Why Does `std::pow` Produce Unexpected Results with Integers in C ?

Explanation of Unexpected Integer Math Result Using std::pow

In C code, integer arithmetic can sometimes behave unexpectedly when combined with floating-point operations. Consider the following code snippet:

#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;
}

This code outputs "122" instead of the expected "123". This is because std::pow() operates with floating-point numbers, which do not possess infinite precision. The implementation of std::pow() may introduce inaccuracies that manifest in integer operations.

Solution Using Custom Integer Power Function

To remedy this issue, it is advisable to define a custom integer power function. This function will provide precise integer calculations. In C 11 and later, you can define a constexpr integer power function:

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

Performance Optimization

For improved performance, you can utilize a tail-recursive version of the function:

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

Conclusion

By employing a custom integer power function, you can accurately perform integer calculations, addressing the precision limitations of floating-point arithmetic and resolving the unexpected results encountered with std::pow() for integer operations.

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