CODEWITHSUNDEEP
perlhash
Matias Irazoqui
Matias Irazoqui

Reputation: 35

Find key in subhash without iterate through the whole hash

I have a hash that looks like this:

my $hash = {
    level1_f1 => {
                  level2_f1 => 'something',
                  level2_f2 => 'another thing'
    },
    level1_f2 => {
                  level2_f3 => 'yet another thing',
                  level2_f4 => 'bla bla'
                  level2_f5 => ''
    }
...
 }

I also got a list of values that correspond to the "level2" keys, which I want to know if thy exist in the hash.

@list = ("level2_f2", "level2_f4", "level2_f99")

I don't know which "level1" key each element of @list belongs to. The only way of finding if they existed I could think was using a foreach loop to go through @list, another foreach loop to go through the keys of %hash and checking 

foreach my $i (@array) {
  foreach my $k (keys %hash) {
     if (exists $hash{$k}{$list[$i]})
 }
}

but I wanted to know if there is a more eficient or maybe a more elegant way to do it. All the answers I found ask you to know the "level1" key, which I don't.

Thanks!!

Upvotes: 3

Views: 277

Answers (3)

ikegami
ikegami

Reputation: 385916

You don't need to iterate over "the entire hash".

You will necessarily over the elements of the outer hash since you want to check the value of each one, but you don't need to iterate over the elements of the inner hashes. Your solution already demonstrates that.

So your solution is as efficient as it can be, at least in terms of how well it scales. You can only perform small optimizations such as stopping as soon as a match is found.

for my $i (@list) {
   while ( my (undef, $inner) = each(%hash) ) {
      if (exists($inner->{$i}) {
         ...
         last;
      }
   }

   keys(%hash);   # Reset iterator since it might not be exhausted.
}

As a micro optimization, it might be beneficial to invert the nesting of the loops.

my %list = map { $_ => 1 } @list;

while ( my (undef, $inner) = each(%hash) ) {
   while (defined( my $k = each(%$inner) )) {
      if ($list{$k}) {
         delete($list{$k});
         ...
         last if !keys(%list);
      }

   }

   keys(%$inner);   # Reset iterator since it might not be exhausted.
   last if !keys(%list);
}

keys(%hash);   # Reset iterator since it might not be exhausted.

If the hashes are small, these changes might actually slow things down.

Honestly, if there's truly a speed issue, the problem is that you used the wrong data structure for the type of query you want to run on it!

Upvotes: 1

Holli
Holli

Reputation: 5072

You have to loop all the level1 keys. But if you don't need to know which keys match and merely care for the existence of any, then you don't have to ask for each member of your list explicitly. You could say

foreach my $k (keys %hash) {
   if ( @{ $hash{$k} }{ @list } )
   {
   }
}

The hash slice will return all values in the subhash which have matching keys in the list. Keys in the list that are not in the subhash get ignored.

Note however, that this does potentially more work than you may really need.

Upvotes: 1

choroba
choroba

Reputation: 241918

Use values:

for my $inner_hash (values %$hash) {
    say grep exists $inner_hash->{$_}, @list;
}

Upvotes: 5

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