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In this tutorial, you will learn how to write a C program to check whether a given number is a prime number or not using a while loop conditional statement. Here, we will go through examples, algorithms, logic, and program explanation for a better understanding of this problem in C programming language.

Required Knowledge

• C Programming Basics
• C Programming Operators
• C printf and Scanf
• While loop in C

Problem Statement

You have to Write a C program that takes a number as input from the user. After taking the integer input our program should check whether it is a prime number or not using a while loop. The program provides a solution to determine the primality of a given number using a while loop in the C programming language.

To understand what we have to achieve, you can see the below example.

Example

Input:
Enter a number: 17

Output:
17 is a prime number.

In the above example, you can see that the user input is 17. And we know that by the definition of prime number, 17 is a Prime number because it is divided by 1 and 17 only.

Algorithm

1. Start the program.
2. Declare the variable num to store the input number, isPrime to store the prime status, and i as a counter.
3. Read the value of num from the user.
4. Initialize isPrime to 1 (true).
5. Initialize i to 2.
6. Enter the while loop with the condition i <= sqrt(num).
   • Check if num is divisible by i without a remainder.
     • If true, set isPrime to 0 (false) and break out of the loop.
     • If false, continue to the next iteration.
   • Increment i by 1.
7. Exit the while loop.
8. Check the value of isPrime.
   • If isPrime is 1, print that num is a prime number.
   • If isPrime is 0, print that num is not a prime number.
9. End the program.

Logic used to solve this Problem

• Declare a variable to hold the input number.
• Prompt the user to enter a number.
• Read and store the entered number in the variable.
• Check if the number is less than 2. If true, it is not a prime number.
• Check if the number is exactly 2. If true, it is a prime number.
• Initialize a divisor variable to 2.
• Use a while loop to iterate while the divisor is less than the square root of the number.
• Inside the loop, check if the number is divisible evenly by the divisor. If true, it is not a prime number.
• Increment the divisor by 1.
• If the loop completes without finding any divisors, the number is a prime number.
• Print the result indicating whether the number is prime or not.
• Finish the program.

• Use a while loop to iterate while the divisor is less than the square root of the number.

C Program to Check whether a given number is Prime or not

#include <stdio.h>
#include <math.h>
int main() {
    int num, isPrime = 1, i = 2;
    printf("Enter a number: ");
    scanf("%d", &num);
    while (i <= sqrt(num)) {
        if (num % i == 0) {
            isPrime = 0;
            break;
        }
        i++;
    }
    if (isPrime == 1) {
        printf("%d is a prime number.\n", num);
    } else {
        printf("%d is not a prime number.\n", num);
    }
    return 0;
}

Output 1

Enter a number: 54
54 is not a prime number

Output 2

Enter a number: 5 
5 is a prime number

Program Explanation

• The program takes input for the number to be checked by the user.
• It initializes a variable isPrime to 1, which represents the initial assumption that the number is prime.
• It initializes a variable divisor to 2, which will be used to check divisibility.
• The program enters a while loop with the condition divisor <= number/2 to iterate through possible divisors.
• Inside the while loop, it checks if the number is divisible by the current value of divisor.
• If the number is divisible, it sets the value of isPrime to 0, indicating that it is not a prime number.
• It then increments the value of divisor by 1 to move on to the next possible divisor.
• The loop continues until divisor exceeds half of the given number.
• After the while loop, it checks the value of isPrime.
• If isPrime is 1, it prints that the given number is prime.
• Otherwise, it prints that the given number is not prime.
• The program ends.

Conclusion

The C program successfully checks whether a given number is prime or not using a while loop. By iterating through the numbers from 2 to the square root of the number and checking for any divisors, it determines the prime status and displays the result on the screen.