Error finding and rectification of my C code for finding prime numbers

Question

The error that I'm struggling, with some examples

I have written a code for finding prime numbers using sieve of Eratosthenes algorithm, but the problem is my code works as it tends to be, but it shows an unspecified or undefined error for some numbers. It shows like "entering initial value (n) infinite times" as the error and for some numbers, I mentioned in the image, it works perfectly. As I started recently studying C in-depth I couldn't find what is going wrong. I request the code-ies to help me in this case, and also help me realize what is the mistake, why it happens and how to prevent it.

Edit: Actually it gets into error zone for smaller numbers, and as the number grows, it works fine and gets again into errors for some, like in the image specified.

Here's the code (modified to avoid garbage values):

#include <stdio.h>
int main()
{
    int n;
    printf("enter number: ");
    scanf("%d",&n);
    int arr[n],pr=2;
    for(int i=0;pr<=n;i++)
    {
        arr[i]=pr;
        pr++;
    }
    int j,k=0;
    while(arr[k]<=n)
    {
        for(j=2;j<n;j++)
        {
            for(k=0;k<n;k++)
            {
                if(arr[k]%j==0 && arr[k]>j)
                    arr[k]=0;
            }
        }
    }
    for(int i=0;i<n;i++)
    {
        if(arr[i]>0)
            printf(" %d",arr[i]);
    }
    printf("\n");
    return 0;
}

Answer 1

There appears to be a mismatch with regard to the number of elements in the array and how the elements of the array are being accessed.

Rather than loading the array with numbers it's simpler to just use the index number of each element of the array to represent each number. Then store either a true or false value -- a flag indicating whether the number represented is or is not prime -- in each array element. That is, for example, if arr[5] held a value of 1 (true) then it represents that the number 5 is considered prime.

Also when using a for loop it's a good idea to have all three of the loop's expressions use the same variable. Your first for loop uses i first. Then pr and n. The loop then uses i again. It's usually better, especially at first, to have a single variable control all three parts of a for loop.

/* Sieve of Eratosthenes.c
   standard implementation without the usual optimizations
*/

#include <stdio.h>

int main(void)
{
    printf("enter number (limit): ");
    int n;
    scanf("%d", &n);

    int arr[n + 1];                   // n + 1 to include array element 0
    for (int i = 2; i <= n; ++i) {    // skip array elements 0 and 1
        arr[i] = 1;                   // mark elements 2 to n as true (prime)
    } 

    for (int i = 2; i <= n; ++i) {    // skip array elements 0 and 1
        if (arr[i]) {                 // if true (prime)
            for (int j = i + i; j <= n; j += i) {  // cross off multiples
                arr[j] = 0;           // mark element as false (not prime)
            }
        }
    }

    printf("primes:");
    for (int i = 2; i <= n; ++i) {    // skip array elements 0 and 1
        if (arr[i]) {                 // if true (prime)
            printf(" %d", i);         //     print array element number  
        }
    }
    printf("\n");

    return 0;
}