What is the logic for random number generator in C programming?

Answer 1

There are many different ways of generating pseudo random numbers. Some of them are good. Some of them are not so good. It depends on the application. For a good read on this topic, I suggest "The Art Of Computer Programming", Donald E. Knuth, Volume 2, Seminumerical Algorithms.

One of the simpler way of generating pseudo random numbers is the linear congruential generator, wherein XN+1 = (AXN + B) mod C. Of course, the choice of A, B, and C is crucial to a successful implementation. Knuth has a lot to say about this generator, including the fact that it is sequentially correlated and, thus, not well suited for some applications. Nevertheless, many random number generators use this method, and they use some techniques for minimizing correlation and maximizing period. The technique used in one of the Microsoft Visual Studio runtime libraries has A=0x343FD, B=0x269EC3, and C=0x100000000, however, the return value is the value of X right shifted 16 places and AND'ed with 0x7FFF, forcing a return value between 0 and 32767, but a period much larger than that.

int rand(unsigned *seed) { /* assumes unsigned is 32 bits */

*seed = *seed * 0x343FD + 0x269EC3;

return (*seed >> 16) & 32767;

}