primepairs

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

#define TRUE   1
#define FALSE  0

int isprime(int n);
int exactprime(int n);
int number_of_primes(int n);

int main()
{
    int n1, n2, n3, c1;
    n1 = 0;
    n2 = 0;
    n3 = 0;

    printf("To generate prime pairs < 10000, enter 1 \nTo check for a prime, enter 2 \nTo determine the number of primes up to a given value, enter 3 \nTo terminate the program, enter 0: ");
    scanf("%d", &n1);

    while (n1 != 0)
    {
        n3 = 0;

        if (n1 == 1)
        {
            while (n3 != 1)
            {
                printf("\nEnter a positive integer > 5 and < 10000: ");
                scanf("%d", &n2);

                if(n2 < 5)
                {
                    printf("Wrong input");
                }

                else if (n2 > 10000)
                {
                    printf("Wrong input");
                }

                else (n3++);

            }

            puts(" ");
            printf("\nThe prime pairs up to %d are: \n\n", n2);

            for(c1 = 3; c1 <= n2; c1++)
            {
                if (isprime(c1) && isprime(c1 + 2))
                    printf("%d  , %d \n", c1, c1 + 2);
            }


        }

        else if (n1 == 2)
        {
            while (n3 != 1)
            {
                printf("\nEnter a positive integer > 5 and < 2,000,000: ");
                scanf("%d", &n2);

                if (n2 < 5)
                    printf("Wrong input.");

                else if (n2 > 2000000)
                    printf("Wrong input.");

                else (n3++);

            }

            puts(" ");
            if (isprime(n2) == TRUE)
                printf("\n%d is a prime number \n\n", n2);

            else
                printf("\n%d is not a prime number \n\n", n2);
        }

        else if (n1 == 3)
       {
           while (n3 != 1)
           {
                printf("\nEnter a positive integer > 5 and < 2,000,000: ");
                scanf("%d", &n2);

                if (n2 < 5)
                    printf("Wrong input.");

                else if (n2 > 2000000)
                    printf("Wrong input.");

                else (n3++);
           }

            puts(" ");
            printf("Exact number of primes up to %d is: %d \n", n2, exactprime(n2));
            puts(" ");
            printf("Approximate number of primes up to %d is: %d \n\n", n2, number_of_primes(n2));

       }

        else
        {    printf("Wrong input. \n");
        }

    printf("\nTo generate prime pairs < 10000, enter 1 \nTo check for a prime, enter 2 \nTo determine the number of primes up to a given value, enter 3 \nTo terminate the program, enter 0: ");
    scanf("%d", &n1);

    }

    printf("\n\n ***Program Terminated*** \n\n");

    return 0;
}



/*-----------------------function isprime---------------
Purpose: To determine if a given number (user input) is a prime number or not
Returns: Either TRUE or FALSE to the main function
------------------------------------------------------*/

int isprime(int n)
{
    int c1;

    for(c1 = 2; c1 < n; c1++)
    {
        if(n%c1 == 0)
            return FALSE;
    }

    return TRUE;
}

 /*-----------------------function number_of_primes--------------
Purpose: To determine the approximate number of primes up to a certain number, determined by user input. Uses a formula to determine this approximate number.
Returns: An integer value to the main function, the integer value being approximately how many primes exist up till the user input
---------------------------------------------------------------*/
int number_of_primes(int n)
{
    int np;
    np = 0;

    np = n / (log(n) - 1);

    return np;
}

int exactprime(int n)
{
    int c1, c2;
    c2 = 1;

    for(c1 = 3; c1 <= n; c1++)
            {
                if (isprime(c1))
                    c2++;
            }
    return c2;
}