Learning Perl on Win32 Systems

Return Values

A subroutine is always part of some expression. The value of the subroutine invocation is called the return value. The return value of a subroutine is the value of the return statement or of the last expression evaluated in the subroutine.

For example, let’s define this subroutine:

sub sum_of_a_and_b {
	return $a + $b;
}

The last expression evaluated in the body of this subroutine (in fact, the only expression evaluated) is the sum of $a and $b, so the sum of $a and $b will be the return value. Here’s that in action:

$a = 3; $b = 4;
$c = sum_of_a_and_b();   # $c gets 7
$d = 3*sum_of_a_and_b(); # $d gets 21

A subroutine can also return a list of values when evaluated in a list context. Consider this subroutine and invocation:

sub list_of_a_and_b {
return	 ($a,$b);
}
$a = 5; $b = 6;
@c = list_of_a_and_b(); # @c gets (5,6)

The last expression evaluated really means the last expression evaluated, rather than the last expression defined in the body of the subroutine. For example, this subroutine returns $a if $a > 0; otherwise, it returns $b:

sub gimme_a_or_b {
		if ($a > 0) {
			print "choosing a ($a)\n";
	return		$a;
		} else {
			print "choosing b ($b)\n";
	return		$b;
		}
}

These examples are all rather trivial. It gets better when we can pass values that are different for each invocation into a subroutine, instead of having to rely on global variables. In fact, this discussion is coming right up.