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

From: Peter Eisentraut <peter_e(at)gmx(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL Development <pgsql-hackers(at)postgreSQL(dot)org>
Subject: Re: Re: [GENERAL] random() function produces wrong range
Date: 2000-08-04 22:03:19
Message-ID: Pine.LNX.4.21.0008050002340.1568-100000@localhost.localdomain (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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
        (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.

Peter Eisentraut                  Sernanders väg 10:115
peter_e(at)gmx(dot)net                   75262 Uppsala            Sweden


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