2️⃣C Program to check Prime Number(With Program and Explanation)

 

C Program to Check Prime Number

🔹 Introduction

A prime number is a natural number greater than 1 that has exactly two distinct positive divisors: 1 and itself. In this post, we will write a C program to check whether a given number is prime or not.

This program is useful for beginners to understand loops, conditions, and basic number logic in C.

🔹 Problem Statement

Write a C program to check whether a given number is a prime number or not.

🔹 Algorithm

  1. Start

  2. Read an integer n

  3. If n ≤ 1, it is not prime

  4. Check divisibility from 2 to n/2

  5. If divisible, number is not prime

  6. Otherwise, number is prime

  7. Stop

🔹 Source Code (C Program)

#include <stdio.h>

int main() {
    int n, i, flag = 0;

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

    if (n <= 1) {
        flag = 1;
    }

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

    if (flag == 0)
        printf("%d is a Prime Number.", n);
    else
        printf("%d is not a Prime Number.", n);

    return 0;
}

🔹 Output

Example 1:

Enter a number: 7
7 is a Prime Number.

Example 2:

Enter a number: 10
10 is not a Prime Number.

🔹 Explanation

  • The program first checks if the number is less than or equal to 1

  • It then checks divisibility from 2 to n/2

  • If the number is divisible by any value in this range, it is not prime

  • Otherwise, it is a prime number

🔹 Time Complexity (Beginners will learn this concept later on)

  • O(n) (can be optimized to √n)

🔹 Optimized Version (Optional – Extra Value for Readers)

   # (Beginners will learn this concept later on)

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

int main() {
    int n, i, flag = 0;

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

    if (n <= 1) {
        printf("%d is not a Prime Number.", n);
        return 0;
    }

    for (i = 2; i <= sqrt(n); i++) {
        if (n % i == 0) {
            flag = 1;
            break;
        }
    }

    if (flag == 0)
        printf("%d is a Prime Number.", n);
    else
        printf("%d is not a Prime Number.", n);

    return 0;
}

🔹 Conclusion

This C program efficiently checks whether a number is prime. Beginners should first understand the basic version and then move to the optimized approach using sqrt(n).

📘 C Programming Series – What’s Next?

👉 In the next post of this series, we will learn how to write a C Program to Check Armstrong Number, with complete explanation and output examples. Stay tuned! 🚀


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