Greatest Common Factors (Version 2) Storing and Comparing Factors

I'm just going to start a new thread for the same project, as I've changed the code a a lot.
Old post: http://www.java-forums.org/new-java/...ng-issues.html

The part of the code I'm working on right now is supposed to save the factors in a 2d array. I've got little to no experience using a 2d array. Here's how I would like it to work. So number[0] is 10. number[0] has 4 factors. We can access the amount of factors number[0] has by using the following method

Code:

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getFactorLength(number[0])

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In this situation though we have 2 numbers. So I get the factors for each number in this for statement.

Code:

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for (int a=0; a<(number.length); a++){
        System.out.println(getFactorLength(number[a]));//0 and 1
        listFactors(number[a]);
}

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Then, the listFactors method prints out the factors of the current number. listFactors uses this code:

Code:

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//*****************************List Factors*******************
private static void listFactors(int number) {
        int numberOfFactors = -1;
        for(int i=1; i<=(number); i++){ //Counting to the current number, ie 1-2-3-4-5-6 etc etc
                if(number % i == 0){
                        System.out.println(i);
                        numberOfFactors++;
                       
                }//end if
        }//end for
        }//end method

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I want to save the factors in the 2d array with this listFactors method

Code:

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//*****************************List Factors*******************
private static void listFactors(int number) {
        int numberOfFactors = -1;
        for(int i=1; i<=(number); i++){ //Counting to 10
                if(number % i == 0){
                        System.out.println(i);
                        numberOfFactors++;
                        factors[number][numberOfFactors] = i; //<-------- Does not work
                       
                }//end if
        }//end for
        }//end method

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With the above method I'm hoping to achieve this if the current number we are using is 10 (number[0])
10's factors are 1,2,5,10
So I want it to save like this:
factors[0][1] = 1
factors[0][2] = 2
factors[0][3] = 5
factors[0][4] = 10

However, running the code with the above method brings this output:
4
1
Exception in thread "main" java.lang.NullPointerException
at Greatest_Common_Factor.listFactors(Greatest_Common _Factor.java:32)
at Greatest_Common_Factor.main(Greatest_Common_Factor .java:16)

Remember, this code is a debug version so it prints out like this when you're not saving it to an array(printing it directly from listfactors() and (getFactorLength())
4 <----- Number of factors of 10
1 <-----First Factor of 10
2 <-----Second Factor of 10
5 <-----Third Factor of 10
10 <----Fourth Factor of 10
4 <-----Number of factors in 15
1 <-----First Factor of 15
3 <-----Second Factor of 15
5 <-----Third Factor of 15
15 <----Final Factor of 15

So any help with the 2d arrays would be greatly appreciated. Here is my full code:

Code:

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public class Greatest_Common_Factor {
static int factors[][];
public static void main (String args[]){   
boolean hasFoundGreatestCommonFactor = false;
int number[];
number = new int[2];
number[0] = 10;
number[1] = 15;



            //while(hasFoundGreatestCommonFactor == false){
                                for (int a=0; a<(number.length); a++){
                                        System.out.println(getFactorLength(number[a]));//0 and 1
                                        listFactors(number[a]);
                                }//end for
    //        }//end while
}//end main
 
 
 
 
 
 //*****************************List Factors*******************
private static void listFactors(int number) {
        int numberOfFactors = -1;
        for(int i=1; i<=(number); i++){ //Counting to 10
                if(number % i == 0){
                        System.out.println(i);
                        numberOfFactors++;
                //        factors[number][numberOfFactors] = i;
                       
                }//end if
        }//end for
        }//end method






//**********************Get Factor Length****************************
    private static int getFactorLength(int currentNumber) {
        int factor1Length = 0;
        for(int i=1; i<=(currentNumber); i++){
            if(currentNumber % i == 0){
            factor1Length++;
            }//end if
        }//end for
        return factor1Length;
    }//end method
 

       
 
}//end class

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thanks!

Anybody understand the problem?

A 2D array is just an array of arrays. Which means that it is a 1D array and each element is another 1D array. Look at the following code:

Code:

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int[] a1 = {1,2,3};
int[] a2 = {9,8,7,6,5,4};
int[][] matrix = new int[2][];
matrix[0] = a1;
matrix[1] = a2;
System.out.println(matrix[0][0]);
System.out.println(matrix[1][3]);

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So, if i've got this right, the issue here is that you can't assign the variables too a two dimensional Array and you're getting a null pointer exception. That usually means an object hasn't been initialised properly. I don't like giving the answer, removes the whole fun of problem solving. Just look at your fields, and keep in mind that something might not have been initialized properly.

