each vs. keys

• This gets the keys all at once, in C:

        for (keys %hash) {
          ...
        }

• This gets the keys one at a time, dispatching Perl operations in between:

        while (my $k = each %hash) {
          ...
        }

• The purpose of each is to conserve space, not time

• You use it when the hash is very large and you don't want to store all the keys at once

  • For example if the hash is tied to a large disk file

• Since it is a space-conserving optimization, you would expect it to be slower than keys

• And so it is

  • Unless you're also interested in the values

  • Or unless the keys call causes your program to become memory-bound

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Copyright © 2003 M. J. Dominus