Question

I am inserting 2-dimensional array references into my heap in Perl.
How should I define the ‘elements’ attribute when constructing my heap so that I can properly use my comparator function?

my $heap = Heap::Simple->new( order     => \&byNumOrStr,
                              elements  => [Array => 0]
                             );

sub byNumOrStr
{
    my ( $a, $b ) = @_;

    $b->[0] <=> $a->[0]  #0-th element is a number. 
            ||
    $a->[1] cmp $b->[1]; #1-st element is a number
}

I keep getting back this error:

Can’t use string (“2.55”) as an ARRAY ref while “strict refs” in use … (This means I might actually have to compare my “number string” numerically)

Answer 1

Well, it’s likely that either $a or $b is being passed in as a string. Try printing out this variables after the assignment.

From what I can see from the documentation, when you pass elements => [ Array => 0 ], unless the 0th item in the array is an array then you’ll only be comparing the values in the first slot of the array.

[Array => $index]

Indicates that the elements are array references, with the key at index $index. So now the element can be not just the key, but also associated data.

This means that if 2.55 is in the array like [ 2.55, … ] then that’s what’s being passed in as $a or $b.

The elements entry tells H::S how you want to derive the key. For a completely generic way, it says that you can pass [Function => $code_ref_for_key]. You could make it like this:

sub first_two_slots {
my $array_ref = shift;
return [ @$array_ref[0,1] ];
}

And then with the order as specified, it would pass that array into your order and specify

my $heap = Heap::Simple->new( order     => \&byNumOrStr,
                              elements  => [Function => \&first_two_slots]
                             );

---

Original comment left in place: (It’s not relevant to how Heap::Simple calls order).

if byNumOrStr is called from sort DON’T assign $a and $b in it. Those values are set by sort. If there is something coming in @_ it’s probably not what you want.