Sorting a two-dimensional array doesn't sort in the right order

Question

I'm trying to sort a two-dimensional array based on the second column, here is the array:

[1.1, 60.0]
[1.2, 66.66]
[1.3, -1.0]
[1.4, 50.0]
[1.5, 100.0]
[2.1, -1.0]
[2.2, -1.0]
[2.3, -1.0]

I looked into sorting mutli-dimensional arrays and created the following code:

    private void sortKnowledgeScores(double[][] knowledgescores){
    Arrays.sort(knowledgescores, new Comparator<double[]>() {
        @Override
        public int compare(double[] o1, double[] o2) {
            double itemOne = o1[1];
            double itemTwo = o2[1];
            // sort on item id
            if(itemOne < itemTwo){
                return (int) itemOne;
            }
            return (int) itemIdTwo;
        }

This rearranges the array to this:

[2.3, -1.0]
[2.2, -1.0]
[2.1, -1.0]
[1.3, -1.0]
[1.1, 60.0]
[1.2, 66.66]
[1.4, 50.0]
[1.5, 100.0]

Which is correct for the first three values however it puts 50 below 60 and 60.66. How can I adjust my sortKnowledgeScores function to sort my array based on the first column in ascending order?

dWinder · Accepted Answer · 2020-11-10 10:20:30Z

3

Comparator should return 1,0,-1 (as bigger, equel, less).

Generally, if the compare function return positive result the first element is bigger, 0 they equal and negative result mean the second obj is bigger.

You returning the values and not the compare result.

You can look at this doc

You need to change your code to:

public int compare(double[] o1, double[] o2) {
        return (int) o1[1] - o2[1];
 }

As @Mick point out - notice that index you using: index 0 is for the first column and index 1 is for the second

edited Nov 10, 2020 at 10:20

answered Nov 10, 2020 at 10:13

dWinder

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3 Comments

Mick Over a year ago

o1[0] - o2[0] , I guess

dWinder Over a year ago

that depend on the OP - I know he said "first" col which mean index 0 but on the code and the explain he talk about the second col so I keep the 1 index here. That is good comment if he actually mean that but I don't think this was his issue

Pzet Over a year ago

That's my bad, I meant the second column which would have an index of 1

Felix · Accepted Answer · 2020-11-10 10:21:56Z

2

First of all, you're using the wrong index. The first element in an array is at index 0

You can use the Comparator-Factory Comparator.comparingDouble:

Comparator.comparingDouble(array -> array[0])

Arrays.sort(knowledgescores, Comparator.comparingDouble(array -> array[0]))

answered Nov 10, 2020 at 10:21

Felix

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Comments

Mick · Accepted Answer · 2020-11-10 10:16:51Z

0

You compare the elements with index "1". This translates to the second column. You should use index "0" if sorting should consider the first column.

Plus - as already mentioned - the compare method works like this:

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

answered Nov 10, 2020 at 10:16

Mick

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