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

From: Peter Eisentraut Tom Lane PostgreSQL Development Re: Re: [GENERAL] random() function produces wrong range 2000-08-04 22:03:19 Pine.LNX.4.21.0008050002340.1568-100000@localhost.localdomain (view raw or whole thread) 2000-08-04 22:03:19 from Peter Eisentraut  2000-08-06 02:05:21 from Tom Lane 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
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.

--
Peter Eisentraut                  Sernanders väg 10:115
peter_e(at)gmx(dot)net                   75262 Uppsala
http://yi.org/peter-e/            Sweden

```

### pgsql-hackers by date

 Next: From: The Hermit Hacker Date: 2000-08-04 22:31:00 Subject: Re: Installation layout idea Previous: From: Peter Eisentraut Date: 2000-08-04 22:01:42 Subject: Re: New Privilege model purposal

Privacy Policy | About PostgreSQL
Copyright © 1996-2016 The PostgreSQL Global Development Group