Quote:

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Originally Posted by Junky

A 2D array is just an array of arrays. Which means that it is a 1D array and each element is another 1D array. Look at the following code:

Code:

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int[] a1 = {1,2,3};
int[] a2 = {9,8,7,6,5,4};
int[][] matrix = new int[2][];
matrix[0] = a1;
matrix[1] = a2;
System.out.println(matrix[0][0]);
System.out.println(matrix[1][3]);

---

---

When I do that, with this new code

Code:

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public class Greatest_Common_Factor {
static int a1[];
static int a2[];
static int[][] factors = new int[2][];
public static void main (String args[]){   
boolean hasFoundGreatestCommonFactor = false;
factors[0] = a1;
factors[1] = a2;

int number[];
number = new int[2];
number[0] = 10;
number[1] = 15;

            //while(hasFoundGreatestCommonFactor == false){
                                for (int a=0; a<(number.length); a++){
                                        System.out.println(getFactorLength(number[a]));//0 and 1
                                        listFactors(number[a]);
                                }//end for
    //        }//end while
}//end main
 
 
 
 
 
 //*****************************List Factors*******************
private static void listFactors(int number) {
        int numberOfFactors = -1;
        for(int i=1; i<=(number); i++){ //Counting to 10
                if(number % i == 0){
                        System.out.println(i);
                        numberOfFactors++;
                        factors[number][numberOfFactors] = i;
                       
                }//end if
        }//end for
        }//end method






//**********************Get Factor Length****************************
    private static int getFactorLength(int currentNumber) {
        int factor1Length = 0;
        for(int i=1; i<=(currentNumber); i++){
            if(currentNumber % i == 0){
            factor1Length++;
            }//end if
        }//end for
        return factor1Length;
    }//end method
 

       
 
}//end class

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it errors :/
4
1
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10
at Greatest_Common_Factor.listFactors(Greatest_Common _Factor.java:35)
at Greatest_Common_Factor.main(Greatest_Common_Factor .java:19)

Ooops!

Now I see it.

Out of curiosity, why is it 2 and 0?

OK what I was suggesting is to use the listFactors method to create and return an array of all the factors of the int passed as a parameter. Then back in the main method assign that returned array to the appropriate element of your 2D array.

BTW the code I posted in reply 3 was just an example. You were supposed to adapt it to your needs not just paste in directly into your code.

Quote:

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Originally Posted by Junky

OK what I was suggesting is to use the listFactors method to create and return an array of all the factors of the int passed as a parameter. Then back in the main method assign that returned array to the appropriate element of your 2D array.

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But doesn't that mean the array then cannot be public?

You can declare the array publicly, and initialize it inside the listFactors() method.

Hur Dur..

Code:

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import java.util.ArrayList;
import java.util.List;

public class Greatest_Common_Factor {
        static int factors[][];
        public static void main (String args[]){   
                boolean hasFoundGreatestCommonFactor = false;
                int[] number = new int[4];
                number[0] = 100;
                number[1] = 10000;
                number[2] = 20;
                number[3] = 250;

                Factors[] theFactors = new Factors[number.length];
                for(int i = 0; i < number.length; i++) {
               
                        theFactors[i] = new Factors();
                        theFactors[i].Number = number[i];
                        theFactors[i].Factors = listFactors(number[i]);
                }//end for



                //loop through all the objects
                for(int i = 0; i < theFactors.length; i++) {
               
                        System.out.print("Factors for " + theFactors[i].Number + ": ");
                        //loop through the factors within this object
                        for(int j = 0; j < theFactors[i].Factors.length; j++) {
                                System.out.print(theFactors[i].Factors[j] + " ");
                        }//end for
                        System.out.println();
                }

                //loop through all of them to find the common factors
                Integer[] commonFactors = null;
                for(int i = 0; i < theFactors.length; i++) {
       
                        if(commonFactors == null)
                        {
                                commonFactors = theFactors[i].Factors;
                                continue;
                        }

                        commonFactors = getCommonFactors(theFactors[i].Factors, commonFactors);


                }


                //we now have an array of common factor, lets print it out!
                System.out.print("The common factors are:");
                int biggestCommonFactor = 1;
                for(int i = 0; i < commonFactors.length; i++)
                {
                        if(commonFactors[i] > biggestCommonFactor)
                        {
                                biggestCommonFactor = commonFactors[i];
                        }

                        System.out.print(commonFactors[i]);

                        if(i+1 != commonFactors.length)
                        {
                                System.out.print(", ");
                        }
                }
                System.out.println();
                System.out.println("The largest common factor is: " + biggestCommonFactor);


        }//end main


        private static Integer[] getCommonFactors(Integer[] arrayA, Integer[] arrayB)
        {
                //we'll put these into an array list, because we don't know how many common factors we'll have
                ArrayList<Integer> commonFactorList = new ArrayList<Integer>();

                //now we use a nested loop to find the common factors
                //loop through the current common factors
                for(int i = 0; i < arrayA.length; i++)
                {
                        //now loop through the candiate factors
                        for(int j = 0; j < arrayB.length; j++)
                        {
                                if(arrayB[j] == arrayA[i])
                                {
                                        commonFactorList.add(arrayA[i]);
                                }
                        }
                }

                //this takes the array list (which is expandable) and turns it into a simple array
                return (Integer[])commonFactorList.toArray(new Integer[commonFactorList.size()]);
        }


        //****************************List Factors*******************************
        private static Integer[] listFactors(int number) {
                //array lists grow, we use them when we don't know how long it will be
                ArrayList<Integer> factors = new ArrayList<Integer>();

                for(int i=1; i<=(number); i++){ //Counting to any number
                        if(number % i == 0){
                                factors.add(i);
                        }//end if
                }//end for

                //this takes the array list (which is expandable) and turns it into a simple array
                return (Integer[])factors.toArray(new Integer[factors.size()]);
        }//end method

        //*********************Get Factor Length*******************************
        private static int getFactorLength(int currentNumber) {
                int factor1Length = 0;
                for(int i=1; i<=(currentNumber); i++){
                        if(currentNumber % i == 0){
                                factor1Length++;
                        }//end if
                }//end for
                return factor1Length;
        }//end method

}//end class

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