Modify subroutine parameter (Perl)

Question

I want to write a Perl subroutine first which takes a list as an input, returns the first element in the list and removes the first element from the list.

Like this:

@list = (1,2,3);
print first(@list); // 1
print @list; // 23

This is not quite working:

sub first(@) {
    return shift @_;
}

What I get is:

print first(@list); // 1
print @list; // 123

The stack variable @_ changes the way I expect it to (first it is (1, 2, 3) then it is (2, 3)), but the list I give as an input (@list) is not changed. I thought the stack variable saves a reference to the variable it refers to.

When I change a list element in the subroutine it also changes something in @list but not the one I wanted to, but that one + 1. So if I in the subroutine I were to write:

@_[0] = "X";

and after executing the subroutine print @list, I would get 2X6.

But why would you like to write the subroutine? When you could simply use shift? — Håkon Hægland
– Håkon Hægland, Commented Jan 6, 2015 at 10:32

Håkon Hægland · Accepted Answer · 2015-01-06 10:55:15Z

2

You need to put a slash in front of the @ prototype to get an array reference, and then modify the reference. If you just use @ you will get a copy of the array @list in the subroutine (and the array in the parent would therefore not be modified). From http://perldoc.perl.org/perlsub.html#Prototypes:

Unbackslashed prototype characters have special meanings. Any unbackslashed @ or % eats all remaining arguments, and forces list context.

So you could write:

use strict;
use warnings;

sub first (\@) {
    my $a = shift;
    return shift @$a;
}

my @list = (1,2,3);
print first(@list) . "\n"; 
print "@list" . "\n";

Output:

1
2 3

edited Jan 6, 2015 at 10:55

answered Jan 6, 2015 at 10:34

Håkon Hægland

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Comments

Dave Cross · Accepted Answer · 2015-01-06 11:57:32Z

1

You don't have lists there, you have arrays. Arrays and lists are different in Perl (as this great blog post explains). If you have an array called @list then you're just guaranteed to confuse yourself (and whoever maintains your code in the future).

answered Jan 6, 2015 at 11:57

Dave Cross

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