C Programming Code Examples C > For Loops and While Loops Code Examples C program to check whether a number is prime number or not C program to check whether a number is prime number or not Write a program in C to input a number and check whether the number is prime number or not using for loop. How to check prime numbers using loop in C programming. What is Prime number? Prime numbers are the positive integers greater than 1 that is only divisible by 1 and self. For example: 2, 3, 5, 7, 11 etc... There are several efficient algorithms for prime test. For this post I am implementing the simplest and easiest algorithm for beginners. If the number is divisible by any number in between 2 to n-1. Then it is composite number otherwise prime. Input a number from user. Store it in some variable say j. Declare and initialize another variable say isPrime = 1. isPrime variable is used as a notification or flag variable. Assigning 0 means number is composite and 1 means prime. Run a loop from 2 to j/2, increment 1 in each iteration. The loop structure should be like for(i=2; i<=j/2; i++). Check, divisibility of the number i.e. if(j%i == 0) then, the number is not prime. Set isPrime = 0 indicating number is not prime and terminate from loop. Outside the loop check the current value of isPrime. According to our assumption if it is equal to 1 then the number is prime otherwise composite. #include <stdio.h> int main() { int i, j, isPrime; /* * isPrime is used as flag variable. * If isPrime = 0, then number is composite * else if isPrime = 1, then number is prime. * Initially I have assumed the number as prime. */ isPrime = 1; /* Input a number from user */ printf("Enter any number to check prime: "); scanf("%d", &j); for(i=2; i<=j/2; i++) { /* Check divisibility of j */ if(j%i==0) { /* Set isPrime to 0 indicating it as composite number */ isPrime = 0; /* Terminate from loop */ break; } } /* * If isPrime contains 1 then it is prime */ if(isPrime == 1) { printf("%d is prime number", j); } else { printf("%d is composite number", j); } return 0; }