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

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.


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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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