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.

---

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.

---

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.

Answer 3

Alternatively:

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

The thing you're fundamentally missing is that an associative array isn't a single argument (though an associative array reference is, as in Paul Tomblin's answer).

Answer 4

It looks like you should pass in a reference to a hash.

sub PrintAA
{
   my $test = shift;
   my $aa = shift;
   if (ref($aa) != "HASH") { die "bad arg!" }
   ....
}

PrintAA($foo, \%bar);

The reason you can't do a

my %aa = shift;

is because Perl flattens all the arguments to a subroutine into one list, @_. Every element is copied, so passing in by reference avoids those copies as well.

Answer 5

As usual there are several ways. Here is what Perl Best Practices, that most revered of style pointers, has to say about passing parameters to functions:

Use a hash of named arguments for any subroutine that has more than three parameters

But since you have only two, you could get away ;) with passing them directly like this:

my $scalar = 5;
my %hash = (a => 1, b => 2, c => 3);

func($scalar, %hash)

And function is defined like this:

sub func {
    my $scalar_var = shift;
    my %hash_var = @_;

    ... Do something ...
}

It could be more useful if you could show some code.

Answer 6

All the above methods work, but this was always the way I preferred to do things like this:

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

Note: I also changed your code a bit. Perl's double-quoted strings will interpret "$test" to be the value of $test rather than the actual string '$test', so you don't need that many .s.

Also, I was wrong about how the prototypes work. To pass a hash, use this:

PrintAA("test", %hash);

To print a hash reference, use this:

PrintAA("test", %$ref_to_hash);

Of course, now you can't modify the hash referenced by $ref_to_hash because you're sending a copy, but you can modify a raw %hash because you're passing it as a reference.

Answer 7

Arguments to functions get flattened into a single array (@_). So it's usually easiest to pass hashes to function by reference.

To create a HASH:

my %myhash = ( key1 => "val1", key2 => "val2" );

To create a reference to that HASH:

my $href = \%myhash

To access that hash by reference;

%$href

So in your sub:

my $myhref = shift;

keys %$myhref;

Answer 8

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.

Answer 9

Use the folowing sub to get hash or hashref - whatever passed :)

sub get_args { ref( $_[0] ) ? shift() : ( @_ % 2 ) ? {} : {@_}; }
sub PrintAA
{
  my $test = shift;
  my $aa = get_args(@_);;
  #then
  $aa->{somearg} #do something
  $aa->{anotherearg} #do something

}

Call your function like this:

printAA($firstarg,somearg=>1, anotherarg=>2)

Or like this(no matter):

printAA($firstarg,{somearg=>1, anotherarg=>2})

Or even like this(no matter):

my(%hash) = ( 'aaa' => 1, 'bbb' => 'balls', 'ccc' => \PrintAA );

PrintAA("test", %hash);

Cheers!