Re: using s/// as map?

I use it a lot in perlgolf, but not in real code since it tends to destroy the string (notice that your code does, though you tried to avoid that I think). In real code using a while on a regex in scalar context is slightly faster and not so dangerous. When programming an inner loop and trying to be blazingly fast, you also should avoid things like $1.$2 since constructing a new value is a relatively expensive operation in perl. So the regex variant I'd use is:

$count{$1}++ while $genome =~ /(?=(..))./g
[download]

But this is still twice as slow as the substr() variant.

Comment on Re: using s/// as map? Download Code

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Re: Re: using s/// as map? by Roy Johnson (Monsignor) on Nov 24, 2003 at 19:29 UTC
This should be a little better. `++$count{$1} while $genome =~ /\G(..)/g;` [download] The PerlMonks advocate for tr///	[reply] [d/l]
Re^3: using s/// as map? by thospel (Hermit) on Nov 24, 2003 at 19:46 UTC
You're only doing half the work now :-) You can make it work like this though: `++$count{$1},pos($genome)-- while $genome =~ /\G(..)/g` [download] It's interesting that this is slightly faster than dropping the \G. This however turns out to be a lot slower (seems to do a complete linear search for the proper string start every time): `$genome =~ /./g; $count{$1}++ while $genome =~ /(.\G.)/g` [download] update This however means that the lookahead version can be speeded up by about 5% too: `$count{$1}++ while $genome =~ /\G(?=(..))./g` [download]	[reply] [d/l] [select]
Re^3a: using s/// as map? by Roy Johnson (Monsignor) on Nov 24, 2003 at 20:05 UTC
You're only doing half the work now :-) But if I understand gene work (not a certainty at all), I'm doing the half they actually want to do. `:-P` A molecule isn't paired with both of its neighbors, they are grouped two-by-two. At least now I understand why they were doing the lookahead. The PerlMonks advocate for tr///	[reply]