C Program to Check Whether a Number is Prime or Not

A prime number is a positive integer that is divisible only by 1 and itself. For example: 2, 3, 5, 7, 11, 13, 17.

Program to Check Prime Number

Output

Enter a positive integer: 29
29 is a prime number.

In the program, a for loop is iterated from i = 2 to i < n/2.

In each iteration, whether n is perfectly divisible by i is checked using:

if (n % i == 0) {
  flag = 1;
  break;
}

If n is perfectly divisible by i, n is not a prime number. In this case, flag is set to 1, and the loop is terminated using the break statement.

Notice that we have initialized flag as 0 during the start of our program.

So, if n is a prime number after the loop, flag will still be 0. However, if n is a non-prime number, flag will be 1.

Visit this page to learn how you can print all the prime numbers between two intervals.