Re^11: How to speed up a nested loop?

Umm, the look-up was already factored out, only with a different syntax, ie.

$stuff = \$stuff2[2];
loop {
    $moo = $$stuff->[3];
}
[download]

instead of

$stuff = $stuff2[2] || [];
loop {
    $moo = $stuff->[3];
}
[download]

Or i'm misunderstanding something here. :)

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Re^12: How to speed up a nested loop? by tilly (Archbishop) on Sep 23, 2008 at 05:40 UTC
I think you're misunderstanding. What you had before calculated $rx in the loop, and then had: `...$tiles[$rx][$ry]...` [download] in the inner loop. Which looks up index $rx, gets an array reference, looks up index $ry, etc. I moved calculating $rx out of the loop and the lookup on $rx out of the loop with: `my $tiles_for_rx = ($tiles[$rx] \|\|= []);` [download] and then in the loop had: `...$tiles_for_rx->[$ry]...` [download] which means that the lookup of index $rx doesn't happen in the inner loop any more. If your inner loop executes 15 times for each iteration of the outer loop, you just lost 14 calculations of $rx, and 14 lookups on index $rx. Less work is generally faster.	[reply] [d/l] [select]

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Re^12: How to speed up a nested loop?
by tilly (Archbishop) on Sep 23, 2008 at 05:40 UTC

...$tiles[$rx][$ry]...
[download]

I moved calculating $rx out of the loop and the lookup on $rx out of the loop with:

my $tiles_for_rx = ($tiles[$rx] ||= []);
[download]

...$tiles_for_rx->[$ry]...
[download]

If your inner loop executes 15 times for each iteration of the outer loop, you just lost 14 calculations of $rx, and 14 lookups on index $rx. Less work is generally faster.

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