Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!eurtrx!euraiv1!reino
From: reino@cs.eur.nl (Reino de Boer)
Newsgroups: comp.lang.pascal
Subject: Re: Random numbers
Message-ID: <1991Mar8.110210.22323@cs.eur.nl>
Date: 8 Mar 91 11:02:10 GMT
References: <1991Mar7.193727.1629@tjhsst.vak12ed.edu> <1991Mar8.031013.14902@casbah.acns.nwu.edu>
Reply-To: reino@cs.eur.nl
Organization: Erasmus University Rotterdam
Lines: 81

In <1991Mar8.031013.14902@casbah.acns.nwu.edu> ajburke@casbah.acns.nwu.edu (Austin Burke) writes:


>In article 1624 John Mitchell writes:

>>   How does TP 5.5 generate random numbers?
> ......
>	There is no internal list. The formula used is, approximately, the
>following:
>     seed = (multiplier * seed) + (incrementor mod modulus)

This probably is:
	seed = ((multiplier * seed) + incrementor) mod modulus
where "modulus == computer word size" to simplify the calculation.

>>P.P.S.  Would anyone be able to send me a formula for generating
>>good random numbers that might be substituted for the TP function
>>call?

Check out Volume 2 of
	Donald E. Knuth -- The Art of Computer Programming
The first half of this book is entirely dedicated to pseudo random
numbers and their generation.

Following are some routines I recently used to check some properties of
prime numbers (this is sun-pascal, although I think it's even level 0):

function random1 : float;

  const a = 1664525.0;
	c = 1.0;
	m = 4294967296.0;

  begin
	seed1 := modulo( seed1 * a + c, m );
	random1 := seed1 / m
  end; { random1 }

function randint1( max : int ) : int;

  begin
	randint1 := floor( random1 * max )
  end; { randint1 }

function random2 : float;

  const a = 17059465.0;
	c = 1.0;
	m = 34359738368.0;

  begin
	seed2 := modulo( seed2 * a + c, m );
	random2 := seed2 / m
  end; { random2 }

function randint2( max : int ) : int;

  begin
	randint2 := floor( random2 * max )
  end; { randint2 }

function randnumber( digits : integer ) : int;

  var	result	: int;
	i	: integer;

  begin
	result := floor( random( 9.0 ) * 9.0 + 1.0 );
	for i := 2 to digits do
	  result := 10.0 * result + floor( random( 10.0 ) * 10.0 );
	randnumber := result
  end; { randnumber }

If needed, I can look up the implementation of modulo() and floor() for
you, although they are fairly trivial.

Hope this helps -- Reino
-- 
Reino R. A. de Boer     "We want to build the right product right, right?"
Erasmus University Rotterdam ( Informatica )
e-mail: reino@cs.eur.nl