An Introduction to Perl

An Introduction to Perl – Part II By: Christopher Todd

Hashes • Hashes are variables that hold lists. • Arrays are also variables that hold lists, but the fundamental difference is how that lists are indexed. • Arrays use numbers for indexing • Hashes use strings for indexing • In a hash, you must declare the index with the variable. There is no range operator for hashes. • Indexes in a Hash are known as “Keys”

Examples of Hashes • To define a Hash, we place a ‘%’ before the name of the Hash. • Ex. If we wish to make a hash named ‘Example’, we declare %Example. • We must also declare the index when we declare the variable. • Ex. We wish to make a hash combining the month name (index/key) and number (data): • %Month (“January”, 1, “February”, 2,…, “December”, 12)

Inversing a Hash • Because a Hash’s indexes and data are both assignable, we can take data from the indexes and switch it with the data. • As in the example before, we can switch the %Month hash so that the indexes are the number representing the month and the names are the data. • %Month_Numbers = reverse %Month

Foreach Control Structure • The Foreach Control Structure is a form of a loop • The loop will go through a array or list, executing one iteration for every value of the list. • The list one at a time until the list is exhausted

Example of a Foreach Command Ex. foreach (1..10) { print " $_\n"; } The command above will go through the list of 1-10 until the list is done. The example above will print the list 1-10, with each number on a separate line.

Split Functions • Split is a function that is used to separate data. • A split can be created using a letter, number, symbol, or any character that can be part of a string. • The split function can be used to break a function into parts and insert them into a list or an array.

Example of a Split Function Ex. $Example_String = “This is an example of a split function”; @Example_Array = split “ “, $Example_String; In the Example above, we make a string, and then split the string into an array. Every space in the string serves a a split to the string and part of the string is inserted into the array.

References • “Perl Basics” • http://www.cs.drexel.edu/~knowak/cs265_fall_2009/perl_basics.pdf • “Introduction to Perl” • http://www.tizag.com/perlT/index.php • “How To Perl” • http://www.perlmeme.org/ • “Perl Tutorial” • http://www.comp.leeds.ac.uk/Perl/

An Introduction to Perl – Part II