Generating random integer values within a range in C

Question

How do I go about generating random integer values between a range (in this case 1-12 including 1 and 12) in the C language?

I've read about seeding (srand()) and using rand() within a range but am unsure about how to go about it.

Edit: Here is what I have so far

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

// Craps Program
// Written by Kane Charles
// Lab 2 - Task 2

// 7 or 11 indicates instant win
// 2, 3 or 12 indicates instant los
// 4, 5, 6, 8, 9, 10 on first roll becomes "the point"
// keep rolling dice until either 7 or "the point is rolled"
//      if "the point" is rolled the player wins
//      if 7 is rolled then the player loses

int wins = 0, losses = 0;
int r, i;
int N = 1, M = 12;
int randomgenerator();


main(void){

  /* initialize random seed: */
  srand (time(NULL));
  /* generate random number 10,000 times: */
  for(i=0; i < 10000 ; i++){
     int r = randomgenerator();
     if (r = 7 || 11) {
        wins++;
     }
     else if (r = 2 || 3 || 12) {
        losses++;
     }
     else if (r = 4 || 5 || 6 || 8 || 9 || 10) {
        int point = r;
        int temproll;
        do
        {
            int temproll = randomgenerator();

        }while (temproll != 7 || point);

        if (temproll = 7) {
            losses++;
        }
        else if (temproll = point) {
            wins++;
        }
     }
  }
    printf("Wins\n");
    printf("%lf",&wins);
    printf("\nLosses\n");
    printf("%lf",&losses);
}

int randomgenerator(){
    r = M + rand() / (RAND_MAX / (N - M + 1) + 1);
    return r;
}

Answer 1

The simple way is

#include <stdlib.h>
#include <sys/time.h>

int main(void)
{
    struct timeval t1;
    gettimeofday(&t1, NULL);
    srand(t1.tv_usec * t1.tv_sec);

    int a = 1, b = 12;

    int val = a + (b-a) * (double)rand() / (double)RAND_MAX + 0.5;

    return 0;
}

Edit, since someone asked: You really do have to use floating point arithmetic to get this to come out right (or as right as it can given rand()'s limitations such as they are). Any solution which relies purely on integer arithmetic and rand() will of necessity use \ or %, and when this happens you will get roundoff error--where c and d are declared int and c = 5 and d = 2, for example, c/d == 2 and d/c == 0. When comes to sampling from a range, what happens is that in compressing the range [0, RAND_MAX] to [a, b], you have to do some kind of division operation since the former is so much larger than the latter. Then roundoff creates bias (unless you get really lucky and things evenly divide). Not a truly thorough explanation but I hope that conveys the idea.

Answer 2

You should use: M + rand() / (RAND_MAX / (N - M + 1) + 1)

Don't use rand() % N (which tries to return numbers from 0 to N-1). It is poor, because the low-order bits of many random number generators are distressingly non-random. (See question 13.18.)

Example code:

#include <stdio.h>      /* printf, scanf, puts, NULL */
#include <stdlib.h>     /* srand, rand */
#include <time.h>       /* time */
int main ()
{
  int r, i;
  int M = 1,
      N = 12;
  /* initialize random seed: */
  srand (time(NULL));

  /* generate number between 1 and 12: */
  for(i=0; i < 10 ; i++){ 
     r = M + rand() / (RAND_MAX / (N - M + 1) + 1);
     printf("\n%d", r);
  }
  printf("\n") ;
  return EXIT_SUCCESS;
}

It's working here at codepad.