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The goal of this assignment is to create a 2D array and then return the row with the maximum value in the array. When I try to call the method in the main method, I get the following:

java.lang.ArrayIndexOutOfBoundsException: 2

At this point, I'm not sure how to proceed.

public class MDArray
{
    private double[][] mdarray;

    public MDArray(double[][] a)
    {
        mdarray = new double[a.length][];
        for(int i = 0; i < a.length; i++)
        {
            mdarray[i] = new double[a[i].length];
            for(int j= 0; j < a[i].length; j++)
            {
                mdarray[i][j] = a[i][j];
            }
        }
    }
    public double[] max()
    {
        double[] maxVal = new double[mdarray.length];
        for(int i = 0, j = i + 1; i < maxVal.length; i++)
        {
            for(int k = 0; k < mdarray[i].length; k++)
            {
                if(mdarray[i][k] > mdarray[j][k])
                {
                    maxVal = mdarray[i];
                }
            }
        }
        return maxVal;
    }
}
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6
  • What is the purpose of your constructor accepting an input 2D array if your code just overwrites it? Commented Dec 8, 2016 at 1:04
  • 1
    Is it really overwriting, or is he manually copying each and every value instead of of just mdarray = a;? Commented Dec 8, 2016 at 1:07
  • @Gendarme right you are Commented Dec 8, 2016 at 1:10
  • I have a test array with these values a = {{3, 4, 1, 8}, {13, 2, 12, 9}}. I'm trying to call it with a.max(). Commented Dec 8, 2016 at 1:12
  • 2
    This works if it's a matrix, but not if it's a 2x4 array like you have here. maxVal.length is 2, not 4. It only has indices 0 and 1, hence the ArrayIndexOutOfBoundsException when you try to assign mdarray[i] which has the size of 4. If you're working with rectangular arrays, you can change double[] maxVal = new double[mdarray.length] to double[] maxVal = new double[mdarray[0].length]. If not, then you'll need another solution. Commented Dec 8, 2016 at 1:13

1 Answer 1

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If I understand what you are trying to do, I would start with a method to get the maximum value from a double[] like

private static double getMaxValue(double[] a) {
    int maxIndex = 0; // <-- start with the first
    for (int i = 1; i < a.length; i++) { // <-- start with the second
        if (a[i] > a[maxIndex]) {
            maxIndex = i;
        }
    }
    return a[maxIndex]; // <-- return the max value.
}

Then you can use that to determine the row with the maximum value (and copy the array) like

public double[] max() {
    int maxIndex = 0; // <-- start with the first
    for (int i = 1; i < mdarray.length; i++) { // <-- start with the second
        double maxValue = getMaxValue(mdarray[maxIndex]);
        double curValue = getMaxValue(mdarray[i]);
        if (curValue > maxValue) {
            maxIndex = i; // <-- The current value is greater, update the index.
        }
    }
    return Arrays.copyOf(mdarray[maxIndex], mdarray[maxIndex].length);
}

Finally, when constructing your MDArray you could also use Arrays.copyOf to simplify the logic like

public MDArray(double[][] a) {
    mdarray = new double[a.length][];
    for (int i = 0; i < a.length; i++) {
        mdarray[i] = Arrays.copyOf(a[i], a[i].length);
    }
}
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1 Comment

@Gendarme I'm scratching my head too. I guess decomposing the problem was all the help OP needed.

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