An associative array differs from an ordinary array in the sense that its size is not fixed and that is indexed by a string, called the key. For example, consider:
variable A = Assoc_Type [Integer_Type];
A["alpha"] = 1;
A["beta"] = 2;
A["gamma"] = 3;
A represents an associative array of integers
(Integer_Type) and three keys have been added to the array.
As the example suggests, an associative array may be created using one of the following forms:
Assoc_Type [type]
Assoc_Type [type, default-value]
Assoc_Type []
Any_Type
objects allowing any type of object to may be stored in
the array.
The form involving a default-value is useful for associating a default value for non-existent array members. This feature is explained in more detail below.
There are several functions that are specially designed to work with associative arrays. These include:
assoc_get_keys, which returns an ordinary array of strings
containing the keys in the array.
assoc_get_values, which returns an ordinary array of the
values of the associative array.
assoc_key_exists, which can be used to determine whether
or not a key exists in the array.
assoc_delete_key, which may be used to remove a key (and
its value) from the array.To illustrate the use of an associative array, consider the problem
of counting the number of repeated occurrences of words in a list.
Let the word list be represented as an array of strings given by
word_list. The number of occurrences of each word may be
stored in an associative array as follows:
variable a, word;
a = Assoc_Type [Integer_Type];
foreach (word_list)
{
word = ();
if (0 == assoc_key_exists (a, word))
a[word] = 0;
a[word]++; % same as a[word] = a[word] + 1;
}
assoc_key_exists was necessary to determine whether
or not a word was already added to the array in order to properly
initialize it. However, by creating the associative array with a
default value of 0, the above code may be simplified to
variable a, word;
a = Assoc_Type [Integer_Type, 0];
foreach (word_list)
{
word = ();
a[word]++;
}