How to use Arrays.sort method to sort a 2D integer array in Java

Question

Suppose I have a 2D integer array of

int[][] arr = new int [][] {
            {1,1},
            {2,222},
            {3,32},
            {4,13},
            {5,1144},
            {6,111},
            {7,12341},
            {8,111}
        };

I want to sort this array in an increasing order of second element of each row.

[[1, 1], [4, 13], [3, 32], [8, 111], [6, 111], [2, 222], [5, 1144], [7, 12341]]

But notice, when the second element's values are same as in

[8, 111] and [6, 111]

The sorting order reversed.

So here is what I attempted

Arrays.sort(arr, (a,b) -> a[1]==b[1]? b[1]-a[1]: a[1]-b[1]);

Whenever second elements are equal, reverse the sorting order to descending order, otherwise keep the ascending order.

After running this code, I get a result of

[[1, 1], [4, 13], [3, 32], [6, 111], [8, 111], [2, 222], [5, 1144], [7, 12341]]

Notice how the

 [6, 111], [8, 111]

are still in the wrong order.

Note how a[1]==b[1]? b[1]-a[1] :a[1]-b[1] will always be 0 or a[1]-b[1] (a[1]==b[1]? b[1]-a[1] is always 0) — knittl
– knittl, Commented Apr 16, 2021 at 13:08
I don't fully understand but you probably need Arrays.sort(arr, (a,b) -> a[1]==b[1]? b[0]-a[0]: a[1]-b[1]); If second elements are equal, compare first elements i.e. elements at 0th index. — SomeDude
– SomeDude, Commented Apr 16, 2021 at 13:10
Your reported output after the change shows a different order but you claim it is the same. — vsfDawg
– vsfDawg, Commented Apr 16, 2021 at 13:13
Are you saying you want a stable sort? That's already what the Java sorting algorithm does, if you just sort on the second element. — k314159
– k314159, Commented Apr 16, 2021 at 13:26

Rocco · Accepted Answer · 2021-04-16 13:07:34Z

2

Maybe this?

Arrays.sort(arr, (a,b) -> a[1]==b[1]? b[0]-a[0]: a[1]-b[1])

answered Apr 16, 2021 at 13:07

Rocco

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2 Comments

k314159 Over a year ago

Comparing using subtraction can result in overflow. You'd think that Integer.MAX_VALUE is bigger than Integer.MIN_VALUE, but Integer.MAX_VALUE - Integer.MIN_VALUE is negative.

WJS Over a year ago

It is very bad technique to subtract values to satisfy the Comparator contract. Try it with values near Integer.MIN_VALUE and Integer.MAX_VALUE and see what you get. Always return 1, -1, or 0. or use Integer.compare() or Integer.compareTo()

WJS · Accepted Answer · 2021-04-16 15:16:00Z

2

I would try it like this using a Comparator. If you want the first numbers in ascending order, then remove the reverseOrder comparator. Otherwise, leave it in. If you want to leave the first numbers in their relatively same positions when the second numbers are equal, then just sort on the second number. This will work since the sorting algorithm is stable.

Arrays.sort(arr, Comparator.comparingInt((int[] a) -> a[1])
        .thenComparing(a -> a[0], Comparator.reverseOrder()));

for (int[] a : arr) {
    System.out.println(Arrays.toString(a));
}

Prints

[1, 1]
[4, 13]
[3, 32]
[8, 111]
[6, 111]
[2, 222]
[5, 1144]
[7, 12341]

edited Apr 16, 2021 at 15:16

answered Apr 16, 2021 at 14:43

WJS

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1 Comment

k314159 Over a year ago

It's a pity there's no thenComparingInt(function, comparator) which forces you to write thenComparing instead, or using the technique in my answer to avoid boxing.

Sergey Afinogenov · Accepted Answer · 2021-04-16 14:50:27Z

1

Using Integer.compare method:

Arrays.sort(arr, (a,b) -> a[1]==b[1]?
                          Integer.compare(b[0], a[0]):
                          Integer.compare(a[1], b[1]));

edited Apr 16, 2021 at 14:50

answered Apr 16, 2021 at 13:46

Sergey Afinogenov

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Comments

k314159 · Accepted Answer · 2021-04-16 14:10:38Z

1

Use this:

    Arrays.sort(arr, Comparator.comparingInt((int[] a) -> a[1])
            .thenComparing(Comparator.comparingInt((int[] a) -> a[0]).reversed()));

edited Apr 16, 2021 at 14:10

answered Apr 16, 2021 at 13:11

k314159

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1 Comment

WJS Over a year ago

Just in case you didn't know, reversed() actually reverses every comparator that precedes it. But since you put in a Comparator within the thenComparing method, it worked. It's a subtlety that I have found that most are not aware of.

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