Ceil function how can we implement it ourselves

Question

I'm aware that C++ has a ceiling function.

I was wondering how we might implement the ceil function in C++ for practise.

The method's signature is public static int ceil (float num)

Please share your thoughts.

I came up with an easy solution:

Convert num to a string, locate the decimal point's index, and see if the decimal component is bigger than zero.

Return num+1 if yes; else, return num.

But I don't want to use the string conversion.

Answer 1

You can disassemble the components of an IEEE754 floating point number and put the logic together yourself:

#include <cstring>

float my_ceil(float f)
{
    unsigned input;
    memcpy(&input, &f, 4);
    int exponent = ((input >> 23) & 255) - 127;
    if (exponent < 0) return (f > 0);
    // small numbers get rounded to 0 or 1, depending on their sign

    int fractional_bits = 23 - exponent;
    if (fractional_bits <= 0) return f;
    // numbers without fractional bits are mapped to themselves

    unsigned integral_mask = 0xffffffff << fractional_bits;
    unsigned output = input & integral_mask;
    // round the number down by masking out the fractional bits

    memcpy(&f, &output, 4);
    if (f > 0 && output != input) ++f;
    // positive numbers need to be rounded up, not down

    return f;
}