Beginning Perl

Chapter 7

Subroutines

WHAT YOU WILL LEARN IN THIS CHAPTER:

• Declaringa subroutine
• Passing data to subroutines
• Returning data from subroutines
• Using prototypes
• Using subroutine references
• Understanding recursion
• Implementing error checking

WROX.COM CODE DOWNLOADS FOR THIS CHAPTER

The wrox.com code downloads for this chapter are found at http://www.wrox.com/WileyCDA/WroxTitle/Beginning-Perl.productCd-1118013847,descCd-DOWNLOAD.html on the Download Code tab. The code for this chapter is divided into the following major examples:

• example_7_1_running_total.pl
• example_7_2_length.pl
• example_7_3_zip.pl
• example_7_4_maze.pl
• listing_7_1_fibonacci.pl
• listing_7_2_binary_search.pl

A subroutine is just a way of providing a “name” to a piece of code. This is useful when you need to execute the same piece of code in several different places in your program, but you don’t want to just “cut-n-drool” the same code all over the place.

Even if you don’t want to reuse a piece of code, applying a name is useful. Compare the following two lines of code:

my $result = 1 + int( rand(6) );
my $result = random_die_roll();

Just by intelligently naming a subroutine, you can see that the second line of code much clearer than the first. Thus, you can use subroutines to make your code more self-documenting. As an added benefit, the name of a subroutine is documentation that you don’t forget to add.

SUBROUTINE SYNTAX

A basic subroutine (often just called a sub) is declared with the syntax of

sub IDENTIFIER BLOCK ...