Random Number Range Problem.

Hello all,
I am trying to create a random number between a range of certain numbers, lets say 1 -10. I am using this below but for some reason it gives me the same number most of the time.

Code:

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int rand_func(int min, int max )
//this function takes 2 arguments: min and max
// and creates random numbers based on min an max and loads it into i

{

int i = 0;
i = rand();

printf("\t%d", i);

i = min + (max - min) * i / RAND_MAX; // min and max forumula
printf("\t%d\n",i);






  return i;

}

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I appreciate any response. Thank You!

I know this one, I KNOW THIS ONE - Wooooooo!!!!!
http://faq.cprogramming.com/cgi-bin/...&id=1043284385

Finally, I get to answer a post where the answer is in the FAQ - rejoice.

and another superb article:
http://www.eternallyconfuzzled.com/articles/rand.html

Quote:

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Originally Posted by xamlit

Hello all,
I am trying to create a random number between a range of certain numbers, lets say 1 -10. I am using this below but for some reason it gives me the same number most of the time.

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It does'nt give you the same number most of the time.
But max isn't behaving.

Code:

---


int rand_func(int min, int max )
{
int i = 0;
i = rand();

/* printf("%d : ", i); */
i = min + (max - min) * i / RAND_MAX;
/* printf("\t%d\n\n",i); */

return i;
}


void calc_avg(int min, int max, int cnt)
{
int  range;
int  i;
int  res;
int  arr[100];


range = max - min + 1;
if ( range > sizeof(arr)/sizeof(arr[0]) )
  {
  fprintf(stderr,"Enlarge arr to %d\n", range);
  return;
  }

memset(arr, 0, sizeof(arr));

for ( i = 0 ; i < cnt ; ++i )
    {
    res = rand_func(min, max);
    ++arr[ res - min ] ;
    }

printf("Resultes with min = %d max = %d. numbers of repeats %d :\n"
        ,min, max, cnt);
printf("Best stat is %.4f%%\n", 1.0/range*100.0);
for ( i = 0 ; i < range ; ++i)
    printf("%4d : %7d  stat = %.4f%%\n",
            i + min, arr[i],
            (arr[i]/(double)cnt)*100.0);
}



int  main(void)
{

calc_avg(6, 8, 9999999);
calc_avg(1, 10, 9999999);
calc_avg(12001, 12009, 9999999);
calc_avg(19001, 19015, 9999999);
getchar();

return(0);
}

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Resultes with min = 6 max = 8. numbers of repeats 9999999 :
Best stat is 33.3333%
6 : 5001173 stat = 50.0117%
7 : 4998537 stat = 49.9854%
8 : 289 stat = 0.0029%
Resultes with min = 1 max = 10. numbers of repeats 9999999 :
Best stat is 10.0000%
1 : 1112069 stat = 11.1207%
2 : 1110040 stat = 11.1004%
3 : 1110804 stat = 11.1080%
4 : 1109341 stat = 11.0934%
5 : 1110923 stat = 11.1092%
6 : 1111714 stat = 11.1171%
7 : 1110934 stat = 11.1093%
8 : 1112303 stat = 11.1230%
9 : 1111575 stat = 11.1158%
10 : 296 stat = 0.0030%
Resultes with min = 12001 max = 12009. numbers of repeats 9999999 :
Best stat is 11.1111%
12001 : 1249934 stat = 12.4993%
12002 : 1248477 stat = 12.4848%
12003 : 1251114 stat = 12.5111%
12004 : 1247631 stat = 12.4763%
12005 : 1250520 stat = 12.5052%
12006 : 1249625 stat = 12.4963%
12007 : 1251098 stat = 12.5110%
12008 : 1251295 stat = 12.5130%
12009 : 305 stat = 0.0031%
Resultes with min = 19001 max = 19015. numbers of repeats 9999999 :
Best stat is 6.6667%
19001 : 712749 stat = 7.1275%
19002 : 712608 stat = 7.1261%
19003 : 714874 stat = 7.1487%
19004 : 714646 stat = 7.1465%
19005 : 715011 stat = 7.1501%
19006 : 714185 stat = 7.1419%
19007 : 715245 stat = 7.1525%
19008 : 714332 stat = 7.1433%
19009 : 714068 stat = 7.1407%
19010 : 713911 stat = 7.1391%
19011 : 714799 stat = 7.1480%
19012 : 714357 stat = 7.1436%
19013 : 714453 stat = 7.1445%
19014 : 714458 stat = 7.1446%
19015 : 303 stat = 0.0030%

@dude543: How did you get this results ?
The rand_func(int min, int max ) as posted can only return 2 values ( min most of the time, max sometimes ).

Kurt

I am not sure if I got you right. but rand_func should return one value in the range min, max.

I do understand what you want it to return but:

Code:

---

i = min + (max - min) * i / RAND_MAX;

---

here you divide the int i by anoter int RAND_MAX that means the result is another int that is always 0 if i is smaller then RAND_MAX.
So this function should return min most of the time.
Kurt

Code:

---

Lets say for exapmle :
min = 6 and max = 8

Lets assume RAND_MAX=13
so we get

i = 6 + 2 * {0,13}/13;

for rand>= 0 && rand<=6  we get i = 6
for rand>= 7 && rand<=12  we get i = 7
for rand>= 13 && rand<=13  we get i = 8

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{0,13}/13; is an integer division -> result is 0 for any value but 13.
it works only if you cast. e.g

Code:

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i = min + (max - min) * (double)i / RAND_MAX;

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try this if you don't believe

Code:

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#include <stdio.h>
#include <stdlib.h>

int rand_func(int min, int max )  {
  int i = 0;
  i = rand();

  i = min + (max - min) * i / RAND_MAX;

  return i;
}

int rand_func_1(int min, int max )  {
  int i = 0;
  i = rand();

  i = min + (max - min) * (double)i / RAND_MAX;

  return i;
}

int main() {
  int i;
  for ( i =0; i < 10; ++i )
      printf( "%d ", rand_func( 5, 10 ) );
  puts("");

  for ( i =0; i < 10; ++i )
      printf( "%d ", rand_func_1( 5, 10 ) );
}

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my output

Code:

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5 5 5 5 5 5 5 5 5 5
9 5 7 9 6 9 5 9 5 9

---

Kurt

They are in red.

Code:

---

Lets say for exapmle :
min = 6 and max = 8

Lets assume RAND_MAX=13
so we get

// i = 6 + 2 * {0,13}/13;

i = 6 + ( 2 * {0,13} )  /13;

for rand>= 0 && rand<=6  we get i = 6
for rand>= 7 && rand<=12  we get i = 7
for rand>= 13 && rand<=13  we get i = 8

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No comment.

I think the best idea is to read the faq, which will inspire something like this:

Code:

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int
get_rand (int min, int max)
{
  static int seeded = 0;
  if (seeded == 0)
  {
    srand (time (NULL));
    seeded = 1;
  }
  return rand() % (max - min + 1) + min;
}

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