This article is Part 6 of our series on Internet Programming. See the Introduction and Part 1, Introduction to HTML. This article covers the Perl "hash" data type.

Introduction to Hashes

A Perl "hash" is very similar to a Perl list -- it is a way to store a lot of data in one named variable. The difference is that where as in lists, the "compartments" where you can store your data are numbered 0, 1, 2, etc., in hashes the compartments each have their own arbitrary name! The "name" of the compartment is called a key and the information stored in a compartment is called a value. So using hashes is all about storing and retrieving keys and values.

Since you are already familiar with lists from Lesson 5, it is easiest to show you examples of using a hash and explain how it is different from how you would use a list....

    %hash = ( "name", "Joe Smith",
              "phone", "617-555-7890",
       	      "state", "Massachusetts",
              "country, "United States of America" );

    print $hash{"name"}, " - ", $hash{phone}, "\n";

    $hash{"state"} = "Maine";

    print $hash{"state"};

Creating a hash: You can create a hash using an assignment just like you would create a list, the differences are:

• The hash variable is prefixed with a percent sign ("%") instead of the at-sign.
• The list of data that you assign to the hash is no longer just a list of values, as in the case of a list. Now it is a list of "key,value" pairs. In the example above, we are creating a hash (called, creatively enough, "hash") which contains 4 keys and 4 values. The keys are "name", "phone", "state", and "country" and the respective values are the items in the list immediately after each key. So, you can create a hash explicitly by assigning a list of the form (key1, value1, key2, value2, ....) to you hash variable.

Reading and writing hash elements: Just like when using a list, when you read a value from or write a value to a hash, you use the dollar sign ("$") as the prefix because you are dealing effectively with a normal simple variable. The only differences between accessing lists and hashes is that you replace the square brackets containing the index of the item, i.e. [3], with curly brackets containing the name of the key, i.e. {"state"}. The quotation marks are not always necessary, but it is good practice to use them so you are not surprised when they are necessary.

Hashes are just like lists, but with names instead of numbers for the indices. You are responsible for making up these names. The names are case sensitive so $hash{State} and $hash{"State"} refer to the values of two completely different and independent hash keys. Use hash elements just like you would use array elements or normal variables.

Hash Functions

Just like with lists, there are several functions built into Perl that are designed to aid in your use of hashes. Some of these are:

keys
@list_of_keys = keys(%my_hash);
This function returns a list of all the keys being used in your hash. It does not return any of the values, just the keys. Also, you have no way of knowing what order it will return the keys in.

values
@list_of_values = values(%my_hash);
This function returns a list of all the values being stored at each key in your hash. It does not return any of the keys, just the values. Also, you have no way of knowing what order it will return the values in (except that it will be in the same other that keys returns the list of keys).

exists
if (exists $my_hash{"questionable_key"}) {
The exists operator is used in conditional statements. It returns "true" if there is such a key in your hash and "false" if your hash has no such key. I.e. "Does this key exist?".

defined
if (defined $my_hash{"questionable_key"}) {
The defined operator is used in conditional statements. It returns "true" if there is such a key in your hash and the value is defined (i.e. the values is NOT undef). It returns "false" otherwise.

For example, a common use of the keys and values functions is to iterate over and display everything in a hash:

    %hash = ( "name", "Joe Smith",
              "phone", "617-555-7890",
              "state", "Massachusetts",
              "country, "United States of America" );

   ## Iterate over and print out all key/value pairs

   @hash_keys = keys(%hash);

   for $key (@hash_keys) {
       print "Key '$key' has value '$hash{$key}'\n";
       }

   ## An alternate way...

   for $key (keys(%hash)) {
       print "Key '$key' has value '$hash{$key}'\n";
       }

   ## An alternate way...

   @hash_keys = keys(%hash);
   @hash_vals = values(%hash);

   for ($i=0;$i<=$#hash_keys;$i=$i+1) {
       print "Key '$hash_keys[$i]' has value '$hash_vals[$i]'\n";
       }

These are very common ways to iterate over a hash and print out all of a hash's keys and values. Note that you don't need to use quotes around the name of the hash key if it is stored in a variable, i.e. $hash{$key}.

Also note that there is no concept of an "index" for a hash element, there are only keys and values and it is up to you to define the keys.

What Good are Hashes?

At this point you may be asking "what good are hashes?" and "why not just use lists for everything?". Well, you could use lists for everything, but if you have a collection of information that naturally has the form of key/value pairs, a hash is often a better choice:

• It is easier to store and retrieve the information
• Using keys rather than numbers makes it easier to read your Perl program and avoid errors.

Good examples of the kinds of things that are "just made for hashes" include:

• Contact information about someone as in the example above
• The information stored in an HTML form (all the fields have names and values)
• Any list where it is easier to refer to the elements by a name rather than a number, such as a list of people and email addresses, or a list of companies and year-end earnings, etc. These can all be represented by a pair of lists, but the information can be retrieved more easily using a hash.

Similar Posts:

• Introduction to Internet Programming: Part 5 – Perl Data Part I – Lists
• Introduction to Internet Programming with Perl and HTML: Part 3 – Introduction to Perl
• Introduction to Internet Programming with Perl and HTML: Part 4 – Basic Perl Flow-Control Syntax
• Introduction to Internet Programming with Perl and HTML: Part 2 – Introduction to Programming Concepts
• Introduction to Internet Programming with Perl and HTML: Part 1 – Introduction to HTML