#include <stdlib.h> i = rand();
"rand" returns a pseudo-random number. Each new call to "rand" returns a new number.
This implementation uses a "linear congruential" generator. These generators are very fast, but have the property that the lowermost bits of the number returned may not be particularly random. The number returned is a "good random number" in the sense that:
rand()/((double)RAND_MAX + 1)
is a good approximation to a uniform random distribution in the interval 0<= r < 1. Thus if you want to generate a floating point number in the range [0,X) (including zero, but strictly less than X), you could use the formula:
X * (rand() / ((double)RAND_MAX + 1))
but the following would be a little better.
rand()/(((double)RAND_MAX + 1) / X)
Thus, if you want to generate a random sequence of integers between 0 and N-1, you could use the formula:
rand()/(int)(((unsigned)RAND_MAX + 1) / N)
However, truncation in the denominator means that this will sometimes produce N. You could produce a floating point number and truncate to an integer.
(int)(rand() / (((double)RAND_MAX + 1)/ N));
but it is better just to discard the occasional N that is generated, as in
while (N <= (val = rand() / (RAND_MAX/N)));
expl c lib srand
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