Question

C PROGRAM TO CHECK WHETHER A GIVEN NUMBER IS PRIME OR NOT

Answer 1

#include <stdio.h>
int main()
{
    int n, i, flag = 0;

    printf("Enter a positive integer: ");
    scanf("%d",&n);

    for(i=2; i<=n/2; ++i)                      /*here actually we are supposed to use (i<=n-1)  instead of  (i<=n/2),but both will give the answer*/
    {
        // condition for nonprime number
        if(n%i==0)
        {
            flag=1;
            break;
        }
    }

    if (flag==0)
        printf("%d is a prime number.",n);
    else
        printf("%d is not a prime number.",n);
    
    return 0;
}

Comments

only one problem:
after a certain number the program outputs a random number, like when i typed 4637375545 it said "342408249 is not a prime number"

Answer 2

#include <stdio.h>

int main()
{
    int num,i,count=0;
    printf ("Enter the number which you want to check whether it is prime number or not \n");
    scanf ("%d",&num);
    for (i=2;i<=num/2;i++)
    {
        if (num%i==0)
        {
            count++;
            break;
        }
    }
    if (count==0 && num!=1)
    {
        printf("%d is prime \n",num);
    }
    else
    printf("%d is not prime \n",num);
        
    return 0;
}