Question 13.16

How can I get random integers in a certain range?

The obvious way,

	rand() % N		/* POOR */

(which tries to return numbers from 0 to N-1) is poor, because the low-order bits of many random number generators are distressingly non-random. (See question 13.18.) A better method is something like

	(int)((double)rand() / ((double)RAND_MAX + 1) * N)

If you're worried about using floating point, you could use

	rand() / (RAND_MAX / N + 1)

Both methods obviously require knowing RAND_MAX (which ANSI #defines in <stdlib.h>), and assume that N is much less than RAND_MAX.

(Note, by the way, that RAND_MAX is a constant telling you what the fixed range of the C library rand function is. You cannot set RAND_MAX to some other value, and there is no way of requesting that rand return numbers in some other range.)

If you're starting with a random number generator which returns floating-point values between 0 and 1, all you have to do to get integers from 0 to N-1 is multiply the output of that generator by N.

References: K&R2 Sec. 7.8.7 p. 168
PCS Sec. 11 p. 172

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