Prime Number Program in C

About this program

This is an example program in c other programs. Read the concept first: If Else Statement in C, then study the code and output below.

Introduction

A prime number is a natural number greater than 1 that has no divisors other than 1 and itself. Checking primality is a classic C exercise that combines loops, conditionals and the modulo operator.

Algorithm

1. If n <= 1, it is not prime.
2. Test divisibility from 2 to n/2 (or up to sqrt(n) for efficiency).
3. If any number divides n evenly, n is not prime.
4. Otherwise, n is prime.

Prime Number Check Program

#include <stdio.h>

int main(void) {
    int n, i, isPrime = 1;

    printf("Enter a positive integer: ");
    scanf("%d", &n);

    if (n <= 1) {
        isPrime = 0;
    } else {
        for (i = 2; i <= n / 2; i++) {
            if (n % i == 0) {
                isPrime = 0;
                break;
            }
        }
    }

    if (isPrime)
        printf("%d is a prime number\n", n);
    else
        printf("%d is not a prime number\n", n);

    return 0;
}

Sample Output

Enter a positive integer: 17
17 is a prime number

Enter a positive integer: 20
20 is not a prime number

Print Primes from 1 to N

#include <stdio.h>

int main(void) {
    int n, num, i, count = 0;

    printf("Enter upper limit: ");
    scanf("%d", &n);

    printf("Prime numbers up to %d:\n", n);
    for (num = 2; num <= n; num++) {
        for (i = 2; i <= num / 2; i++)
            if (num % i == 0) break;
        if (i > num / 2) {
            printf("%d ", num);
            count++;
        }
    }
    printf("\nTotal: %d primes\n", count);
    return 0;
}

Sample Output

Enter upper limit: 30
Prime numbers up to 30:
2 3 5 7 11 13 17 19 23 29
Total: 10 primes

Related Programs

See also Odd Or Even and Print 1 to N.