Lists and Arguments in Perl

Lists Are One Dimensional

Recall that all lists and all arrays are one dimensional. If we have this list:

(@a, @b)

it becomes a one-dimensional list containing the contents of@afollowed by the contents of@b. This is an important rule when it comes to passing arrays into functions, since they will be passed in as a one-dimensional list. This is illustrated in the following example:

#!/usr/bin/perl -w
# passarrays.pl

use strict;

my(@nums1, @nums2);
@nums1 = (2, 4, 6);
@nums2 = (8, 10, 12);

process_arrays(@nums1, @nums2);

sub process_arrays {
my(@a, @b) = @_;

print "contents of \@a\n";
print "[$_] " foreach @a;
print "\n\n";

print "contents of \@b\n";
print "[$_] " foreach @b;
print "\n";
}

This program creates two 3-element arrays,@nums1and@nums2. These arrays are then passed intoprocess_arrays()and are immediately copied into two arrays,@aand@b. We might think that@areceives the contents of@nums1and@breceives the contents of@nums2, but that is not what happens. Since the arguments are passed in as

process_arrays(@nums1, @nums2);

the elements are flattened into this one-dimensional list:

(2, 4, 6, 8, 10, 12)

and this list is passed in and assigned to the assignable list:

my(@a, @b) = @_;

Since this assignable list contains an array,@a, it will consume all the elements that are assigned to it. Therefore,@bwill be empty because there are no elements remaining to assign to it. So, when we execute this program, we will see that@acontains all the elements passed in and@bcontains no elements:

$ perl passarrays.pl
contents of @a
[2] [4] [6] [8] [10] [12]

contents of @b

$

Later, when we discuss references in Chapter 11, we will see how to pass two arrays (or hashes) into a function and treat them as two separate variables.