Question

In C Program

The first 11 prime integers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, and 31.

A positive integer between 1 and 1000 (inclusive), other than the first 11 prime integers, is prime if it is not divisible by 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, and 31.

Write a program that prompts the user to enter a positive integer between 1 and 1000 (inclusive) and that outputs whether the number is prime.

If the number is not prime, then output all the numbers, from the list of the first 11 prime integers, which divide the number.

Example (Numbers with underscore indicate an input):

Enter an integer between 1 and 1000 (inclusive): 62
62 is divisible by 2, 31.
62 is not prime.

--------------------------------------------

Enter an integer between 1 and 1000 (inclusive): -1
You must enter a number between 1 and 1000 (inclusive).

--------------------------------------------

Enter an integer between 1 and 1000 (inclusive): 1206
You must enter a number between 1 and 1000 (inclusive).

Answer 1

#include<stdio.h>
int isPrime(int num)
{
   int i;
   if (num <= 1) return 0;
   if (num % 2 == 0 && num > 2) return 0;
   for (i = 3; i < num / 2; i += 2)
   {
      if (num % i == 0)
         return 0;
   }
   return 1;
}

int main() {
   int n, i, f;
   printf("Enter an integer between 1 and 1000 (inclusive): ");
   scanf("%d", &n);
   
   if(n>=1 && n<=1000){
      if(isPrime(n)){
         printf("%d is prime.\n",n);
      }
      else{
         f = 0;
         printf("%d is divisible by ",n);
            
         for(i = 2;i<n;i++){
            if(n%i==0){
               if(f == 0){
                  printf("%d",i);    
               }
               else{
                  printf(", %d",i);  
               }
               f = 1;
            }
         }
         printf(".\n");
         printf("%d is not prime.\n",n);
      }
   }
   else{
      printf("You must enter a number between 1 and 1000 (inclusive).\n");
   }
   
   return 0;
}