Rand_c

In translating one of my pet programs from SuperBasic (first it started in Pascal), I needed a number of random numbers. Not finding a random number generator in Small-C, I decided to write the program in Lattice C. But, Lattice C does not really have any QL graphic functions. So back I go to Small-C to find a random number generator (RNG).

I tried a number of RNGs but had no luck. I was limited by Small-C handling only 16 bit integers with a range from -32K to 32K. Herb Schaaf found that floating point numbers go beyond the 16 bit limitation. Soon, he sent me a RNG and a test program. I took the RNG and turned it into a seperate C function and file that can be included into your C programs.

The seed for the RNG is taken from the date. This works well if you need a large number of random numbers. But if you run the program again, you will start with the same seed. If you need to have a program run numerous times, either build it into the code so it only initialises the RNG once, or have the user type in a seed at runtime.

/*  rand_c

     For QL Small-C with floating point library.
     Gets seed from date().  To make more random you
     could call initrand() before every occurance of
     rand().

     USAGE:
        int num1[3], temp[3], num2;

        initrand();             * must call at least once *
        *temp = float(100);       * 100 in float *
        fmove(rand(),num1);       * put rand in num1 *
        *num1 = fmult(num1,temp); * num1 * 100 *
        num2 = int(num1);

*/

/*
  Global variables used for functions
  Small-C does not support Static Variables
*/

int rt, rnd[3], ra[3], rd[3], rm[3], rr[3];

initrand()
{
int t[2];

  *rm = float(125);
  *rd = atof("2796203");
  *t = date();
  *ra = float(t[1]);
}

rand()
{

  *ra = fmult(ra,rm);
  *ra = fdiv(ra,rd);
  rt = int(ra);
  *rr = float(rt);
  *rnd = fsub(ra,rr);
  *ra = fmult(rnd,rd);
  return rnd;
}

Herb, not one to say “good enough”, has also come up with another RNG. This one is based on the algorithm that the ZX81 and T/S 2068 use in thier RNG’s. This program is not set up to be included in your program, but with some minor work can be made to do so.

/*  ZXrand_c    April 26, 1991 11am
*        Using QL Small-C to
*     Generate Random Numbers insame way as ZX-81 & TS 2068
*/

#include <stdio_h>


int a[3], b[3], m[3], d[3];
int seed[3], random[3], i;
char c, strg[6], *ow; 

main()   {
  initz();
  printf("Choose an integer seed in the range from 0 to 65535 \n");
  gets(strg);
  *seed = atof(strg);
  printf("The seed will be %s\n",ftoa(seed,strg));
  printf("touch [SPACE BAR] to begin, q to quit\n");
  while((getc(c)) != 'q')   {
     rand();
     printf("%6s",ftoa(seed,strg));
  }
  printf("\nPress any key to exit\n");
  beep(10,1);
  for(i=0; i<20480; i++) {
     ;
  }
  beep(10,2);
  getc();
}

initz()  {
  *a = atof("65536");
  *b = atof("65537");   
  *m = atof("75");
  *d = atof("74");
}

rand()  {
  *random = fmult(m,seed);
  *random = fadd(random,d);
  *random = fmod();
  fmove(seed,random);
  }

fmod()   {
  while(fcmp(random,b) >0 ) {
     *random = fsub(random,b);
  }
return (*random);
}

_console() {
 mode(4);
 fopen(ow,"con","w");
 window(ow,512,256,0,0);
 paper(ow,0);
 ink(ow,4);
 border(ow,7,2);
 cls(ow);
}

/*  End of ZXrand_c Apr 26, 1991 noon **  */ 

Now that I know how to beat the 16 bit limit with floating point numbers, I have a few other RNGs that could be ported to Small-C. Plus, Herb still has a few more up his sleeves. If there is interest, I will publish these other RNGs.