Will strlen be calculated multiple times if used in a loop condition?

c++ c gcc optimization strlen
daisy picture daisy · Jul 6, 2012 · Viewed 7.1k times · Source

I'm not sure if the following code can cause redundant calculations, or is it compiler-specific?

for (int i = 0; i < strlen(ss); ++i)
{
    // blabla
}

Will strlen() be calculated every time when i increases?

Answer

Mike Seymour picture Mike Seymour · Jul 6, 2012

Yes, strlen() will be evaluated on each iteration. It's possible that, under ideal circumstances, the optimiser might be able to deduce that the value won't change, but I personally wouldn't rely on that.

I'd do something like

for (int i = 0, n = strlen(ss); i < n; ++i)

or possibly

for (int i = 0; ss[i]; ++i)

as long as the string isn't going to change length during the iteration. If it might, then you'll need to either call strlen() each time, or handle it through more complicated logic.

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