How to Identify a Prime Number Using C Program

In this blog, we will learn the secrets of using while loops, functions, and recursion. Let us decode the complexities of prime numbers and improve your C programming abilities. Together, we will uncover the simplicity behind the complexity and give you the tools you need to effectively use prime numbers.

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C Program to Check Whether a Number is Prime or Not

A prime number is a positive integer greater than 1 with no positive divisors other than 1 and itself. In simpler terms, a prime number is a whole number greater than 1 that cannot be formed by multiplying two smaller natural numbers other than 1 and itself. 

For example, 2, 3, 5, 7, 11, and 13 are prime numbers.

In C programming, there are different techniques to check whether a number is prime. Some of them are mentioned below.

Using While Loop

Refer to the given code:

#include <stdio.h>
int main() {
    int num, i = 2;
    int isPrime = 1;  // Assume the number is prime initially
    // Input from user
    printf("Enter a positive integer: ");
    scanf("%d", &num);
    // Special case: 0 and 1 are not prime numbers
    if (num < 2) {
        isPrime = 0;
    } else {
        // Check for factors from 2 to the square root of the number
        while (i * i <= num) {
            if (num % i == 0) {
                // If the number is divisible by i, it's not prime
                isPrime = 0;
                break;
            }
            i++;
        }
    }
    // Output the result
    if (isPrime) {
        printf("%d is a prime number.\n", num);
    } else {
        printf("%d is not a prime number.\n", num);
    }
    return 0;
}

Output:

Enter a positive integer: 33 is a prime number.

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Using Functions

Refer to the given code:

#include <stdio.h>
#include <stdbool.h>
// Function to check whether a number is prime
bool isPrime(int num) {
    if (num <= 1) {
        return false; // 0 and 1 are not prime numbers
    }
    for (int i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            return false; // If num is divisible by any number from 2 to sqrt(num), it's not prime
        }
    }
    return true; // If no divisors found, num is prime
}
int main() {
    int number;
    // Input from user
    printf("Enter a positive integer: ");
    scanf("%d", &number);
    // Check and display the result
    if (isPrime(number)) {
        printf("%d is a prime number.\n", number);
    } else {
        printf("%d is not a prime number.\n", number);
    }
    return 0;
}

Output:

Enter a positive integer: 11 is not a prime number.

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Using Recursion

Refer to the given code:

#include <stdio.h>
// Function to check if a number is prime
int isPrime(int num, int i) {
    // Base cases
    if (i == 1) {
        return 1; // Prime
    } else {
        // If the number is divisible by any number other than 1 and itself, it's not prime
        if (num % i == 0) {
            return 0; // Not prime
        } else {
            // Recursively check for the next divisor
            return isPrime(num, i - 1);
        }
    }
}
int main() {
    int num;
    // Input a number from the user
    printf("Enter a positive integer: ");
    scanf("%d", &num);
    // Check if the number is greater than 1
    if (num > 1) {
        // Call the recursive function to check if the number is prime
        if (isPrime(num, num / 2)) {
            printf("%d is a prime number.\n", num);
        } else {
            printf("%d is not a prime number.\n", num);
        }
    } else {
        printf("Please enter a positive integer greater than 1.\n");
    }
    return 0;
}

Output:

Enter a positive integer: 6767 is a prime number.

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Wrap-Up

Understanding prime numbers in C is essential for many programming applications. In this blog, we’ve explored the concept of prime numbers. Through a step-by-step guide and a C code snippet, we’ve shown how to determine whether a number is prime or not using a basic algorithm. This knowledge is fundamental not only for identifying prime numbers but also for various mathematical computations, cryptography, and problem-solving within the domain of computer science. Mastering prime number detection in C lays a strong foundation for tackling more complex programming challenges.

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