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How Perl can push array into array and then how retrieve

by Anonymous Monk
on Nov 25, 2021 at 03:59 UTC ( #11139097=perlquestion: print w/replies, xml ) Need Help??

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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

Please help out, very thanked in advance

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Re: How Perl can push array into array and then how retrieve
by kcott (Bishop) on Nov 25, 2021 at 04:30 UTC

    I'm guessing somewhat regarding what you want to achieve.

    Always put strict and warnings at the top of your code. In this instance, that would have alerted you to the fact that there was something wrong with $f.

    I was unsure why you wanted to increment $i inside the loop. In my version, I changed $i+=2 to $i+2.

    Here's a couple of ways of doing what I think you want:

    #!/usr/bin/env perl use strict; use warnings; my @f; # Modification of your code for my $i (0..2) { my @e = ($i+2, $i+1); push @f, [@e]; } # A more succinct way to achieve it for my $i (3..5) { push @f, [$i+2, $i+1]; } # For demo purposes use Data::Dump; dd \@f;

    Output:

    [[2, 1], [3, 2], [4, 3], [5, 4], [6, 5], [7, 6]]

    See also: "perlintro: Arrays"; "perldsc - Perl Data Structures Cookbook"; "perllol - Manipulating Arrays of Arrays in Perl".

    — Ken

      Just as an afterthought, an even more succinct way is

      ... my @f; push @f, [$_+2, $_+1] for 0 .. 5; ...

      which produces identical output.

      — Ken

        Or:

        ... my @f = map [$_+2, $_+1], 0 .. 5; ...
Re: How Perl can push array into array and then how retrieve
by Marshall (Canon) on Nov 25, 2021 at 07:53 UTC
    You understand what an array is. If you push an array onto another array, you just wind up with a single bigger single dimensional array. A multi dimensional array is an array of references to array. Push a reference to the @e array onto the @f array. push @f,\@e; I suppose you could write, push @f,[@e], but that would expand @e into an anonymous array and then make a reference to that newly created anon array.

    BTW, there can be huge problems if you don't use "my" variables. "my @e" creates a brand new @e array every time this is seen. The push, pushes a reference to the "current @e array". When the loop comes around again, a whole new different @e array is created.

    use strict; use warnings; use Data::Dump qw(pp); my @f; for my $i (0..40) { my @e=($i+=2, $i+1); #same as ($i+2, $i+3) push(@f,\@e); } pp \@f; __END__ Not sure if this is what you want, but this code results in: [ [2, 3], [3, 4], [4, 5], [5, 6], [6, 7], [7, 8], [8, 9], [9, 10], [10, 11], [11, 12], [12, 13], [13, 14], [14, 15], [15, 16], [16, 17], [17, 18], [18, 19], [19, 20], [20, 21], [21, 22], [22, 23], [23, 24], [24, 25], [25, 26], [26, 27], [27, 28], [28, 29], [29, 30], [30, 31], [31, 32], [32, 33], [33, 34], [34, 35], [35, 36], [36, 37], [37, 38], [38, 39], [39, 40], [40, 41], [41, 42], [42, 43], ]
    Update: Now that I think about it, $i+=2 is an extremely bad idea. $i is the loop variable. You are asking for big trouble if you attempt to modify the loop variable while you are within the for loop. It does work here, but in general, I would avoid it and go with my ($i+2, $i+3) formulation.
      "Now that I think about it, $i+=2 ..."
      I think OP was meaning to have step in Perl for loop.
        There is more than one way to do it.

        You can compute calculate the numbers from a simple sequence:

        my @f; for my $i (0 .. 20) { my @e = (2 * $i, 2 * $i + 1); push @f, \@e; } print map "(@$_)", @f
        or you can use the C-style loop to skip over the unwanted numbers:
        for (my $i = 0; $i <= 40; $i += 2) { my @e = ($i, $i + 1); push @f, \@e; }
        or you can use grep to filter the numbers you want:
        for my $i (grep 0 == $_ % 2, 0 .. 40) { my @e = ($i, $i + 1); push @f, \@e; }
        etc.

        map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

        here is another way to have a step in this kind of loop:

        for my $i (map { $_ * 2 } 0..40/2){ ... }
Re: How Perl can push array into array and then how retrieve
by LanX (Sage) on Nov 25, 2021 at 08:28 UTC
    If you know any other language better, please tell. So I can explain in analogy.

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery

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