What is wrong in this program ?

Question

import java.util.*;
class Twin
{
int n=0;
int c=0;
int Is_Prime(int a)
{
for(int i=1;i<=a;i++)
{
if(a%i==0)
c=c+1;
}
if(c==2)
return 1;
else
{
return 2;
}
}
}
public class Twin_prime
{
public static void main()
{
Scanner obj=new Scanner(System.in);
Twin t=new Twin();
System.out.println("Enter a number");
int a=obj.nextInt();
System.out.println("Enter another number");
int b=obj.nextInt();
int c=t.Is_Prime(a);
if(c==1)
c=a;
else
c=0;
int d=t.Is_Prime(b);
if(d==1)
d=b;
else
d=0;
int min=c>d?d:c;
int max=c>d?c:d;
if(Math.abs(max-min)==2)
System.out.println("Twin Primes");
else
System.out.println("Not Twin Primes");
}
}

Answer 1

Your function for checking the prime number has TWO problems...

your loop counter should start at 2, not 1 and end with a-1;

Actually , you do not need to check them all... you only need to go half way.

If you do not find a divisor by then, there aren't any.

But as written a%1 will ALWAYS be zero because anything divides by 1 evenly

Here is the revamped code.....

======================================

import java.io.*;

class TwinPrimes

{

private Console console;

private int num1, num2;

//constructor

TwinPrimes()

{

console = System.console();

num1 = num2 = 0;

}

// only need to go half way : if you don't find a divisor by then, there aren't any !!!

private boolean IsPrime ( int X)

{

int N = X/2;

boolean boolReturn=true; //assumes it is prime until proven otherwise;

for (int i=2; i<=N; i++)

{

if ((X % i)==0)

{

boolReturn=false;

}

}

return(boolReturn);

}

//returns 1 if the two numbers are twin primes, 0 if they are not twin primes, -1 on error

int Go()

{

String inbuff;

int iReturn = 0; //begins pessimisticly, assumes they are not twin primes until it is proven that they are

try

{

//perhaps you can trap for number format exception if the user inputs something funny

System.out.print(" Please input the first prime number :>");

inbuff = console.readLine();

num1 = Integer.parseInt(inbuff);

System.out.print(" Please input the second prime number :> ");

inbuff = console.readLine();

num2 = Integer.parseInt(inbuff);

//swaps them if out of order

if (num1 < num2)

{

int temp = num2;

num2 = num1;

num1 = temp;

} //swap

if (

(IsPrime(num1)) &&

(IsPrime(num2)) &&

((num1-num2)==2)

)

{

iReturn = 1;

} //if isPrime and they differ by 2

}//try

catch (Exception ex)

{

iReturn = -1;

System.out.println(" Input error has occured !!! ");

}

return(iReturn);

}

public static void main( String args[])

{

TwinPrimes myTwinPrimes = new TwinPrimes();

int iReturn = myTwinPrimes.Go();

switch (iReturn)

{

case 0:

{

System.out.println(" They are not twin primes !!!");

break;

}

case 1:

{

System.out.println(" They are twin primes !!! ");

break;

}

case -1:

{

System.out.println(" Input error has occured !!! ");

break;

}

} // switch

} //main

} //class TwinPrimes