On Fri, May 08, 2026 at 03:20:00PM +0200, Alejandro Colomar via Mutt-dev wrote:
>> I'd like to see evidence that compilers are unable to perform this
> optimization these days.
Sure!
$ gcc -O2 -o loop_vs_strspn loop_vs_strspn.c
$ ./loop_vs_strspn /////12345 /
Loop method time: 0.178080 seconds
strspn method time: 0.314601 seconds
Test program attached. The if block is necessary so that the compiler
doesn't completely optimize away the loop in the strspn case. It's
realistic since you're going to actually use the value in real code.
Note that I chose the values so the printf would not actually execute.
> But even if I saw evidence (which I doubt), I think the fact that it
> doesn't allow silly mistakes when writing the loop already counters the
> small efficiency theoretical gains.
I don't know what you mean by silly mistakes; regardless the code I
wrote was unit-tested and has no silly mistakes.
On Fri, May 08, 2026 at 03:16:24PM +0200, Alejandro Colomar via Mutt-dev wrote:
> The name means "string span". It is the span (length) of the substring
> composed exclusively of characters in the second parameter.
I know what it is and what it does. But it has a stupid name and it
is not used often, regardless of why. [By comparison, C++ has
std::string::find_first_not_of, which is a much clearer name.]
> I believe it's not often used because few people know it, and thus few
> people spread the knowledge of when it's useful and how.
Which is a great reason not to use it. The while loop solution is
both more efficient and more explicit. That means it wins every time
in my book.
> There are mainly two cases where it's useful:
There's really one, in my opinion: when you want to find the first
character not in a list of more than two or three characters.
Otherwise I'd use the loop.
> Then there are a few other cases, but these two cover the most common
> ones. Once you learn this function, it crams a lot of logic into a
> single function, making the code more compact and readable.
No it doesn't, as demonstrated by my test program. The while loop
version takes up almost exactly the same amount of space.
--
Derek D. Martin http://www.pizzashack.org/ GPG Key ID: 0xDFBEAD02
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#include <stdio.h>
#include <string.h>
#include <time.h>
#define ITERATIONS 100000000
int main(int argc, char **argv) {
char *str = argv[1];
char *accept = argv[2];
clock_t start, end;
double cpu_time_used[2];
// Timing the loop method
start = clock();
for (int i = 0; i < ITERATIONS; i++) {
char *ptr = str;
while (*ptr && *ptr == *accept) ptr++;
if (*ptr != '1') printf("%s\n", ptr);
}
end = clock();
cpu_time_used[0] = ((double)(end - start)) / CLOCKS_PER_SEC;
// Timing the strspn method
start = clock();
for (int i = 0; i < ITERATIONS; i++) {
char *ptr = NULL;
ptr = str + strspn(str, accept);
if (*ptr != '1') printf("%s\n", ptr);
}
end = clock();
cpu_time_used[1] = ((double)(end - start)) / CLOCKS_PER_SEC;
printf("Loop method time: %f seconds\n", cpu_time_used[0]);
printf("strspn method time: %f seconds\n", cpu_time_used[1]);
return 0;
}
