Write a function isprime() which takes a number and returns 1 if the number is prime and 0 otherwise.

  What is a prime number in definition?

A prime number is a whole number is greater than 1whose only factors are 1 and itself. A factors a whole number that can be  divided evenly into another number. The first few prime number are : 2, 3, 5, 7, 11, 13, 17, 19 and  23  etc. 

The following program check the input number is prime or not.

#include<stdio.h>

int isprime(int);

int main()

{

    int num;

    printf("Enter the Number check it

is prime or not :");

    scanf("%d",&num);

    if(isprime(num))

    {

        printf("%d is prime number.\n",

num);

    }

    else

    {

        printf("%d is not a prime number.

        \n",num);

    }

    return 0;

}

int isprime(int n)

{

    int i = 2;

    while(i<n)

    {

        if(n%i==0) /*number is completely

divisible by smaller no.

        so it not a prime number*/

            return 0;

        i++;

    }

    if(i==n) // prime number is divisible

by itself.

        return 1;

}

Output :

Enter the Number check it is prime or

not :7 7 is prime number. Enter the Number check it is prime or

not :9 9 is not a prime number.

The above table show prime number between 1 to 100.