Random float number generation

Question

How do I generate random floats in C++?

I thought I could take the integer rand and divide it by something, would that be adequate enough?

Answer 1

If you know that your floating point format is IEEE 754 (almost all modern CPUs including Intel and ARM) then you can build a random floating point number from a random integer using bit-wise methods. This should only be considered if you do not have access to C++11's random or Boost.Random which are both much better.

float rand_float()
{
    // returns a random value in the range [0.0-1.0)

    // start with a bit pattern equating to 1.0
    uint32_t pattern = 0x3f800000;

    // get 23 bits of random integer
    uint32_t random23 = 0x7fffff & (rand() << 8 ^ rand());

    // replace the mantissa, resulting in a number [1.0-2.0)
    pattern |= random23;

    // convert from int to float without undefined behavior
    assert(sizeof(float) == sizeof(uint32_t));
    char buffer[sizeof(float)];
    memcpy(buffer, &pattern, sizeof(float));
    float f;
    memcpy(&f, buffer, sizeof(float));

    return f - 1.0;
}

This will give a better distribution than one using division.