A hash (associative array) is a structure that stores data in the form of key/value pairs. The keys are unique strings, and the values can be any scalar value (string, number, or reference). In essence, a hash is a look-up table, or, in some people’s terms, a dictionary.

Hashes have a % prefix. You can assign a simple list to a hash and it will be interpreted as sequential key/value pairs. Consider a set of translations of color names from English to French:

    my %color_map = ('red', 'rouge', 'blue', 'bleu', 'green', 'vert');

In the above hash the string ‘red’ is the key for the value ‘rouge’. Perl gives us an alternative to the comma that is particularly useful in making hash assignments more readable — this is the ’=>’ or fat-comma:

    my %color_map = ( red => 'rouge', blue => 'bleu', green => 'vert');

The => operator is a synonym for an ordinary comma with one extra feature: It automatically quotes the string on the left. The second example makes it much more clear that we are assigning 3 associations to the hash rather than a 6-element list. You can access individual elements of a hash using the $ symbol and using curly braces to denote which key you want to access:

    print "$color_map{red}\n";  # prints: rouge

Now, consider a simple program that asks a user to input a color name and which then prints out the French translation. Without hashes you will probably wind up using a series if if/elsif tests to discover what color was entered and print out the appropriate translation. However, with the %color_map hash we need use a single if/else construct to see if the word has a translation in our hash and act accordingly:

    my %color_map = ( red    => 'rouge',
                      blue   => 'bleu',
                      green  => 'vert',
                      black  => 'noir',
                      purple => 'violet',
                      yellow => 'jeune',
                      white  => 'blanc',
                      brown  => 'brun',
                      grey   => 'gris',
                    );
    print "Enter a color in English: ";
    chomp( my $color = <STDIN>);
    $color = lc($color);
    if ( $color_map{$color} ) {
        print "$color translates to '$color_map{$color}'\n";
    } else {
        print "No translation available for '$color'\n";
    }

One thing to notice is that keys in a hash are case-sensitive, hence we used the lc() function to lowercase the user’s input before we looked up the color in the hash.

An important distinction between hashes and arrays are that hashes are not ordered — in the above example the key ‘red’ is not the first element in the hash. We can use the keys() function to retrieve the list of keys in a hash, and the values() function to retrieve the list of values. To walk through a hash and print out each pairing you can simply iterate over the keys:

    foreach my $key (keys %hash) {
        print "$key : $hash{$key}\n";
    }

The each() function returns key/value pairs in scalar context so you may also iterate over the hash like so:

    while( my($key, $val) = each(%hash) ) {
        print "$key : $val\n";
    }

A couple of earlier articles describe ways of using hashes and you might want to look them up in the archive (and archive link is provided below in the resources section) — see the articles dated July 20, 2000 and August 31, 2000.

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We’ve looked at arrays and some of the basic array operators in a previous article (back in March 2000), so in this article we’ll revisit arrays from a slightly different angle — when and how to use them.

Often I see cases where a list of data is assigned to an equal list of scalar variables as in the following hypothetical database reformatting task:

    #!/usr/bin/perl -w
    use strict;
    while(<>){
        chomp;
        my ($id, $lname, $fname, $department,
            $location, $shift) = split /:/;
        print "$lname|$fname|$id|$shift|$department|$location\n";
    }
    __END__

While assigning each field to a named scalar can be beneficial when many of them will be used in various places in the rest of the block (self documenting code), the above usage is trivial and unnecessary. If we are merely reformatting the data, or perhaps storing it in a larger data structure for later use, we do not need to create all of these variables:

    #!/usr/bin/perl -w
    use strict;
    while(<>){
        chomp;
        my @fields = split /:/;
        print join('|', @fields[1,2,0,5,3,4]),"\n";
    }
    __END__

In this case, we are just reformatting the data so rather than specify each field name in the new ordering, we need only re-join the fields in their new order using an array slice.

Another good place to use an array is in building a hash. That’s right, we can easily use an array as a list of keys when building a hash by using a hash-slice. Consider reading in the same type of data above but this time we wish to build a hash using field names for keys:

    #!/usr/bin/perl -w
    use strict;
    my @fields = qw/id lname fname department location shift/;
    while(<>){
        chomp;
        my %data;
        @data{@fields} = split /:/;
        process(\%data); # do something with %data hash
    }
    # process subroutine definition ...
    __END__

Besides being easy to follow, an advantage of the above method is that if you want to eventually process the data fields in the order they occur, you can loop through the array of keys rather than using the keys() function (which does not return keys in the order they were entered in the hash).

As a final array example, let pick a random element from an array:

    #!/usr/bin/perl -w
    use strict;
    my @array = qw/andrew john greg brad/;
    my $draw  =  @array[rand @array];
    print "I picked $draw\n";
    __END__

This works because the rand() function returns a number from 0 up to but not including the argument passed to it. In the above, it returns a number from 0 to 3.999… — and an array index always uses the integer value of the number provided, so in the above case the index can be 0, 1, 2, or 3. If you want to randomize an entire array in place then the perlfaq’s have a good example of a shuffle routine called the fisher_yates shuffle — see the entry in perlfaq4 entitled:

    How do I shuffle an array randomly?

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