Bash program to check if the Number is a Prime or not

A prime number is a natural number greater than 1 that has exactly two factors: 1 and itself. Examples include 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, etc. In this article, we'll write a C program to check if a given number is prime or not.

Syntax

for (i = 2; i <= n/2; i++) {
    if (n % i == 0) {
        // Not prime
        break;
    }
}

Algorithm

• Step 1 − Read the number from user input
• Step 2 − Loop from 2 to n/2 to check for divisors
• Step 3 − If any divisor is found, the number is not prime
• Step 4 − If no divisors are found, the number is prime

Example 1: Basic Prime Check

This program checks if a number is prime by testing divisibility from 2 to n/2 −

#include <stdio.h>

int main() {
    int n = 29, i, flag = 0;
    
    if (n <= 1) {
        printf("%d is not a prime number<br>", n);
        return 0;
    }
    
    for (i = 2; i <= n / 2; i++) {
        if (n % i == 0) {
            flag = 1;
            break;
        }
    }
    
    if (flag == 0) {
        printf("%d is a prime number<br>", n);
    } else {
        printf("%d is not a prime number<br>", n);
    }
    
    return 0;
}

29 is a prime number

Example 2: Optimized Prime Check

This optimized version checks divisibility only up to the square root of n −

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

int isPrime(int n) {
    if (n <= 1) return 0;
    if (n == 2) return 1;
    if (n % 2 == 0) return 0;
    
    for (int i = 3; i <= sqrt(n); i += 2) {
        if (n % i == 0) {
            return 0;
        }
    }
    return 1;
}

int main() {
    int num = 97;
    
    if (isPrime(num)) {
        printf("%d is a prime number<br>", num);
    } else {
        printf("%d is not a prime number<br>", num);
    }
    
    return 0;
}

97 is a prime number

Key Points

• Numbers less than or equal to 1 are not considered prime
• The number 2 is the only even prime number
• For optimization, check divisibility only up to ?n
• Skip even divisors after checking for 2

Conclusion

Prime number checking in C can be implemented efficiently using modulo operations and loop optimization. The square root approach significantly reduces the number of iterations needed for large numbers.