Check if a Number is Prime

This program checks if a given number is prime.

Program in C:

#include <stdio.h>
#include <stdbool.h>

// Function to check if a number is prime
bool isPrime(int num) {
    // Handling special cases
    if (num <= 1) {
        return false;
    }
    if (num <= 3) {
        return true;
    }

    // Check for divisibility from 2 to sqrt(num)
    for (int i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            return false;
        }
    }
    
    return true;
}

int main() {
    int number;
    
    // Input number from user
    printf("Enter a number: ");
    scanf("%d", &number);
    
    // Check if the number is prime
    bool prime = isPrime(number);
    
    // Print the result
    if (prime) {
        printf("%d is a prime number.\n", number);
    } else {
        printf("%d is not a prime number.\n", number);
    }

    return 0;
}
        

Output:

Enter a number: 7
7 is a prime number.

Explanation:

1. isPrime Function:
   • bool isPrime(int num): This function takes an integer num as input and returns true if num is a prime number, and false otherwise.
   • It first handles special cases where numbers less than or equal to 1 are not prime (return false).
   • For numbers 2 and 3 (num <= 3), it directly returns true.
   • For other numbers (num > 3), it checks divisibility from 2 up to the square root of num. If num is divisible by any number in this range, it returns false. If no divisors are found, it returns true.
2. Main Function:
   • int main(): This is the main function where the program starts execution.
   • It prompts the user to enter a number and reads the input using scanf.
   • It calls the isPrime function to check if the entered number is prime.
   • Based on the return value of isPrime, it prints whether the number is prime or not.