Chapter: Random Numbers

Here is the code for pseudorandom.c:

/* RPJ 25viii05 -- from code by Rahul Simha */

/* Uniform [0,1] using the standard Lehmer generator. 
   See http://www.seas.gwu.edu/~simhaweb/java/lectures/appendix/random.html
   for more details. */

#include < stdio.h>

const long m = 2147483647;
const long a = 48271;
const long q = 44488;
const long r = 3399;

long r_seed = 4071776L;  /* change this to anything you like */

double prandom ()
{
  long t, lo, hi;

  hi = r_seed / q;
  lo = r_seed - q * hi;
  t = a * lo - r * hi;
  if (t > 0)
    r_seed = t;
  else
    r_seed = t + m;
  return ( (double) r_seed / (double) m );
}

/* Uniform random number generator - returns a number between a and b */ 

double prandom_range (double a, double b)
{
  if (b > a)
    return ( a + (b-a) * prandom() );
  else { 
    printf ("ERROR: in prandom.prandom_range(): a=%lf b=%lf\n",a,b); return 0;
  }
}

/* generate randoms 0-99 to test */
main(argc, argv) 
     int argc;
     char *argv[];
{
  if (argc>1) r_seed = atoi(argv[1]);
  do {
    printf("%d\n", (int)prandom_range(0.0, 100.0));
    printf("ENTER to continue, q to quit > ");
  }
  while(getchar() != 'q');
}

If you compile this via gcc -o pseudorandom pseuedorandom.c and run it by simply typing pseudorandom you will get a sequence of pseudo-random numbers in the range 0-99. It will always be exactly the same sequence because the initial seed is fixed. By using argc and argv and atoi() you can select a new intial seed for each run. Typing pseudorandom 19071961 you will get a different sequence.

Q. 1

What is the point of this?

One last exercise. A good idea is to modify the above code.

Exercise 6

> cards 47
ace of clubs
ENTER to continue, q to quit > 
Joker
ENTER to continue, q to quit > 
deuce of diamonds
ENTER to continue, q to quit > 
ten of spades
ENTER to continue, q to quit > 
ace of hearts
ENTER to continue, q to quit > 
knave of spades
ENTER to continue, q to quit > 
queen of clubs
ENTER to continue, q to quit > 
king of hearts
ENTER to continue, q to quit > 
7 of diamonds
ENTER to continue, q to quit > q
>