C Program to Find Prime Number

Find Prime Number

In this Program, you’ll learn how to find Prime number entered by User.

What is the prime number?
To understand this example, you should have the knowledge of C programming :
A number is considered as the prime number when it satisfies the below conditions.

  • It should be the whole number
  • It should be greater than 1
  • It should have only 2 factors. They are 1 and the number itself.

Example for prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23 etc.

Why 4, 6, 8, 9, 10, 12, 14, 15, 16 etc are not prime numbers?

Because the number 4 can be factored as 2*2 and 1*4. As per the rule of prime number, there should be 2 factors only. They are 1 and the number itself. But, number 4 has 2*2 also. Like this, all remaining numbers 6, 8, 9, 10, 12, 14, 15, 16 have factors other than 1 and the number itself. So, these are not called as prime numbers.

Program to find Prime Numbers Entered by User

#include <stdio.h>
int main()
    int n, i, flag = 0;

    printf("Enter a positive integer: ");

    for(i=2; i<=n/2; ++i)
        // condition for nonprime number

    if (flag==0)
        printf("%d is a prime number.",n);
        printf("%d is not a prime number.",n);
    return 0;


Please enter a number: 13
Entered number is 13 and it is a prime number.

C Program to find Prime Number
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