Here is a page from Sams Teach Yourself Perl in 21 Days that talks about this subject:
Limitations of Array Variables
In the array variables you've seen so far, you can access an element of a stored list by specifying a subscript. For example, the following statement accesses the third element of the list stored in the array variable @array:
$scalar = $array[2];
The subscript 2 indicates that the third element of the array is to be referenced.
Although array variables are useful, they have one significant drawback: it's often difficult to remember which element of an array stores what. For example, suppose you want to write a program that counts the number of occurrences of each capitalized word in an input file. You can do this using array variables, but it's very difficult. Listing 10.1 shows you what you have to go through to do this.
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Listing 10.1. A program that uses array variables to keep track of capitalized words in an input file.
1: #!/usr/local/bin/perl
2:
3: while ($inputline = <STDIN>) {
4: while ($inputline =~ /\b[A-Z]\S+/g) {
5: $word = $&;
6: $word =~ s/[;.,:-]$//; # remove punctuation
7: for ($count = 1; $count <= @wordlist;
8: $count++) {
9: $found = 0;
10: if ($wordlist[$count-1] eq $word) {
11: $found = 1;
12: $wordcount[$count-1] += 1;
13: last;
14: }
15: }
16: if ($found == 0) {
17: $oldlength = @wordlist;
18: $wordlist[$oldlength] = $word;
19: $wordcount[$oldlength] = 1;
20: }
21: }
22: }
23: print ("Capitalized words and number of occurrences:\n"
;
24: for ($count = 1; $count <= @wordlist; $count++) {
25: print ("$wordlist[$count-1]: $wordcount[$count-1]\n"
;
26: }
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$ program10_1
Here is a line of Input.
This Input contains some Capitalized words.
^D
Capitalized words and number of occurrences:
Here: 1
Input: 2
This: 1
Capitalized: 1
$
This program reads one line of input at a time from the standard input file. The loop starting on line 4 matches each capitalized word in the line; the loop iterates once for each match, and it assigns the match being examined in this particular iteration to the scalar variable $word.
Once any closing punctuation has been removed by line 6, the program must then check whether this word has been seen before. Lines 7-15 do this by examining each element of the list @wordlist in turn. If an element of @wordlist is identical to the word stored in $word, the corresponding element of @wordcount is incremented.
If no element of @wordlist matches $word, lines 16-20 add a new element to @wordlist and @wordcount.
Definition
As you can see, using array variables creates several problems. First, it's not obvious which element of @wordlist in Listing 10.1 corresponds to which capitalized word. In the example shown, $wordlist[0] contains Here because this is the first capitalized word in the input file, but this is not obvious to the reader.
Worse still, the program has no way of knowing which element of @wordlist contains which word. This means that every time the program reads a new word, it has to check the entire list to see if the word has already been found. This becomes time-consuming as the list grows larger.
All of these problems with array variables exist because elements of array variables are accessed by numeric subscripts. To get around these problems, Perl defines another kind of array, which enables you to access array variables using any scalar value you like. These arrays are called associative arrays.
And here is the same word counting program using a hash:
1: #!/usr/local/bin/perl
2:
3: while ($inputline = <STDIN>) {
4: while ($inputline =~ /\b[A-Z]\S+/g) {
5: $word = $&;
6: $word =~ s/[;.,:-]$//; # remove punctuation
7: $wordlist{$word} += 1;
8: }
9: }
10: print ("Capitalized words and number of occurrences:\n"
;
11: foreach $capword (keys(%wordlist)) {
12: print ("$capword: $wordlist{$capword}\n"
;
13: }
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