Prime Number in C

A prime number is a positive integer greater than 1 that has no positive integer divisors other than 1 and itself. To check if a number is prime in C, you can write a program that iterates through all the numbers from 2 to the square root of the number you want to check and checks for divisibility. If the number is divisible by any other number in that range, it’s not prime; otherwise, it’s prime. Here’s a C program to determine if a number is prime:

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

int isPrime(int num) {
    if (num <= 1) {
        return 0; // 0 and 1 are not prime
    }
    if (num == 2) {
        return 1; // 2 is prime
    }
    if (num % 2 == 0) {
        return 0; // Even numbers other than 2 are not prime
    }

    // Check for divisibility from 3 to the square root of num
    for (int i = 3; i <= sqrt(num); i += 2) {
        if (num % i == 0) {
            return 0; // Not prime
        }
    }

    return 1; // Prime
}

int main() {
    int num;

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

    if (isPrime(num)) {
        printf("%d is a prime number.\n", num);
    } else {
        printf("%d is not a prime number.\n", num);
    }

    return 0;
}

In this program:

• The isPrime function takes an integer num as input and returns 1 if num is prime and 0 if it’s not prime.
• We handle some special cases first: 0 and 1 are not prime, 2 is prime, and even numbers greater than 2 are not prime.
• We then iterate through odd numbers from 3 to the square root of num. We only check odd numbers because even numbers greater than 2 are already excluded as mentioned earlier. If num is divisible by any of these numbers, it’s not prime.
• Finally, we call the isPrime function in the main function to check if the entered number is prime and display the result.

Compile and run the program, and it will determine if the entered integer is prime or not.