How to generate an array of random & unique integers in C?

Question

I am creating a C-prog that requires an array of 17 integers, all being less than 18 and unique. This is what I could do until now:

int ques_arr[17];
int x,y;
time_t t;
srand((unsigned)time(&t));

for(int a=0; a<17; a++)
{
x=rand()%18; //Assume that srand() has been declared in the program
 for(int aa=0; aa<17; aa++)
 {
  if(x==ques_arr[aa])
   { do{
        y=0;
        y=rand()%18;
        }while(y==ques_arr[aa]);
      x=y; 
      ques_arr[a]=x;
    }else ques_arr[a]=x;
  }
}

My current algorithm is that everytime rand() generates a number, that number will be checked throughout array whether same number already exists or not, if it does, rand() keeps on generating a number until a unique number is obtained and then it is stored in the array.If such a number doesn't exist in the array, it's directly fed in to it.

As of now, numbers stored in the array are not unique.

Any help would be appreciated.

Answer 1

This is not an optimal solution, your time complexity once you have fixed the problem in your code is O(n²), you can reduce the time complexity to O(n) using the "Knuth Shuffle algorithm":

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

static int rrand(int value)
{
    return (int)((double)value * (rand() / (RAND_MAX + 1.0)));
}

static void randomize(int arr[], int size)
{
    for (int idx = 0; idx < size; idx++)
    {
        arr[idx] = idx;
    }
    for (int idx = size; idx > 1; idx--)
    {
        int num = rrand(idx);
        int tmp = arr[idx - 1];

        arr[idx - 1] = arr[num];
        arr[num] = tmp;
    }
}

int main(void)
{
    srand((unsigned)time(NULL));

    int arr[17] = {0};
    int size = sizeof arr / sizeof *arr;

    randomize(arr, size);
    for (int idx = 0; idx < size; idx++)
    {
        printf("%d\n", arr[idx]);
    }
    return 0;
}