C Exercises: Check whether a given number is prime or not

Last update on July 31 2025 05:46:28 (UTC/GMT +8 hours)

32. Prime Number Determination

Write a C program to determine whether a given number is prime or not.

The program should prompt the user for an integer input, then determine if the number has any divisors other than 1 and itself. If the number has no other divisors, it is prime; otherwise, it is not. The result should be displayed to the user.

Visual Presentation:

Check whether a given number is prime or not

Sample Solution:

C Code:

#include <stdio.h> // Include the standard input/output header file.

void main(){

    int num,i,ctr=0; // Declare variables for user input, loop control, and a counter.

    printf("Input  a number: "); // Prompt the user to input a number.
    scanf("%d",&num); // Read the input number from the user.

    for(i=2;i<=num/2;i++){ // Start a loop to check for factors of the input number.
        if(num % i==0){ // If the input number is divisible by 'i'.
            ctr++; // Increment the counter.
            break; // Exit the loop since a factor has been found.
        }
    }

    if(ctr==0 && num!= 1) // If no factors were found and the number is not 1.
        printf("%d is a prime number.\n",num); // Print that the number is prime.
    else
        printf("%d is not a prime number",num); // Print that the number is not prime.
}

Output:

Input  a number: 13                                                                                           
13 is a prime number.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to determine if a number is prime using the Euclidean algorithm for optimization.
Write a C program to check if a number is prime using recursion.
Write a C program to test primality by checking divisibility only up to the square root of the number.
Write a C program to check if a number is prime and, if not, display its factors.

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