Efficiency of vector index access vs iterator access?

Without seeing the test harnesses, the compiler options, and how you measured the time, it's hard to say anything. Also, a good compiler may be able eliminate the loop in one case or the other, since the loop has no effect on the value returned. Still, depending on the implementation, it wouldn't surprise me to see iterators significantly faster than indexing (or vice versa).

When I compile both programs with -O2 (Linux, GCC 4.6.1), they run equally fast.

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