Re: string.sub vs string.byte

["Ashwin Hirschi" <deery@...>]

> I am writing an app that must test every character in an input stream.
> As i understand it, "s1 = string.sub(s,n,n)" must construct a new string
> each time it is called.

Yes.

> Would "b = string.byte(s,n)" be appreciably faster ?

It's an interesting thought. I feel that stuff like this can usually best be benchmarked. So I wrote a little script to time for-loops containing just the strbyte and strsub calls [yes, I'm still using Lua4].

As it turns out strsub is about 1.4 times slower than strbyte. I expect Lua5 to give you approximately the same results. But you can easily test this yourself.

Obviously, the difference I measured may not matter at all to your particular application. The effect of this type of peep-hole optimisation can vary tremendously, depending on the rest of your system.

> (although all my comparisons would then be number, rather than string,
> and i believe slower)

It looks like your beliefs might be wrong on my machine [;-)].

As a side-note: if you intend to do *lots* of byte fetching you might want to do something like

	local strbyte = string.byte

before entering your byte-fetching section. It will save you time.

Ashwin.
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