Aim:

Design and develop a function isprime (x) that accepts an integer argument and returns 1 if the argument is prime and 0 otherwise. The function is to use plain division checking approach to determine if a given number is prime. Invoke this function from the main with different values obtained from the user and print appropriate messages.

Summary:

A Prime number is a number which can be divisible by 1 and itself, 1 is neither considered as prime nor non-prime(composite). Here a plain division checking approach is used to determine whether a given number is prime or not. The given number is check for divisibility from 1 to itself, if it is not divisible by any other number other than 1 and itself, then that number is a prime number or else it is not a prime number.

Algorithm:

1. Start
2. Take the input integer to be checked
3. Call the isPrime function
4. isPrime function when value=1, print it is niether prime nor composite from i=2 to value perform x (mod) i when x%i is equal to 0, return 0 else return 1
5. when value return is 1, print number is prime else step 6.
6. i.e. The return is 0, print number is not prime.
7. Stop

Program: isprime.c

#include<stdio.h>
#include<stdlib.h>

int isPrime(int);

int main(void)
{
        int val,flag;

        printf("\nEnter the value to be checked\n");
        scanf("%d",&val); //Taking input//

        flag = isPrime(val);//Calling isprime function//

        if(flag==1)  //Prints if the number is prime//
        printf("\nThe entered value %d is a prime number\n",val);

        else   //Prints if the number is not prime//
         printf("\nThe entered value %d is not a prime number\n",val);

        return 0;
}

int isPrime(int x)
{
        int i;
        if(x==1)  //Executes if the given value is 1//
        {
              printf("\n1 is neither prime nor composite\n");
          exit(0);
        }
        for(i = 2; i<x ;i++)
        {
                if(x % i==0)// Checks for divisibility//
                return 0;
        }
        return 1;
}

Output:

Run the following commands in your terminal:

gcc isprime.c

./a.out

Enter the value to be checked
1
1 is neither prime nor composite

gcc isprime.c

./a.out

Enter the value to be checked
27
The entered value 27 is not a prime number

gcc isprime.c

./a.out

Enter the value to be checked
29
The entered value 29 is a prime number