# Display the prime factors of a number using sets

Joe Fett
Greenhorn
Posts: 9
This program is supposed to do two things:
1) determine if a user input number is prime and if it is, the output will say as much (this works okay)
2) if it is not prime, it should display the number and its prime factors using a set(I'm lost)

Here is the code and pseudocode I have for finding the prime factors. I'm just not understanding what I need to do for getting the prime factors. I'm not looking for free code because I want to understand this as learning Java has been frustrating for me. Any nudge of assistance may turn the light on for me so I'd appreciate anything you folks can help me with.

Thanks!

<blockquote>code:
<pre name="code" class="core"> // Program finds the prime factors of a number using sets.
import java.util.Scanner;
import java.util.HashSet;

public class PrimeFactors
{
public PrimeFactors()
{
Scanner scanner = new Scanner( System.in ); // create scanner
System.out.println( "Please enter a number, -1 to terminate:" );
int inputNumber = scanner.nextInt(); // get number
boolean prime = true;

// process user input numbers
while ( inputNumber != -1 )
{
HashSet< Integer > factorSet = new HashSet< Integer >();
factorize( inputNumber, factorSet );

// Now print

for (int i=2; i < inputNumber/2; i++)
{
if (inputNumber%i==0)
prime = false;
}
if (prime)
System.out.println(inputNumber + " is prime");
//If a prime, print the number is a prime

else
System.out.println(inputNumber + " is not prime and its factors are:\n");

//If not a prime, print the prime factors for the number

// Now get the next number
System.out.println("Please enter a number, -1 to terminate:" );
inputNumber = scanner.nextInt(); // get number

} // end while
} // end constructor PrimeFactors

// find prime factors
public boolean factorize( int inputNumber, HashSet< Integer > set )
{

return false;
} // end method factorize

public static void main( String args[] )
{
new PrimeFactors();
} // end main
} // end class PrimeFactors
</pre>
</blockquote>

Rajah Nagur
Ranch Hand
Posts: 239
I had written the below code to print prime factors for a euler problem.
The program prints all the prime factors of a large number - 600851475143.

The logic is in the while i.e. keep repeatedly dividing until the remainder is zero.

<blockquote>code:
<pre name="code" class="core">
package euler;

public class Problem3 {
/**
* @param args
*/
public static void main(String[] args) {
long N = 600851475143l;
for (long i = 2 ; i <= N ; i++){
while (N % i == 0){
System.out.println(i);
N = N/i;
}
}
}
}
</pre>
</blockquote>

Jesper de Jong
Java Cowboy
Saloon Keeper
Posts: 15451
42
Rajah, instead of testing all numbers between 2 and N, you only need to test all primes between 2 and the square root of N.

Joe Fett
Greenhorn
Posts: 9
It turns out the prime number testing portion does not even work well so I tried a few things and got mixed results at best.

Anyone have an ideas how I can get my brain back in the game?

<blockquote>code:
<pre name="code" class="core">// Program finds the prime factors of a number using sets.
import java.util.Scanner;
import java.util.HashSet;

public class PrimeFactors
{
public PrimeFactors()
{
boolean prime = true;
Scanner scanner = new Scanner( System.in ); // create scanner
System.out.println( "Please enter a number, -1 to terminate:" );
int inputNumber = scanner.nextInt(); // get number

// process user input numbers
while ( inputNumber != -1 )
{
HashSet< Integer > factorSet = new HashSet< Integer >();
factorize( inputNumber, factorSet );

// Now print

int root = (int) Math.sqrt( inputNumber );

for (int i=2; i <= root; i++)
{
if (inputNumber % i == 0)
{
// System.out.println(inputNumber + " is not prime");
prime = false;
continue;
}
}

if (prime)
System.out.println(inputNumber + " is prime");
//If a prime, print the number is a prime

else
System.out.println(inputNumber + " is not prime");

//If not a prime, print the prime factors for the number

// Now get the next number
System.out.println("Please enter a number, -1 to terminate:" );
inputNumber = scanner.nextInt(); // get number

} // end while

} // end constructor PrimeFactors

// find prime factors
public boolean factorize( int inputNumber, HashSet< Integer > set )
{

return false;
} // end method factorize

public static void main( String args[] )
{
new PrimeFactors();
} // end main
} // end class PrimeFactors
</pre>
</blockquote>

Piet Verdriet
Ranch Hand
Posts: 266
Here's how you can do it:

I made little changes to the actual code, I only added a couple of extra methods. The logic is still the same as in your last post.

Good luck!
[ July 18, 2008: Message edited by: Piet Verdriet ]

Joe Fett
Greenhorn
Posts: 9
Originally posted by Piet Verdriet:
Here's how you can do it:

I made little changes to the actual code, I only added a couple of extra methods. The logic is still the same as in your last post.

Good luck!

[ July 18, 2008: Message edited by: Piet Verdriet ]

I really appreciate your help Piet, breaking it into different methods makes it much saner. I'm still having an issue with the set portion though. I used your pseudo code to get it started but I think I'm making some rookie mistakes and its giving me incompatibility errors.

Piet Verdriet
Ranch Hand
Posts: 266
Getting there!

You already declared 'div' as an integer, so you can remove the declaration. Also, you will need to utilize your isPrime(...) method in that if-statement and lastly: by doing 'dic = 0' you're assigning the value 0 (zero) to 'div', not testing for equality.

No, here you need the divide operator, not the modulo operator.

Joe Fett
Greenhorn
Posts: 9
Originally posted by Piet Verdriet:
Getting there!

No, here you need the divide operator, not the modulo operator.

Piet, thanks again. I was able to get rid of the last bit of problems in the factorize method. It compiled and ran fine. I feel like an elephant stopped sitting on my back now. Thanks so much for your assistance! Breaking this program into several methods really made the difference for me. Great idea on your part.

Cheers to ya!

Piet Verdriet
Ranch Hand
Posts: 266
Good to hear that Joe.
And you're most welcome!