How do I pass a hash to a function in Perl?

Question

I have a function that takes a variable and an associative array, but I can\'t seem to get them to pass right. I think this has something to do with function declarations, h

Answer 1

All the other replies here so far seem rather complicated to me. When I write Perl function I usually "expand" all the passed arguments in the first line of the function.

sub someFunction {
    my ( $arg1, $arg2, $arg3 ) = @_;

This is similar to other languages, where you declare functions as

... someFunction ( arg1, arg2, arg3 )

And if you do it that way and pass the hash as the last argument, you'll be fine without any tricks or special magic. E.g.:

sub testFunc {
    my ( $string, %hash ) = @_;
    print "$string $hash{'abc'} $hash{'efg'} $string\n";
}

my %testHash = (
    'abc' => "Hello",
    'efg' => "World"
);
testFunc('!!!', %testHash);

The output is as expected:

!!! Hello World !!!

This works becaus in Perl arguments are always passed as an array of scalar values and if you pass a hash, it's key value/pairs are added to that array. In the sample above, the arguments passed to the function as array (@_) are in fact:

'!!!', 'abc', 'Hello', 'efg', 'World'

and '!!!' is simple assigned to %string, while %hash "swallows" all the other arguments, always interpreting one as a key and the next one as value (until all elements are used up).

You cannot pass multiple hashes that way and the hash cannot be the first argument, as otherwise it would swallow everything and leaves all other arguments unassigned.

Of course exactly the same works for array as a last argument. The only difference here is that arrays don't distinguish between keys and values, for them all arguments left over are values and just get pushed to the array.