Why Does `std::pow` Give Unexpected Results with Integer Math?

Why Integer Math with std::pow Produces Unexpected Results

When using std::pow for integer math, you may encounter unexpected outputs. In this scenario, a code snippet intending to set i = 23 1 * 10^2 results in 122, not 123.

This behavior stems from the floating-point nature of std::pow, which inherently lacks infinite precision. Implementations may exacerbate this issue through inefficient implementations.

Integer Power Function Implementation

To address this, a custom integer power function can be defined. In C 11 and later, a constexpr function ensures that the result can be computed at compile-time:

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

Tail-Recursive Variation

An alternative tail-recursive form, attributed to Dan Nissenbaum, allows for more efficient computation:

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

These alternatives provide accurate integer power computations, resolving the issue encountered with std::pow.

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