how to realize the floor division of Python in CPP? [duplicate]

Question

now i have a segment of code running online:

def div1(val, min, max, period):
    der = (max - min) / period # 0.005
    return val // der  # 41

# call div1
div1(0.21, 0, 1, 200)

when i call the function with div1(0.21, 0, 1, 200), i get the result of 41.

Now, i want to transform it to C++ language.

But in C++, i get 42 instead of 41.

int Box(double val, double min, double max, double period) {
    double der = (max - min) / period; // 0.005
    int final_val = floor(val / der); // 42
    return final_val;
}

Box(val, 0, 1, 200) // get 42

any thing wrong with my code? and how to fix it if i want to get the same result in C++?

ps: the code of Python CAN NOT be modified...