Re: Count string in string with loop increase 2, why is this not faster?

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Count string in string with loop increase 2, why is this not faster?
Date	Wed, 07 Feb 2018 15:20:16 -0800
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Bart Smissaert <bart.smissaert@gmail.com> writes:
> Managed to get it quite a bit faster still:
>
> SQLITE_API int sqlite3_CountCharInString(const wchar_t *strSearch, int nCount, const wchar_t *strChar)
> {
> 	int i;
> 	int total;
> 	total = 0;
>
> 	for (i = 0; i < nCount; i++)
> 	{
> 		if (*strSearch == *strChar) total++;
> 		total = total + (*strSearch == *strChar);
> 		strSearch++;
> 	}
> 	return total;
> }
>
> This is the line that speeds things up:
>
> total = total + (*strSearch == *strChar);

Here:

    if (*strSearch == *strChar) total++;
    total = total + (*strSearch == *strChar);

you're doing the same thing twice.  Did you accidentally leave the old
line in place when you added the modified version?

That's more briefly written as:

    total += (*strSearch == *strChar);

(the parentheses aren't strictly necessary), but the generated code
should be the same.

I might write it like this:

    for (i = 0; i < nCount; i++, strSearch++) {
        total += (*strSearch == *strChar);
    }

since i++ and strSearch++ are both part of the iteration logic.  (Others
might run screaming at the sight of the comma operator being used like
that.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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