in reply to Hash optimization in Perl?

There are really only two optimizations you can do with perl hashing - speed or size.

If you know the approximate size of your hash and want to speed up access, you can presize your hash 

  keys(%hash) = 1024;

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Given the set of keys you have, that may or may not really speed things up. The theory is that by allocating space up front, you prevent slow downs when the hash needs to be re-allocated.

For memory size, if you think your hash is going to consume more memory than you have, you should look into the various Tie::Hash modules that have a db backend.

Either way, both are probably the last places I would look for optimizations.

-derby

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Re^2: Hash optimization in Perl?
by ferreira (Chaplain) on May 02, 2007 at 11:23 UTC

    For in-memory hashes with smaller size requirements, you may try Tie::GHash whose proposal is to offer smaller (but possibly slower) hashes. That could be useful, if your system has available the Gnome glib library from where this module's hashes come from.

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Re^2: Hash optimization in Perl?
by Zaxo (Archbishop) on May 02, 2007 at 06:28 UTC

    Are you sure keys is an lvalue function?

    I get no complaint in scalar assignment, but no indication it worked, either. List assignment gives, 

    $ perl -Mwarnings -Mstrict -e'keys( my %foo) = qw/foo bar baz/;print %foo'
Useless use of a constant in void context at -e line 1.
Useless use of a constant in void context at -e line 1.
Argument "baz" isn't numeric in scalar assignment at -e line 1.
$

    After Compline,
    Zaxo

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      perldoc -f keys
      As an lvalue "keys" allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say 
          keys %hash = 200;

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      then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do "%hash = ()", use "undef %hash" if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using "keys" in this way (but you needn't worry about doing this by accident, as trying has no effect).
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