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C Program to Check Whether a Number is Prime or Not

In this example, you will learn to check whether an integer entered by the user is a prime number or not.

To understand this example, you should have the knowledge of the following C programming topics:


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

#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 non-prime
    if (n % i == 0) {
      flag = 1;
      break;
    }
  }

  if (n == 1) {
    printf("1 is neither prime nor composite.");
  } 
  else {
    if (flag == 0)
      printf("%d is a prime number.", n);
    else
      printf("%d is not a prime number.", n);
  }

  return 0;
}

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.

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