Perl hash when both the keys and the values are array references

Question

I have a problem where pairs of numbers map to other pairs of numbers. For instance, (1,2)->(12,97). Some pairs may map to multiple other pairs, so what I really need is the ability to map a pair into a list of lists, like (1,2)->((12,97),(4,1)). At the end of the day I want to process each of the values (i.e., each list of lists) separately.

In Python, I could do this by simply saying:

key = ( x, y )
val = [ a, b ]
if (x,y) not in my_dict:
    my_dict[ (x,y) ] = []
my_dict[ (x,y) ].append( [a,b] )

However, in Perl, I have to use refs for the keys and values. So I can certainly say:

$keyref = [ x1, y1 ]
$valref = [ a, b ]
%my_hash = { $keyref => $valref }

But what happens when another pair (x2,y2) comes along? Even if x2==x1 and y2==y1, $keyref=[x2,y2] will differ from the previous keyref generated, so I do not see a way to do the lookup. Of course, I could compare (x2,y2) with each dereferenced hash key, but after all, God gave us hash tables precisely to avoid the need to do so.

Is there a Perl solution?

Thanks,

-W.

Answer 1

In Perl, all hash keys are strings, or are "stringified" before lookup. Using an array reference as a key is usually the wrong approach.

What about using a "two-dimensional" hash?

$hash{$x1}{$y1} = [ $a, $b ];
# or
%hash = ( $x1 => { $y1 => [ $a, $b ] } );


($x2,$y2)=($x1,$y1);
print @{$hash{$x2}{$y2}};   # will print $a and $b

Answer 2

Like most things in Perl, TMTOWTDI.

Option 1: Use multidimensional array emulation

$hash{$x,$y} = [$a, $b];

See also the documentation for the built-in variable $;.

Option 2: Use the Hash::MultiKey module

tie %hash, 'Hash::MultiKey';
$hash{[$x, $y]} = [$a, $b];

Option 3: Use a HoH (hash of hashes) instead

$hash{$x}{$y} = [$a, $b];