Modifying array in subroutine

Question

I've got some code that doesn't work the way I expect, and I'd appreciate some help figuring out how to get it working the way I DO expect.

I'd like to use a subroutine to modify an input array. I figured that if I passed in a reference to the array, any changes I made to it would be reflected in the caller's version, too. But it apparently doesn't work that way.

my @test_array = qw (zero one two three);

shift_array(\@test_array);
print "POST SUBROUTINE: $test_array[0]\n";

sub shift_array {
    my @array = @{(shift)};
    shift @array;
    print "AFTER SHIFT IN SUB: $array[0]\n";
}

This prints:

AFTER SHIFT IN SUB: one
POST SUBROUTINE: zero

I expected it to print one both times.

So my question is two-fold:

1) Why isn't it behaving the way I think it should? Does passing a reference to an array create a copy of the array?

2) How do I get the behavior I WAS expecting? How do I I get a subroutine to slide one or more elements off of the front of caller's copy of an input array?

Thanks in advance for any insight you can offer.

Answer 1

Because my @array is a new array that you assigned the dereferenced original array to. It's a copy.

Instead do this:

sub shift_array {
    my $array = shift;
    shift( @$array );
}

Edit: I originally said dereferencing it creates the problem but using @$array is still dereferencing it. As @mob pointed out, assigning the variable is what creates the new array instead of updating the existing reference.

Answer 2

This is one of the (tiny number) of cases where Perl prototypes are useful.

#!/usr/bin/perl

use strict;
use warnings;

# Prototype forces the first argument to be a reference to an array    
sub shift_array (\@) {
  my $array = shift;
  shift(@$array);
  print "AFTER SHIFT IN SUB: @$array[0]\n";
}

my @test_array = qw (zero one two three);

shift_array(@test_array);
print "POST SUBROUTINE: $test_array[0]\n";