C Program to Check Whether a Number is Prime or Not

Introduction

In number theory, a prime number is a number greater than 1 that has no divisors other than 1 and itself. Detecting whether a number is prime is a foundational concept in programming. This post provides a simple and efficient C program to check whether the given number is prime or not.

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Problem Statement

Write a C program that takes an integer input from the user and checks if it is a prime number. A prime number should be divisible only by 1 and itself.

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C Program to Check Prime Number

#include <stdio.h>

int main() {
    int num, i, isPrime = 1;

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

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

    if (isPrime)
        printf("%d is a Prime Number.\n", num);
    else
        printf("%d is Not a Prime Number.\n", num);

    return 0;
}

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Code Explanation

• Input: The user provides an integer number.
• Prime Check Logic:
  • If the number is less than or equal to 1, it’s not a prime.
  • The loop checks divisibility from 2 to num/2.
  • If divisible by any number in that range, it’s not a prime.
• Output: Displays whether the number is prime or not.

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Sample Output

Enter a number: 7  
7 is a Prime Number.

Enter a number: 12  
12 is Not a Prime Number.

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Why Use This Code

This program is useful for understanding how loops and conditionals work together. It is often used in introductory computer science and competitive programming tasks.

Conclusion

The C program to check prime number demonstrates fundamental logic and control flow. Understanding prime number logic is essential in many areas of algorithm development, including encryption and data processing.