Thread: How to create a No-repeat random array

Try this. It generates the number of random numbers you want, within the range of the numbers you want. If you want 10 random numbers, the range will be 0-9. If you want 100 random numbers, the range will be 0-99, etc.

Each number it gives, is unique. Edit: but see the next post for a better algorithm.

Code:

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

#define SIZE 10

int main() {
  int i, n; 
  int numbers[SIZE];
  time_t t;
  printf("\n\n\n");
  for(i=0;i<SIZE;i++) {  //initalize numbers
    numbers[i] = i;
  }
  srand((unsigned) time(&t));
  printf("\nUnique random numbers:\n");
  for(i=0;i<SIZE;i++) {
    n=rand() % SIZE; 
    if(numbers[n]<0)     //not unique, try again
      --i;
    else  {
      printf("%d\n", numbers[n]);   //got one
      numbers[n]=-1;     //mark that index taken, 
    }                    //thus marking that number
  }

  printf("\n\n\t\t\t     press enter when ready");
  (void) getchar(); 
  return 0;
}

Output:

Unique random numbers:
3
8
4
6
5
7
0
1
2
9

I just printed out the random numbers, but assigning them into another array, is nbd.

Originally Posted by Adak

Only one comparison is needed.

I'm not sure that's true, unless I'm missing your meaning. You say yourself in the code: "not unique, try again". The same index can be hit multiple times even though it's already been used.

Adak's way works, but here's a solution that doesn't require the algorithm to "try again". This solution would have a huge performance gain over Adak's for a large set of numbers. For a set of 4 numbers like the OP wants it really doesn't matter.

Code:

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

#define SIZE 10

int main(void)
{
  int numbers[SIZE];
  int i, n, tmp;

  srand(time(NULL));

  // Initialize the array
  for(i = 0;i < SIZE;++i)
    numbers[i] = i;

  // Shuffle the array
  for(i = 0;i < SIZE;++i)
  {
    n = rand() % SIZE;
    tmp = numbers[n];
    numbers[n] = numbers[i];
    numbers[i] = tmp;
  }

  // Iterate through the array. Your numbers are already random
  for(i = 0;i < SIZE;++i)
    printf("%d\n", numbers[i]);

  return 0;
}

A couple of my outputs:

Output 1:
4
1
2
9
8
6
3
5
0
7

Output 2:
7
9
0
2
6
1
3
4
8
5

The core of itsme86's suggestion is to shuffle the array, but note that the shuffle used is biased. If that is undesirable, you could write:

Code:

/* Shuffle the array */
for (i = SIZE; i > 1;)
{
    n = rand() % i; /* ignore the possible slight bias introduced with this range reducing method */
    --i;
    tmp = numbers[n];
    numbers[n] = numbers[i];
    numbers[i] = tmp;
}