Advanced Perl Programming

Using References

References are absolutely essential for creating complex data structures. Since the next chapter is devoted solely to this topic, we will not say more here. This section lists the other advantages of Perl’s support for indirection and memory management.

Passing Arrays and Hashes to Subroutines

When you pass more than one array or hash to a subroutine, Perl merges all of them into the @_ array available within the subroutine. The only way to avoid this merger is to pass references to the input arrays or hashes. Here’s an example that adds elements of one array to the corresponding elements of the other:

@array1 = (1, 2, 3); @array2 = (4, 5, 6, 7);
AddArrays (\@array1, \@array2); # Passing the arrays by reference.
print "@array1 \n";
sub AddArrays 
{
		my ($rarray1, $rarray2) = @_;
		$len2 = @$rarray2;  # Length of array2
		for ($i = 0 ; $i  < $len2 ;  $i++) {
			$rarray1->[$i] += $rarray2->[$i];   
		}
}

In this example, two array references are passed to AddArrays which then dereferences the two references, determines the lengths of the arrays, and adds up the individual array elements.

Performance Efficiency

Using references, you can efficiently pass large amounts of data to and from a subroutine.

However, passing references to scalars typically turns out not to be an optimization at all. I have often seen code like this, in which the programmer has intended to minimize copying while reading lines from a file:

while ($ref_line = GetNextLine()) { ..... ..... } sub GetNextLine () ...