Xref: news2.mv.net comp.os.msdos.djgpp:8039
From: wtanksle AT sdcc15 DOT ucsd DOT edu (William Tanksley)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: random() and rand() question
Date: 29 Aug 1996 20:21:25 GMT
Organization: University of California, San Diego
Lines: 44
Message-ID: <504u45$e83@sdcc12.ucsd.edu>
References: <DwvF3A DOT H2r AT interactive DOT net> <3225ABDD DOT 7AF48E03 AT alcyone DOT com>
NNTP-Posting-Host: sdcc15.ucsd.edu
To: djgpp AT delorie DOT com
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In article <3225ABDD DOT 7AF48E03 AT alcyone DOT com> Erik Max Francis <max AT alcyone DOT com> writes:
>J.J. Pierson wrote:

>> Silly question... How would I go about getting a random number from say 1
>> to a specified number?

>For a number between 1 and n, use 
>    rand()%n + 1.
>If n is an even power of 2, then bitwise-anding a mask will be faste:
>    rand()&(n - 1) + 1.

You'd be suprised at how bad these two methods are, especially with 
rand().  If you're using any congruential RNG you'll have to expect the 
lower bits to contain lots of nasty patterns.

A better plan in certain circumstances is to either use a different RNG 
(RC4 is wonderful, and very simple) or set up your conditions a little 
better.  For example, if speed isn't an issue, try

	rand()/(float)UINT_MAX * n

If speed is an issue, but you only need a certain range of n, try

	// throw away as many bad bits as you can afford,
	// never more than the maximum place value of your n;
	// for this example, n must be < 32.
	r = rand() >> 5;
	// factor the n into the equation
	r = r*n;
	// finally, divide the rest of the bits out, leaving
	// only the random bits affected by n.
	r = r >> (32-5);

This gives approximately the same results within its range as the floating 
point method, but is faster.  The more bits you can throw away (the 
number 5 in this example) the better for a congruential RNG, such as 
ANSI C uses.

If it was up to me, I'd use RC4.  I've developed a distrust of 
congruential RNGs, even Berkeley's random().

>Erik Max Francis, &tSftDotIotE   http://www.alcyone.com/max/   max AT alcyone DOT com

-Billy