[Programming] Question 14: Write a Program to check whether a number is prime or not

Learn how to write a C program to determine whether a given number is prime. This guide provides a step-by-step explanation of how to implement a function that checks for primality by testing divisibility from 2 up to the square root of the number. The tutorial includes sample code, explanations of the logic used, and tips for optimizing the program.

#include <stdio.h>

int main() {
	int num, i, flag = 0;

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

	for (i = 2; i <= num / 2; ++i) {
		if (num % i == 0) {
			flag = 1;
			break;
		}
	}

	if (num == 1) {
		printf("1 is neither prime nor composite.\n");
	} 
	else {
		if (flag == 0)
			printf("%d is a prime number.\n", num);
		else
			printf("%d is not a prime number.\n", num);
	}

	return 0;
} 

Inside the main() function:

- An integer variable num is declared to store the input number.

- The user is prompted to enter a positive integer.

- A for loop is used to iterate from 2 to num/2.

- Within the loop, it checks if the number is divisible by any number other than 1 and itself.

- If the number is prime, it sets flag to 0, otherwise to 1.

- Finally, it prints whether the number is prime or not based on the value of flag.