Question

I’m still learning Perl, so there’s probably a more efficient way of doing this. I’m trying to take a hash, reverse it so $values => $keys, grab the new key (the old value) and then sort those keys.

Here’s the code in question:

foreach my $key (sort keys reverse %hash){

...}

What I’m expecting to happen is that reverse %hash will return a hash type, which is what keys is looking for. However, I get the following error:

Type of arg 1 to keys must be hash (not reverse)

I’ve tried putting parentheses around reverse %hash, but still get the same thing.

Any ideas why this wouldn’t work?

Answer 1

Perl functions can either return scalar values or lists; there is no explicit hash return type
(You can call return %hash from a subroutine, but Perl implicitly unrolls the key-value pairs from the hash and returns them as a list).

Therefore, the return value of reverse %hash is a list, not a hash, and not suitable for use as an argument to keys. You can force Perl to interpret this list as a hash with the %{{}} cast:

foreach my $key (sort keys %{{reverse %hash}}) { ... 

You could also sort on the values of the hash by saying

foreach my $key (sort values %hash) { ...

Using values %hash is subtly different from using keys %{{reverse %hash}} in that keys %{{reverse %hash}} will not return any duplicate values.