The rand () function generates a pseudo-random number in the range [0, RAND_MAX]. You can initialize it with srand (seed). In most cases, however, we want random numbers in a much smaller range [0, M). What's the best way to do this?

Points to note:

• Most implementations of rand () are linear-congruential generators. They generate a pre-defined sequence from the given seed that exhibits some "randomness".
• rand (), therefore, should not be used when we require "serious" randomness (say for cryptographic applications).
• rand () also should not be used when we depend upon large set of random numbers. As a rule of thumb you can generate 1/10 of RAND_MAX before the series stops exhibiting random behaviour.

When we need to adjust the range from [0, RAND_MAX] to [0, M) | (M<RAND_MAX) we should not use:
        num = rand () % M;
as the result will focus on the lower bits of the rand () result, which are much less random than the higher bytes (for a linear congruential generator).

The recommended way to get a random number in the required range is:

        num = (int)(( ((double)rand ()) / ((double)(RAND_MAX) + 1.0) ) * M);

Points to note:

• We should not use the simpler: (rand () * M)/(RAND_MAX + 1) as either (or both) of (rand () * M) and (RAND_MAX + 1) could overflow.
• 1.0 is a literal double, hence no cast is required.