Random float number generation

rand() can be used to generate pseudo-random numbers in C++. In combination with RAND_MAX and a little math, you can generate random numbers in any arbitrary interval you choose. This is sufficient for learning purposes and toy programs. If you need truly random numbers with normal distribution, you'll need to employ a more advanced method.

This will generate a number from 0.0 to 1.0, inclusive.

float r = static_cast <float> (rand()) / static_cast <float> (RAND_MAX);

This will generate a number from 0.0 to some arbitrary float, X:

float r2 = static_cast <float> (rand()) / (static_cast <float> (RAND_MAX/X));

This will generate a number from some arbitrary LO to some arbitrary HI:

float r3 = LO + static_cast <float> (rand()) /( static_cast <float> (RAND_MAX/(HI-LO)));

Note that the rand() function will often not be sufficient if you need truly random numbers.

Before calling rand(), you must first "seed" the random number generator by calling srand(). This should be done once during your program's run -- not once every time you call rand(). This is often done like this:

srand (static_cast <unsigned> (time(0)));

In order to call rand or srand you must #include <cstdlib>.

In order to call time, you must #include <ctime>.

Take a look at Boost.Random. You could do something like this:

float gen_random_float(float min, float max)
{
    boost::mt19937 rng;
    boost::uniform_real<float> u(min, max);
    boost::variate_generator<boost::mt19937&, boost::uniform_real<float> > gen(rng, u);
    return gen();
}

Play around, you might do better passing the same mt19937 object around instead of constructing a new one every time, but hopefully you get the idea.

Call the code with two float values, the code works in any range.

float rand_FloatRange(float a, float b)
{
    return ((b - a) * ((float)rand() / RAND_MAX)) + a;
}

On some systems (Windows with VC springs to mind, currently), RAND_MAX is ridiculously small, i. e. only 15 bit. When dividing by RAND_MAX you are only generating a mantissa of 15 bit instead of the 23 possible bits. This may or may not be a problem for you, but you're missing out some values in that case.

Oh, just noticed that there was already a comment for that problem. Anyway, here's some code that might solve this for you:

float r = (float)((rand() << 15 + rand()) & ((1 << 24) - 1)) / (1 << 24);

Untested, but might work :-)

rand() return a int between 0 and RAND_MAX. To get a random number between 0.0 and 1.0, first cast the int return by rand() to a float, then divide by RAND_MAX.

#include <cstdint>
#include <cstdlib>
#include <ctime>

using namespace std;

/* single precision float offers 24bit worth of linear distance from 1.0f to 0.0f */
float getval() {
    /* rand() has min 16bit, but we need a 24bit random number. */
    uint_least32_t r = (rand() & 0xffff) + ((rand() & 0x00ff) << 16);
    /* 5.9604645E-8 is (1f - 0.99999994f), 0.99999994f is the first value less than 1f. */
    return (double)r * 5.9604645E-8;
}

int main()
{
    srand(time(NULL));
...

I couldn't post two answers, so here is the second solution. log2 random numbers, massive bias towards 0.0f but it's truly a random float 1.0f to 0.0f.

#include <cstdint>
#include <cstdlib>
#include <ctime>

using namespace std;

float getval () {
    union UNION {
        uint32_t i;
        float f;
    } r;
    /* 3 because it's 0011, the first bit is the float's sign.
     * Clearing the second bit eliminates values > 1.0f.
     */
    r.i = (rand () & 0xffff) + ((rand () & 0x3fff) << 16);
    return r.f;
}

int main ()
{
    srand (time (NULL));
...