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. Logic to check prime numbers in C programming.

**Example**

**Input**

Input any number: 17

**Output**

17 is prime number

## Required knowledge

Basic C programming, If else, For loop

### 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...

## Logic to check prime number

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.

Step by step descriptive logic to check prime number.

- Input a number from user. Store it in some variable say
`num`. - 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`num/2`

, increment 1 in each iteration. The loop structure should be like`for(i=2; i<=num/2; i++)`

. - Check, divisibility of the number i.e.
`if(num%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.

## Program to check prime number

```
/**
* C program to whether a number is prime number or not
*/
#include <stdio.h>
int main()
{
int i, num, 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", &num);
for(i=2; i<=num/2; i++)
{
/* Check divisibility of num */
if(num%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", num);
}
else
{
printf("%d is composite number", num);
}
return 0;
}
```

Enhance your skills by learning this program using functional approach.

Learn more - Program to check prime using functions.

Output

Enter any number to check prime: 7 7 is prime number

Happy coding 😉

<pre><code> ----Your Source Code---- </code></pre>