Subroutines

Until now, all the Perl programs that we have written have simply a set of instructions, line by line. Like any good language, Perl allows one to write modular code. To do this, at the very least, the language must allow the programmer to set aside subroutines of code that can be reused. Perl, of course, provides this feature.

Note that many people call Perl subroutines "functions". We prefer to use the term "functions" for those routines that are built in to Perl, and "subroutines" for code written by the Perl programmer. This is not standard terminology, so you may hear others use subroutines and functions interchangeably, but that will not be the case in this book. We feel that it is easier to make the distinction if we have two different terms for functions and subroutines.

Note that user subroutines can be used anywhere it is valid to use a native Perl function.

Defining Subroutines

Defining a subroutine is quite easy. You use the keyword sub, followed by the name of your subroutine, followed by a code block. This friendly subroutine can be used to greet the user:

use strict;
sub HowdyEveryone {
   print "Hello everyone.\nWhere do you want to go with Perl today?\n";
}

Now, anywhere in the code where we want to greet the user, we can simply say:

&HowdyEveryone;

and it will print that message to the user. In fact, in most cases, the & for invoking subroutines is optional.

Returning Values

Perhaps we did not want our new subroutine to actually print the message. Instead, we would like it to return the string of the message, and then we will call print on it.

This is very easy to do with the return statement.

use strict;
sub HowdyEveryone {
   return "Hello everyone.\nWhere do you want to go with Perl today?\n";
}
print &HowdyEveryone;

Using Arguments

A subroutine is not much good if you cannot give it input on which to operate. Of course, Perl allows you to pass arguments to subroutines just like you would to native Perl functions.

At the start of each subroutine, Perl sets a special array variable, @_, to be the list of arguments sent into the subroutine. By standard convention, you can access these variables through $_[0 .. $#_]. However, it is a good idea to instead immediately declare a list of variables and assign @_ to them. For example, if we want to greet a particular group of people, we could do the following:

use strict;
sub HowdyEveryone {
   my($name1, $name2) = @_;
   return "Hello $name1 and $name2.\n" .
          "Where do you want to go with Perl today?\n";
}
print &HowdyEveryone("bart", "lisa");

Note that since we used my, and we are in a new block, the variables we declared will live only as long as the subroutine execution.

This subroutine leaves a bit to be desired. It would be nice if we could have a custom greeting, instead of just "Hello". In addition, we would like to greet as many people as we want to, not just two. This version fixes those two problems:

use strict;
sub HowdyEveryone {
   my($greeting, @names) = @_;
   my $returnString;

   foreach my $name (@names) {
       $returnString .= "$greeting, $name!\n";
   }

   return $returnString .
          "Where do you want to go with Perl today?\n";
}
print &HowdyEveryone("Howdy", "bart", "lisa", "homer", "marge", "maggie");

We use two interesting techniques in this example. First of all, we use a list as the last parameter when we accept the arguments. This means that everything after the first argument will be put into @names. Note that had any other variables followed @names, they would have remained undefined. However, scalars before the array (like $greeting) do receive values out of @_. Thus, it is always a good idea to only make the array the last argument.

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