        ];

You can clearly see the individual "sub-arrays" inside your main array.

There's more about this in the perllol manual page and you can find out far more about references in perlreftut and perlref.

CodePudding user response：

Do you mean something like this?

#! /usr/bin/env perl

use warnings;
use strict;
use utf8;
use feature qw<say>;
use Data::Dumper;

my @f = ();

for my $i (0..4) {
    my @e = ($i =2, $i 1);
    push(@f,\@e);
}

say Dumper(\@f);

exit(0);