此代码中的哪个位置生成一个随机数？

Question

I came across rgba 's floating point random number generator:

http://rgba.org/articles/sfrand/sfrand.htm

The explanation is clear and the code is simple. There is one issue: I cannot figure out where the random bits come from. Code snippet below:

static unsigned int mirand = 1;

float sfrand( void )
{
    unsigned int a;
    mirand *= 16807;
    a = (mirand&0x007fffff) | 0x40000000;
    return( *((float*)&a) - 3.0f );
}

I did try to compile this function and always get a result of -0.995993 , which makes sense if mirand starts out as 1 . What am I missing, or is mirand simply the seed?

Answer 1

Truth is, that code does NOT generate random numbers... it generates pseudo-random numbers. Assuming you start with the same seed, a layman's way to imagine what's happening is that you are just going through a HUGE list of "random" numbers. Eventually, the numbers will repeat themselves, but a good formula is one that makes this repetition happen at a large interval.

To clarify, take a look at the rand formula that Excel uses: http://support.microsoft.com/kb/828795

Answer 2

The reason you're getting the same number every time is because this is in fact a pseudo random number generator just like regular ones. It has no seed so you will always get the same value on the nth call. If you make several subsequent calls to sfrand you'll see the results, which are uniform between -1 and 1:

-0.995993
0.347347
-0.130602
0.970146
-0.749159
0.883045

Just like if you were to call the normal rand() function without seeding it you'd see the same sequence. As discussed the number 16807 was chosen for good reason, so you can seed this number by calling the function a random number of times:

static unsigned int mirand = 1;

float sfrand(double seed)
{
    unsigned int a;
    mirand *= seed;
    a = (mirand&0x007fffff) | 0x40000000;
    return( *((float*)&a) - 3.0f );
}

int main()
{
    srand(time(NULL));
    int count = rand() % 1000 + 1
    for(int i = 0; i < count; ++i)
        sfrand();
}

This will just discard the first count values while still giving you a random seed, and all subsequent calls will still gain the performance boost intended by the function. Subsequent calls now return unique values:

codys-macbook-pro:~ cody$ ./a.out
0.166836
codys-macbook-pro:~ cody$ ./a.out
0.256372
codys-macbook-pro:~ cody$ ./a.out
-0.194259
codys-macbook-pro:~ cody$ ./a.out
-0.556834