paulf@htsa.aha.nl (Paul Ferron) writes:
: could anybody out here mail me a true random number generator algorithm.
:
: I've found that the random number generators in C++  are not al that random.
:
: thanx
:
: Paul
:
: --
: --------------------------------------------------------------------------------
:    Kevin Landwaster son of Lorric        No smart comments are mine.
:       Arch-mage of Threefork.                  aka paulf@solist.htsa.aha.nl
: --------------------------------------------------------------------------------

This comes up so often that a solution should perhaps be in the FAQ.


Here's a copy of random.c as I used it in one of
my programs, together with reference s to the original article,
(which you *should* read -- it says a lot about
the care and feeding of this --and other -- random number generaE
tors.

#if 0


Ths following are the original random variations from
Random Number Generators: Good ones are hard to find,
by Stephen K. Park and Keith W. Miller,
in Communications of the ACM, October 1988 Volume 31 Number 10,
pages 1192 - 1201.

function Random : real;
  (* Integer version 1: maxint must be 2**46-1 or larger *)
const
  a = 16807;
  m = 2147483647;
begin
  seed := (a * seed) mod m;
  Random := seed / m
end;


correctness: The following should yield 1043618065

seed := 1;
for n := 1 to 10000 do
  u := Random;
Writeln('The current value of seed is : ', seed);



var seed : real;

function Random : real;
  (* real version 1: required 46-bit or larget mantissa
                                   (excluding the sign bit)
                *)
const
  a = 16807.0;
  m = 2147483647.0;
var
  temp : real;
begin
  temp := a * seed;
  seed := temp - m * Trunc(temp / m);
  Random := seed / m
end;



function Random : real;
  (* Integer Version 2 : required maxint of 2**31 or larger *)
const
  a = 16807;
  m = 2147483647;
  q = 127773;  (* m div a *)
  r = 2836;    (* m mod a *)
var
  lo, hi, test : integer;
begin
  hi := seed div q;
  lo := seed mod q;
  test := a * lo - r * hi;
  if test > 0 then
    seed := test
  else
    seed := test + m;
  Random := seed / m
end;

function random : real;
  (* Real version 2: requires 32-bit or larger mantissa;
                                   requires maxint at least 16807
                *)
const
  a = 16807.0;
  m = 2147483647.0;
  q = 127773.0; (* m div a *)
  r = 2836.0; (* m div a *)
var
  lo, hi, test : real;
begin
  hi := Trunc(seed / q);
  lo := seed - q * hi;
  test := a * lo - r * hi;
  if test > 0.0 then
    seed := test
  else
    seed := test + m;
  Random := seed / m
end;


TurboPascal version 3.0 needs 'real version 2' of random.
The '87' version of TurboPascal can use either 'real version'.

#endif

long seed = 1;

double Random(Void)
  /* Integer Version 2 : required maxint of 2**31 or larger */
#define  a 16807
#define  m 2147483647
#define  q 127773
 /* m div a */
#define  r 2836
 /* m mod a */
  { long int lo, hi, test;
    hi = seed / q;
    lo = seed % q;
    test = a * lo - r * hi;
    if(test > 0)
      seed = test;
    else
      seed = test + m;
    return ((double)seed) / ((double)m);
  }

main()
  { int i;
    double u;
    for(i = 1; i <= 10000; i++)
      { u = Random();
      }
    printf("\nThe current value of seed is %ld.", seed);
    printf("\nIt should be 1043618065.\n");
  }

    Hendrik Boom     CAM.ORG!ozrout!topoi!hendrik
    4133 Marcil      hendrik@topoi.uucp
    Montreal      topoi!hendrik@uunet.uu.net
       topoi!hendrik@vedge.com
       vedge!topoi!hendrik@sobeco.com


--
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at work: hendrik@vedge.com,  iros1!vedge!hendrik
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could anybody out here mail me a true random number generator algorithm.

I've found that the random number generators in C++  are not al that random.

thanx

Paul

--
--------------------------------------------------------------------------------
   Kevin Landwaster son of Lorric        No smart comments are mine.
      Arch-mage of Threefork.                  aka paulf@solist.htsa.aha.nl
--------------------------------------------------------------------------------