Display the prime factors of a number using sets

[Joe Fett]

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]

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]

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]

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]

You are almost there. I suggest you re-factor your code a bit by adding some additional methods. It will enhance the readability of your code.
Here's how you can do it:

import java.util.*;
public class PrimeFactors {
public PrimeFactors() {
int number = 0;
while(number != -1) {
number = readIntFromUser();
if(isPrime(number)) {
System.out.println(number+" is a prime!");
}
else {
System.out.println(number+"'s prime factors are: "+factorize(number));
}
}
System.out.println("Bye!");
}
public Set<Integer> factorize(int number) {
Set<Integer> primeFactors = new TreeSet<Integer>();
/*
'div' <- 2
WHILE number is mor than one
IF 'div' is a prime number AND 'number' modulus 'div' equals zero
add 'div' to your set
divide 'number' with 'div'
ELSE
increase 'div' by one
END IF
END WHILE
*/
return primeFactors;
}
public boolean isPrime(int number) {
int root = (int)Math.sqrt(number);
for(int i = 2; i <= root; i++) {
if(number%i == 0) {
return false;
}
}
return true;
}
public int readIntFromUser() {
Scanner scanner = new Scanner(System.in);
System.out.print("Please enter a number, -1 to terminate:" );
return scanner.nextInt();
}
public static void main(String[] args) {
new PrimeFactors();
}
}

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.
In addition, I added some pseudo code in your factorize(...) method. Try to finish it now.

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

[Joe Fett]

 

Originally posted by Piet Verdriet:
You are almost there. I suggest you re-factor your code a bit by adding some additional methods. It will enhance the readability of your code.
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.
In addition, I added some pseudo code in your factorize(...) method. Try to finish it now.

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.

public Set<Integer> factorize(int number)
{
Set<Integer> primeFactors = new TreeSet<Integer>();
int div = 2; //'div' <- 2
while (number > 1) //WHILE number is more than one
{
if (int div == isPrime && number % div = 0) //IF 'div' is a prime number AND 'number' modulus 'div' equals zero
{
primeFactors.add(new Integer(div));//add 'div' to your set
number = number % div; //divide 'number' with 'div'
}
else
div++; //increase 'div' by one
} // end while
return primeFactors;
} // end method factorize

[Piet Verdriet]

Getting there!

...
if (int div == isPrime && number % div = 0)
...

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.

...
number = number % div; //divide 'number' with 'div'
...

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

[Joe Fett]

 

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]

Good to hear that Joe.
And you're most welcome!