A.18 Subroutines

Subroutines are defined by the keyword sub , followed by the name of the subroutine, followed by a block enclosed by curly braces containing the body of the subroutine. The following is a simple example.

 sub a_subroutine {     print "I'm in a subroutine\n"; } 

In general, you can call subroutines using the name of the subroutine followed by a parenthesized list of arguments:

 a_subroutine(); 

Arguments can be passed into subroutines as a list of scalars. If any arrays are given as arguments, their elements are interpolated into the list of scalars. The subroutine receives all scalar values as a list in the special variable @_ . This example illustrates a subroutine definition and the calling of the subroutine with some arguments:

 sub concatenate_dna {     my($dna1, $dna2) = @_;     my($concatenation);     $concatenation = "$dna1$dna2";     return $concatenation; } print concatenate_dna('AAA', 'CGC'); 

This prints:

 AAACGC 

The arguments ' AAA ' and ' CGC ' are passed into the subroutine as a list of scalars. The first statement in the subroutine's block:

 my($dna1, $dna2) = @_; 

assigns this list, available in the special variable @_ , to the variables $dna1 and $dna2 .

The variables $dna1 and $dna2 are declared as my variables to keep them local to the subroutine's block. In general, you declare all variables as my variables; this can be enforced by adding the statement use strict; near the beginning of your program. However, it is possible to use global variables that are not declared with my , which can be used anywhere in a program, including within subroutines.

The statement:

 my($concatenation); 

declares another variable for use by the subroutine.

After the statement:

 $concatenation = "$dna1$dna2"; 

performs the work of the subroutine, the subroutine defines its value with the return statement:

 return $concatenation; 

The value returned from a call to a subroutine can be used however you wish; in this example, it is given as the argument to the print function.

If any arrays are given as arguments, their elements are interpolated into the @_ list, as in the following example:

 sub example_sub {     my(@arguments) = @_;     print "@arguments\n"; } my @array = (`two', `three', `four'); example_sub(`one', @array, `five'); 

This prints:

 one two three four five 

Note that the following attempt to mix arrays and scalars in the arguments to a subroutine won't work:

 # This won't work!! sub bad_sub {     my(@array, $scalar) = @_;     print $scalar; } my @arr = ('DNA', 'RNA'); my $string = 'Protein'; bad_sub(@arr, $string); 

In this example, the subroutine's variable @array on the left side in the assignment statement consumes the entire list on the right side in @_ , namely (' DNA ', ' RNA ', ' Protein '). The subroutine's variable $scalar won't be set, so the subroutine won't print ' Protein ' as intended. To pass separate arrays and hashes to a subroutine, you need to use references. Here's a brief example:

 sub good_sub {     my($arrayref, $hashref) = @_;     print "@$arrayref", "\n";     my @keys = keys %$hashref;     print "@keys", "\n"; } my @arr = ('DNA', 'RNA'); my %nums = ( 'one' => 1, 'two' => 2); good_sub(\@arr, \%nums); 

This prints:

 DNA RNA one two