Re: Re: [GENERAL] random() function produces wrong range

Tom Lane writes:

> Oh, that's interesting. What platform do you use? If RAND_MAX applies
> to random() on some machines that'd probably explain why the code is
> written like it is. But on my box (HPUX) the rand() function is old
> and crufty and considerably different from random().

rand() and RAND_MAX are defined by ANSI C, random() is a BSD-ism. I
suggest you use the former. Also, while you're at it, this is a snippet
from the C FAQ:

13.16: How can I get random integers in a certain range?

A: 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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Peter Eisentraut Sernanders väg 10:115
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