map for hashes in Perl

Question

Is there a hash equivalent for map?

my %new_hash = hash_map { new_key($a) => new_val($b) } %hash; 

I know that I could loop through the keys.

Answer 1

List::Pairwise claims to implement exactly that syntax -- see mapp, grepp. I haven't used it though.

Also, you can do it as

%new_hash = map { new_key($_) => new_value($hash{$_}) } keys %hash;  

which I admit looks clumsier if %hash is really a $deeply->{buried}->{hash}. I prefer using $temp = ...; map {...} keys %$temp in such cases.

Answer 2

I really can’t see what you are trying to do here. What does “a hash equivalent for map” even mean? You can use map on a hash just fine. If you want the keys, just use keys; for example"

@msglist = map { "value of $_ is $hash{$_}" } keys %hash     

although usually

say "value of $_ is $hash{$_}"  keys %hash; 

is just fine.

If you want both, then use the whole hash.

For assignment, what’s wrong with %new_hash = %old_hash?

Do you have deep-copy issues? Then use Storable::dclone.

Do you want both key and value available in the closure at the same time? Then make a bunch of pairs with the first map:

@pairlist = map { [ $_ => $hash{$_} ] } keys %hash   

I need to see an example of what you would want to do with this, but so far I can see zero cause for using some big old module instead of basic Perl.