#include <stdio.h>

#define FALSE 0
#define TRUE 1   /* we define 0 and 1 as false and true so we can use them later, so it's more obvious what the code means.*/

int main()
{
   int myNumber; /* the number the user enters is stored in this*/
   int isPrime = TRUE; /* we will set this to true, and if we find it not to be prime, we change it to false.*/
                        /* if after we've checked all divisors it is still true, we know it to be prime*/
   int i = 0; /*this variable is used as a counter*/
   printf("Please enter a positive integer: "); /* ask the user to enter a number to test the primality of */
   scanf("%d", &myNumber);
   
   if(myNumber == 1) /* first we deal with special cases and tell the user off if he/she enters a silly number*/
   {
      printf("1 is not prime.");
      return 0; /* return 0 ends the program */
   }
   else if(myNumber <= 0)
   {
      printf("Error. Please enter a positive number.");
      return 0;
   }
   
   for(i=2;i<myNumber;i++) /*in this for-loop we test to see whether every number below our number is a factor of it */
   {                       /*if any number is a factor, we deduce we don't have a prime*/
                           /*i successively takes values from 2 to myNumber-1.*/
      if(myNumber%i == 0) /*here, % calculates the remainder. so if our remainder is zero, we know we have a factor*/
      {
         printf("%d is divisible by %d so it is not prime.", myNumber, i);
         isPrime = FALSE;
         break; /*we've realized we don't have a prime, so this breaks the for-loop as we can stop searching for factors.*/
      }
   }
   
   if(isPrime) /*if-statements treat all non-zero values as true. and we defined TRUE as 1.*/
      printf("%d is prime!", myNumber);   
         
   return 0; /* exit the program */
}