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, however I can't figure out how they work in Perl. Is there a good reference for this and how do I accomplish what I need?

I should add that it needs to be passed by reference.

sub PrintAA {     my $test = shift;     my %aa   = shift;     print $test . "\n";     foreach (keys %aa)     {         print $_ . " : " . $aa{$_} . "\n";         $aa{$_} = $aa{$_} . "+";     } } 

Answer 1

Pass the reference instead of the hash itself. As in

PrintAA("abc", \%fooHash);  sub PrintAA {   my $test = shift;   my $aaRef = shift;    print $test, "\n";   foreach (keys %{$aaRef})   {     print $_, " : ", $aaRef->{$_}, "\n";   } } 

See also perlfaq7: How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?

Answer 2

This code works:

#!/bin/perl -w  use strict;  sub PrintAA {     my($test, %aa) = @_;     print $test . "\n";     foreach (keys %aa)     {         print $_ . " : " . $aa{$_} . "\n";     } }  my(%hash) = ( 'aaa' => 1, 'bbb' => 'balls', 'ccc' => \&PrintAA );  PrintAA("test", %hash); 

The key point is the use of the array context in the my() 'statement' in the function.

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What does the array context business actually do?

Succinctly, it makes it work correctly.

It means that the first value in the @_ array of arguments is assigned to $test, and the remaining items are assigned to the hash %aa. Given the way I called it, there is an odd number of items in the @_, so once the first item is assigned to $test, there is an even number of items available to assign to %aa, with the first item of each pair being the key ('aaa', 'bbb', 'ccc' in my example), and the second being the corresponding value.

It would be possible to replace %aa with @aa, in which case, the array would have 6 items in it. It would also be possible to replace %aa with $aa, and in that case, the variable $aa would contain the value 'aaa', and the remaining values in @_ would be ignored by the assignment.

If you omit the parentheses around the variable list, Perl refuses to compile the code. One of the alternative answers showed the notation:

my $test = shift; my(%aa) = @_; 

This is pretty much equivalent to what I wrote; the difference is that after the two my statements, @_ only contains 6 elements in this variation, whereas in the single my version, it still contains 7 elements.

There are definitely other questions in SO about array context.

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Actually, I wasn't asking about the my($test, %aa) = @_; I was asking about my(%hash) = ( 'aaa' => 1, 'bbb' => 'balls', 'ccc' => \&PrintAA ); versus my %hash = { 'aaa' => 1, ... };

The difference is that the { ... } notation generates a hash ref and the ( ... ) notation generates a list, which maps to a hash (as opposed to hash ref). Similarly, [ ... ] generates an array ref and not an array.

Indeed, change the 'main' code so it reads: my(%hash) = { ... }; and you get a run-time (but not compile time) error - treat line numbers with caution since I've added alternative codings to my file:

Reference found where even-sized list expected at xx.pl line 18. ... Use of uninitialized value in concatenation (.) or string at xx.pl line 13.