Why am I getting a different result from std::fmod and std::remainder

Question

In the below example app I calculate the floating point remainder from dividing 953 by 0.1, using std::fmod

What I was expectin

Answer 1

Because they are different functions.

std::remainder(x, y) calculates IEEE remainder which is x - (round(x/y)*y) where round is rounding half to even (so in particular round(1.0/2.0) == 0)

std::fmod(x, y) calculates x - trunc(x/y)*y. When you divide 953 by 0.1 you may get a number slightly smaller than 9530, so truncation gives 9529. So as the result you get 953.0 - 952.9 = 0.1