If the hash is big enough it does make a difference:

C:\test>p1
$t = time; my %h; keys %h = 10e6; $h{$_}=1 for 1 .. 10e6; printf "%.6f
+\n", time() -$t;;
15.792654

$t = time; my %i; $i{$_}=1 for 1 .. 10e6; printf "%.6f\n", time() -$t;
+;
18.772236

[download]

That's about an 18% saving. And if you use hashes with 100s of millions of keys as I routinely do, then the saving get bigger.

Perl's hashes start off with room for 8 keys and then double and double again as they grow. Each time it doubles, new memory is allocated and the keys must be rehashed and the values copied over. By preallocating a 10 million key hash that process is avoided 21 times.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: Does "preallocating hash improve performance"? Or "using a hash slice"? by BrowserUk
in thread Does "preallocating hash improve performance"? Or "using a hash slice"? by vr

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