How to manipulate array reference in subroutine in perl

Question

Im passing array by reference to a subroutine:

my @template_ops;
handle_templates_with_post_mod(\@template_ops);

sub handle_templates_with_post_mod
{
    my @my_template_ops    = @{$_[0]};
    unshift (@my_template_ops), 'O';
}

but after calling the subroutine the array still empty, I have tried:

sub handle_templates_with_post_mod
{
    my $my_template_ops    = shift;
    unshift (@$my_template_ops), 'O';
}

but I get this error: Can't use string ("...") as an ARRAY ref while "strict refs"

tobyink · Accepted Answer · 2017-08-08 07:54:47Z

Four issues:

ALWAYS turn on use strict; use warnings;
Then unshift (@$my_template_ops), 'O'; would have produced Useless use of unshift with no values at ./t.pl line 17.. You have set the parens () wrong. Either write unshift (@$my_template_ops, 'O'); or just leave them off completely: unshift @$my_template_ops, 'O';
The code my @my_template_ops = @{$_[0]}; generates a copy of the supplied array reference and you are manipulating the copy. Thus you don't see the effect at top level.
The 2nd version of your function works because you operate on the supplied array ref and just dereference it for the unshift call.

Working version:

use strict;
use warnings;
use Data::Dumper;

my @template_ops;
handle_templates_with_post_mod(\@template_ops);

print Data::Dumper::Dumper( \@template_ops );

sub handle_templates_with_post_mod
{
    my $my_template_ops    = shift;
    unshift @$my_template_ops, 'O';
}

Output:

$VAR1 = [
          'O'
        ];

piojo · Accepted Answer · 2017-08-07 10:26:47Z

2

When you give the dereferenced array a name, it becomes a copy. To operate on an array ref and affect the original array, you either need to use it as a ref, or dereference it without giving it a new name (using the @ operator to dereference).

my $arrayRef = ...;
push @{$arrayRef}, 42;
my $hashRef = ...;
delete $hashRef->{key};

edited Aug 7, 2017 at 10:26

answered Aug 7, 2017 at 10:12

piojo

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1 Comment

piojo Over a year ago

@PerlDuck Thanks. I changed it to a hash ref, since deleting hash elements is safe and still illustrates the syntax.

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