Passing arrays to functions in Perl

Question

I think I have misunderstood some aspects of argument passing to functions in Perl. What\'s the difference between func(\\@array) and func(@array)?

Answer 1

It's impossible to pass arrays to subs. Subs take a list of scalars for argument. (And that's the only thing they can return too.)

You can pass a reference to an array:

func(\@array)

You can pass the elements of an array:

func(@array)

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When should we use which?

If you want to pass more than just the elements of the array (e.g. pass $x, $y and @a), it can become tricky unless you pass a reference.

If you're going to process lists (e.g. sum mysub grep { ... } ...), you might not want to pass a reference.

If you want to modify the array (as opposed to just modifying the existing elements of the array), you need to pass a reference.

It can be more efficient to pass a reference for long arrays, since creating and putting one reference on the stack is faster than creating an alias for each element of a large array. This will rarely be an issue, though.

It's usually decided by one of the first two of the above. Beyond that, it's mostly a question of personal preference.

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Also, how do I imitate pass-by-value in Perl?

sub foo {
   my ($x) = @_;   # Changing $x doesn't change the argument.
   ...
}

sub foo {
   my @a = @_;   # Changing @a or its contents
   ...           #    doesn't change the arguments.
}