CODEWITHSUNDEEP
javaarrays
charcoalite
charcoalite

Reputation: 43

ERROR when combining 2 String[][] and 1 String[] become one String[][]

needs help on my problem, I need to combine 3 array into one multidimensional array. but I always encounter error which some value return null. the data in first array (String[][]dataset) was:

001200700001        70        70        70        55        70        
        001200700002        70        70        60        60        60        
        001200700003        85        85        85        60        85        
        001200700004        60        70        85        70        85        
        001200700005        70        60        60        60        85

data for String[][]predictiondata was:

 15        74.25        64.25        64.25        64.25        89.25        
 14        74.25        64.25        64.25        64.25        89.25

and for Means[] was:

String[]Means={"Means","0","0","0","0","0"};

so here's my code so far and the error output:

public void hashtree(String[][]dataset, String[][]predictiondata){
        //======fetch data from database then add 1 morre row for calculation

        String[]Means={"Means","0","0","0","0","0"};

        int heightofarray = (dataset.length)+(predictiondata.length)+(Means.length);
        int lengthofarray=(dataset[0].length);
        System.out.println("heightofarray: "+heightofarray+" lengthofarray: "+lengthofarray);
        String[][]toCalculate = new String[heightofarray][lengthofarray];


        for(int a=0;a<dataset.length;a++){
            for(int b=0;b<dataset[0].length;b++){
                toCalculate[a][b]=dataset[a][b];
            }
        }
        for(int a=0;a<predictiondata.length;a++){
            for(int b=0;b<predictiondata[0].length;b++){
                toCalculate[(dataset.length)+a][b]=predictiondata[a][b];
            }
        }
        for(int a=0;a<Means.length;a++){
            toCalculate[heightofarray-1][a]=Means[a];
        }


        System.out.println("should print String[][]toCalculate");
        for(int a=0;a<toCalculate.length;a++){
            for(int b=0;b<toCalculate[0].length;b++){
                System.out.print(toCalculate[a][b]+"\t");
            }System.out.println("");
        }

The order for the new combined array ( String[][]toCalculate ) is:

String[][]dataset

String[][]predictiondata

String[]Means

but however I always get the output

System.out.println("should print String[][]toCalculate");
    001200700001        70        70        70        55        70        
    001200700002        70        70        60        60        60        
    001200700003        85        85        85        60        85        
    001200700004        60        70        85        70        85        
    001200700005        70        60        60        60        85        
    15        74.25        64.25        64.25        64.25        89.25        
    14        74.25        64.25        64.25        64.25        89.25        
    null        null        null        null        null        null        
    null        null        null        null        null        null        
    null        null        null        null        null        null        
    Exception in thread "main" java.lang.NullPointerException
    null        null        null        null        null        null        
    null        null        null        null        null        null        
    Means        0        0        0        0        0

Thank you very much for the solving

Upvotes: 0

Views: 63

Answers (2)

Adam Liss
Adam Liss

Reputation: 48330

It will probably be helpful for you to learn some debugging techniques, so you can solve problems like this when you see them in the future.

To that end, here's one way to go about debugging:

Looking at the output, you can see that the dataset and the predictiondata are stored correctly. It's the Means that are stored in the wrong place.

The for loop that stores the Means is different from the others. The first two loops calculate the first available row in toCalculate by adding the number of rows that have already been populated. But the last one counts backwards from the end. That must be the problem.

The size of toCalculate is given by

  int heightofarray = (dataset.length)+(predictiondata.length)+(Means.length);

The Means are stored exactly Means.length - 1 rows beyond the proper location. That's because heightofarray is calculated incorrectly as the sum of the height of the dataset array, the height of the predictiondata array, and the length of the Means.

The correct calculation is

int heightofarray = dataset.length + predictiondata.length + 1;

Hope that helps!

Upvotes: 1

Jim
Jim

Reputation: 22656

int heightofarray = (dataset.length)+(predictiondata.length)+(Means.length);

This should be:

int heightofarray = (dataset.length)+(predictiondata.length)+(1);

The height of Means is 1

Upvotes: 3

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