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From: John Forkosh <forkosh@panix.com>
Newsgroups: comp.lang.c
Subject: Re: Compute Unique Numbers in a Set
Date: Sun, 15 Jan 2023 02:49:50 -0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
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Albert <invalid@gmail.com> wrote:
> Is this the best way to generate unique random numbers in a set of 6 
> numbers? <snip>

I think what you're looking for (not sure whether or not it's already
been mentioned in this long thread) is a "partial permutation",
swapping used numbers with unused ones, but we don't need to save the
used ones. Here's my general solution, and cc -DTESTDRIVE for the
main() test driver...

/* ---
 * uniqrand.c
 * Generate nuniq random numbers in the range lorand...hirand
 * ---
 * (optional) command-line arguments:
 *    ./uniqrand  nuniq  lorand  hirand    seedval
 *                  100       1     999  864297531  <-- default test values
 * ------------------------------------------------------------------------ */

/* --- standard headers --- */
#include <stdio.h>
#include <stdlib.h>
#include <sys/param.h> /* for MIN(i,j) macro */

/* --- standard headers --- */
#include <stdio.h>
#include <stdlib.h>
#include <sys/param.h> /* for MIN(i,j) macro */

/* --- entry point --- */
int *uniqrand ( int nuniq, int lorand, int hirand ) {
  /* --- allocations and declarations --- */
  int  iuniq = 0;                       /* index 0...nuniq-1 */
  int  range = 1+abs(hirand-lorand);    /* random number range */
  int  *randvals  = NULL,               /* nuniq ints returned to caller */
       *rangevals = NULL;               /* all numbers lorand...hirand */
  /* --- initialization --- */
  if ( nuniq<1 || nuniq>range ) goto end_of_job; /* input error */  
  randvals  = (int *)malloc((nuniq+99)*sizeof(int)); /* caller free()'s */
  rangevals = (int *)malloc((range+99)*sizeof(int)); /* we free() work area */
  if ( randvals==NULL || rangevals==NULL ) goto end_of_job; /* malloc error */
  rangevals[0] = MIN(lorand,hirand);    /* in case hirand<lorand */
  for ( iuniq=1; iuniq<range; iuniq++ ) /* all numbers lorand...hirand... */
    rangevals[iuniq] = rangevals[0] + iuniq; /* ...or vice versa */
  /* --- generate nuniq unique random numbers from lorand to hirand --- */
  for ( iuniq=0; iuniq<nuniq; iuniq++ ) {
    int  thisrand = rand()%range;       /* random number 0...range-1 */
    randvals[iuniq] = rangevals[thisrand]; /* unique random lorand...hirand */
    rangevals[thisrand] = rangevals[--range]; } /* replace used number */
  /* --- end-of-job --- */
  end_of_job:
    if ( rangevals != NULL ) free((void *)rangevals);
    return ( randvals );
  } /* --- end-of-function uniqrand() --- */

#if defined(TESTDRIVE)
/* --- entry point --- */
int main ( int argc, char *argv[] ) {
  /* --- command-line arguments --- */
  int    nuniq = ( argc>1? atoi(argv[1]) :       100 ),
        lorand = ( argc>2? atoi(argv[2]) :         1 ),
        hirand = ( argc>3? atoi(argv[3]) :       999 ),
       seedval = ( argc>4? atoi(argv[4]) : 164897532 );
  /* --- allocations and declarations --- */
  int *uniqrand(), *randvals=NULL;      /* randvals from uniqrand() */
  int iuniq = 0;                        /* randvals[] index */
  /* --- initialization --- */
  if ( seedval > 0 ) srand(seedval);    /* initialize rand() */
  /* --- call uniqrand() and display results --- */
  if ( (randvals=uniqrand(nuniq,lorand,hirand)) == NULL ) goto end_of_job;
  for ( iuniq=0; iuniq<nuniq; iuniq++ )
    printf("%7d%s",randvals[iuniq],
           (iuniq==nuniq-1?"\n":((iuniq+1)%8==0?",\n":",")) );
  /* --- end-of-job --- */
  end_of_job:
    if ( randvals != NULL ) free((void *)randvals);
    exit ( 0 );
  } /* --- end-of-function main() --- */
#endif
/* --- end-of-file uniqrand.c --- */

John Forkosh  ( mailto:  j@f.com  where j=john and f=forkosh )