# C Program to determine whether given number is prime or not

A natural number which is perfectly divisible by 1 and the number itself then that number is called as prime. So, it has only two divisors i.e., 1 and the number itself. This program finds whether given number is prime or not by checking the remainder of that number when divided by a number(every natural number) starting from 2 to (n/2). So if we get remainder as zero means it has divisors other than 1 and the number itself, Which implies that the given number is not prime number otherwise given number is a prime number.

# Source Code :

#include<stdio.h> int main( ) { int n, r, d=2; printf( "\n Enter the number :"); scanf("%d", &n); // Step 1 r = 1; while(d <= n/2) // Step 2 { r = n%d; // Step 2 (i) if(r == 0) break; // Step 2 (ii) d++; // Step 2 (iii) } if(r==0) printf("\n IT IS NOT A PRIME"); // Step 3 else printf("\n IT IS A PRIME"); // Step 3 return 0; }

# Sample Test cases :

1. Enter the number : 5 IT IS A PRIME 2. Enter the number : 10 IT IS NOT A PRIME 3. Enter the number : 11 IT IS A PRIME

## Explanation for Prime Number:

Step 1 : The program reads a number from the user using scanf() function.

Step 2 : The while loop is iterated from 2 to (n/2) to find out remainder when given number is divided by every natural number staring from 2 to (n/2). Generally you’ll find programs checking till n which is of no use ðŸ˜‰

Step 2 (i) : The remainder, When given number is divided by the ‘d’ is found and stored in ‘r’.

Step 2 (ii) : If we get ‘r’ as zero in middle this implies that given number is not a prime number, So we exit from the loop and move to step3.

Step 2 (iii): If remainder is non-zero then we increment the loop counter to next number.

Step 3 : Until loop ends if ‘r’ is non-zero then it implies that given number is prime number.

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