C Programming Code Examples C > Mathematics Code Examples C Program to Check Whether a Number is Prime or Not C Program to Check Whether a Number is Prime or Not A prime number is a positive integer which is divisible only by 1 and itself. For example: 2, 3, 5, 7, 11, 13 #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) { // 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; } If the for loop terminates when the test expression of loop i <= n/2 is false, the entered number is a prime number. The value of flag is equal to 0 in this case. If the loop terminates because of break statement inside the if statement, the entered number is a nonprime number. The value of flag is 1 in this case.
