C Program to Print Prime Numbers up to n

Prime numbers are fundamental in computer science, mathematics, and cryptography. If you’re learning C programming, understanding how to work with prime numbers is a must. In this blog, we’ll walk through a simple yet efficient C program to print all prime numbers from 1 to a user-specified number n.

Whether you’re a student or preparing for coding interviews, this practical guide will help you grasp the concept of prime number logic and enhance your C programming skills.

βœ… What is a Prime Number?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. Examples include 2, 3, 5, 7, 11, etc.

Note: 1 is not a prime number, and 2 is the smallest (and only even) prime number.

🧠 Logic to Find Prime Numbers

To determine if a number is prime:

  1. Skip numbers less than or equal to 1.
  2. For numbers greater than 2, check if they are divisible by any number from 2 to √n.
  3. If divisible, it’s not a prime number.
  4. If not divisible by any, it’s a prime number.

πŸ’» C Program to Print Prime Numbers from 1 to n

cCopyEdit#include <stdio.h>
#include <math.h>

int isPrime(int num) {
    if (num <= 1) return 0;
    if (num == 2) return 1;

    for (int i = 2; i <= sqrt(num); i++) {
        if (num % i == 0) return 0;
    }
    return 1;
}

int main() {
    int n;

    printf("Enter the value of n: ");
    scanf("%d", &n);

    printf("Prime numbers from 1 to %d are:\n", n);
    for (int i = 1; i <= n; i++) {
        if (isPrime(i)) {
            printf("%d ", i);
        }
    }

    return 0;
}

πŸ“Œ Sample Output

sqlCopyEditEnter the value of n: 20  
Prime numbers from 1 to 20 are:  
2 3 5 7 11 13 17 19

πŸš€ Key Takeaways

  • Prime number checking is a classic C programming problem.
  • Always optimize using sqrt(n) for better performance.
  • Wrap logic in a reusable function for cleaner code.

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