Hash Functions

Hash in List Context

When we discussed lists and arrays, we spent a lot of time talking about the difference between list and scalar context. Let’s look at what happens when we evaluate a hash in list context. This is demonstrated with the following program:

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

use strict;

my %person = (
    name  => 'John Doe',
    age   => 39,
    phone => '555-1212',
    city  => 'Chicago'
);

my @data = %person;

print "list context: ", join("|", @data), "\n";
print "another way: ", %person, "\n";

This program takes the hash in list context in two ways. First, it assigns it to an array:

my @data = %person;

then the array is printed by joining its contents with the string “|” (more on thejoin()function in Chapter 7):

print "list context: ", join("|", @data), "\n";

The second way is to simply print it:

print "another way: ", %person, "\n";

Recall that all arguments to theprint()function are treated in list context.

When executed, we can see that a hash variable in list context is a list of the key/value pairs in the order stored in memory (not necessarily in the order in which the hash was created):

$ perl listcontext.pl
list context: age|39|city|Chicago|phone|555-1212|name|John Doe
another way: phone555-1212age39cityChicagonameJohn Doe
$

We see a key (phone), followed by its value (555-1212), followed by a key (age), followed by its value (39), etc.