C Programming for Prime Numbers

Prime numbers are natural numbers greater than 1 that are only divisible by 1 and themselves. In this article, we will discuss how to write C code to identify prime numbers using different approaches.

Approach 1: Brute force method The brute force method involves checking if a number n is divisible by any number between 2 and n-1. If n is divisible by any number between 2 and n-1, then it is not a prime number. Otherwise, it is a prime number.

Code implementation: The following code implements the brute force method to identify prime numbers:

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

int isPrime(int n)
{
    int i;

    // check if n is a multiple of 2
    if (n % 2 == 0) {
        return 0;
    }

    // check for all odd numbers
    for (i = 3; i <= sqrt(n); i += 2) {
        if (n % i == 0) {
            return 0;
        }
    }

    return 1;
}

int main()
{
    int n, flag;

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

    if (n == 1) {
        printf("1 is neither prime nor composite.");
    }
    else {
        flag = isPrime(n);
        if (flag == 1) {
            printf("%d is a prime number.", n);
        }
        else {
            printf("%d is not a prime number.", n);
        }
    }

    return 0;
}

In this code, we first take input from the user and check if the number is 0 or 1 (which are not prime numbers). Then we iterate from 2 to n/2 and check if n is divisible by any number between 2 and n/2. If n is divisible, we set the flag to 1 and break out of the loop. Finally, we check the value of the flag to determine if n is a prime number.

Approach 2: Optimized brute force method The optimized brute force method involves checking if a number n is divisible by any prime number between 2 and sqrt(n). If n is divisible by any prime number between 2 and sqrt(n), then it is not a prime number. Otherwise, it is a prime number.

Code implementation: The following code implements the optimized brute force method to identify prime numbers:

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

int isPrime(int n)
{
    int i;

    // check if n is a multiple of 2
    if (n % 2 == 0) {
        return 0;
    }

    // check for all odd numbers
    for (i = 3; i <= sqrt(n); i += 2) {
        if (n % i == 0) {
            return 0;
        }
    }

    return 1;
}

int main()
{
    int n, flag;

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

    if (n == 1) {
        printf("1 is neither prime nor composite.");
    }
    else {
        flag = isPrime(n);
        if (flag == 1) {
            printf("%d is a prime number.", n);
        }
        else {
            printf("%d is not a prime number.", n);
        }
    }

    return 0;
}

In this code, we first take input from the user and check if the number is 1. If the number is 1, we print a message saying that 1 is neither prime nor composite. Otherwise, we call the isPrime function to determine if the number is prime or not. In the isPrime function, we first check if the number is a multiple of 2. If