Sorting a 2d array where all values are doubles in Java

Question

I guess this is kind of a common question, but I looked at some other responses and couldn't get my code to work.

I have a 2d array of doubles, two columns and a bunch of rows, and I want to sort the array by the first column in ascending order.

My array is called ratio[][], and I tried this code:

  Arrays.sort(ratio, new Comparator<double[]>() {

    @Override
    public double compare(double[] o1, double[] o2) {
        return valueOf(o1[0]).compareTo(valueOf(o2[0]));
    }

  });

The compiler is finding a bunch of errors. I've imported java.util.Arrays and java.util.Comparator. What am I doing wrong?

You are just comparing two double values again and again, not array of doubles. — Shashwat
– Shashwat, Commented Oct 30, 2012 at 5:12

Ted Hopp · Accepted Answer · 2012-10-30 05:25:13Z

3

If you want to define the ordering of your double arrays by the first element (which seems like what you are trying to do), then all you need to do is:

Arrays.sort(ratio, new Comparator<double[]>() {
    @Override
    public double compare(double[] o1, double[] o2) {
        return Double.compare(o1[0], o2[0]);
    }
});

There's no need to construct a Double object for each comparison (or to convert your arrays to type Double[]).

edited Oct 30, 2012 at 5:25

answered Oct 30, 2012 at 5:20

Ted Hopp

235k48 gold badges412 silver badges533 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rohit Jain · Accepted Answer · 2012-10-30 05:58:15Z

2

Your valueOf is a static method of Double class. It should be invoked on it.

If you are just sorting the 2-D array on the basis of first column of each array, it would be like this: -

Arrays.sort(ratio, new Comparator<double[]>() {
        @Override
        public int compare(double[] o1, double[] o2) {
            return Double.valueOf(o1[0]).compareTo(Double.valueOf(o2[0]));
        }
});

And, you can use Comparator on type double[] since an array is an object only. So, you don't need to convert it to Double[][].

UPDATE: -

You can also use Double.compare(double, double) method as in @Ted's answer, because that will not need you to create two double objects. But, that's not going to matter much on small scale, if you are building a large application having lots of comparison, then you should definitely go this one.

edited Oct 30, 2012 at 5:58

answered Oct 30, 2012 at 5:14

Rohit Jain

214k45 gold badges419 silver badges534 bronze badges

15 Comments

Ed Cota Over a year ago

That did it, I'm surprised I was that close... Thank you!

MadProgrammer Over a year ago

Maybe I'm missing the point, but this won't sort the array. Have a look at my answer for examples (yes, I just tried your answer it still won't sort the array)

Rohit Jain Over a year ago

@MadProgrammer. I want to sort the array by the first column in ascending order. This is what OP wants. Now you can try the code and see whether it does it. I think you misunderstood the question. Even I got it wrong the first time. :)

MadProgrammer Over a year ago

@RohitJain I am missing something, sleep. I've being trying to sort the inner array not the outer :P

Ted Hopp Over a year ago

No worries about trying to use compareTo on primitives. :) Regarding your edit--using o1[0] - o2[0] has several defects: it can overflow; it can fall victim to catastrophic round-off errors; it does not correctly handle differences less than 1; and it does not correctly handle NaN values. Much better to use Double.compare(double,double).

|

slipperyseal · Accepted Answer · 2012-10-30 05:45:07Z

0

just a suggestion..

  class Row {
      public double one;
      public double two;
  }  

  Row[] rows = // create this

  Arrays.sort(rows, new Comparator<Row>() {
      public int compare(Row o1, Row o2) {
          double c = o1.one - o2.one;
          return c < 0.0 ? -1 : c > 0.0 ? 1 : 0;
      }
  }

edited Oct 30, 2012 at 5:45

answered Oct 30, 2012 at 5:21

slipperyseal

2,7881 gold badge16 silver badges15 bronze badges

12 Comments

Ted Hopp Over a year ago

This can have a problem with underflow/overflow and does not correctly handle NaN.

slipperyseal Over a year ago

Ahhh... Also, if MadProgrammer's answer works I retract my answer :)

Rohit Jain Over a year ago

@catchpolenet. No, his answer is not going to work. It will throw an Exception at runtime.

Rohit Jain Over a year ago

@catchpolenet. You're very first line is incorrect. We can create Comparator on double[]. An array is an object right?

MadProgrammer Over a year ago

@RohitJain Apart from sorting the wrong array index (:P), why would it have thrown an exception?

|

Collectives™ on Stack Overflow

Sorting a 2d array where all values are doubles in Java

3 Answers 3

Comments

15 Comments

12 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

15 Comments

12 Comments

Your Answer

Sign up or log in

Post as a guest

Related