Why is my integer math with std::pow giving the wrong answer?

Question

Consider the following piece of code:

#include <iostream> #include <cmath>  int main() {     int i = 23;     int j = 1;     int base = 10;     int k = 2;     i += j * pow(base, k);     std::cout << i << std::endl; } 

It outputs "122" instead of "123". Is it a bug in g++ 4.7.2 (MinGW, Windows XP)?

Answer 1

std::pow() works with floating point numbers, which do not have infinite precision, and probably the implementation of the Standard Library you are using implements pow() in a (poor) way that makes this lack of infinite precision become relevant.

However, you could easily define your own version that works with integers. In C++11, you can even make it constexpr (so that the result could be computed at compile-time when possible):

constexpr int int_pow(int b, int e) {     return (e == 0) ? 1 : b * int_pow(b, e - 1); } 

Here is a live example.

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Tail-recursive form (credits to Dan Nissenbaum):

constexpr int int_pow(int b, int e, int res = 1) {     return (e == 0) ? res : int_pow(b, e - 1, b * res); }