Java: sort list of lists

Question

i have a list of lists

private LinkedList<LinkedList<Double>> data = 
new LinkedList<LinkedList<Double>>();

Those lists represent columns(outer list) and rows(inner list). Now i'd like to sort the rows by the values of column 0.

EDIT

New example:

row0 = [4,2]

row1 = [1,3]

After sort it should look like this.

row0 (old row1) = [1,3]

row1 (old row0) = [4,2]

I tried this: creating new Columns and adding values. data.get() indexes a column data.get().get() indexes a row

for (int i = 0; i < 2; i++) {
    LinkedList<Double> newCol = new LinkedList<Double>();
    data.add(newCol);
}
data.get(0).add(4D);
data.get(0).add(1D);
data.get(1).add(2D);
data.get(1).add(3D);

To sort i wrote this code

Collections.sort(data, new Comparator<List<Double>>()
            {
              public int compare(List<Double> o1, List<Double> o2)
              {
                @Overrides
                return o1.get(0).compareTo(o2.get(0));
              }
            });

But this code sorts the columns by the values of row0. That is not what i want.

System.out.println(data.get(0).get(0) + " " + data.get(1).get(0));
System.out.println(data.get(0).get(1) + " " + data.get(1).get(1));

The output is:

row0 = [2, 4]

row1 = [3, 1]

Anyone knows how to do this?

Can you provide your Comparator's code ? That seems the right way to do it, so you must have gotten wrong somewhere. — Alexandre FILLATRE
– Alexandre FILLATRE, Commented May 8, 2014 at 21:01
i was only able to write a code that sort the cols by the values of row 0. But i want it the other way round.. — user3618164
– user3618164, Commented May 8, 2014 at 21:19

Alexandre FILLATRE · Accepted Answer · 2014-05-08 21:32:47Z

Try that

public void testSO() {
    LinkedList<LinkedList<Double>> data =
            new LinkedList<>();

    LinkedList l1 = new LinkedList<Double>();
    l1.add(1D);
    l1.add(5D);
    l1.add(3D);

    LinkedList l2 = new LinkedList<Double>();
    l2.add(0D);
    l2.add(3D);
    l2.add(1D);

    LinkedList l3 = new LinkedList<Double>();
    l3.add(2D);
    l3.add(3D);
    l3.add(1D);

    data.add(l1);
    data.add(l2);
    data.add(l3);

    System.out.println("Before sorting :");
    for (LinkedList<Double> list : data) {
        for (Double d : list) {
            System.out.print(d + " ");
        }
        System.out.println();
    }

    Collections.sort(data, new Comparator<LinkedList<Double>>() {
        @Override
        public int compare(LinkedList<Double> o1, LinkedList<Double> o2) {
            try {
                return o1.get(0).compareTo(o2.get(0));
            } catch (NullPointerException e) {
                return 0;
            }
        }
    });

    System.out.println("After sorting :");
    for (LinkedList<Double> list : data) {
        for (Double d : list) {
            System.out.print(d + " ");
        }
        System.out.println();
    }
}

Community · Accepted Answer · 2017-05-23 10:31:24Z

1

Use a comparator Sorting a list of non-comparable elements This is the complete example you are looking for.

public class Test {
    public static void main(String args[])
    {
        LinkedList<LinkedList<Double>> data =
                new LinkedList<LinkedList<Double>>();
        LinkedList<Double> row1 = new LinkedList<>();
        row1.add(1d); row1.add(5d); row1.add(3d);
        LinkedList<Double> row2 = new LinkedList<>();
        row2.add(0d); row2.add(3d); row2.add(1d);
        LinkedList<Double> row3 = new LinkedList<>();
        row3.add(2d); row3.add(3d); row3.add(1d);
        data.add(row1); data.add(row2); data.add(row3);
        System.out.println("Before the sort:"+ data);
        Collections.sort(data, new Comparator<LinkedList<Double>>() {
            @Override
            public int compare(LinkedList<Double> o1, LinkedList<Double> o2) {
                return (int)(o1.get(0) - o2.get(0));
            }
        });
        System.out.println("After the sort:"+ data);
    }
}

edited May 23, 2017 at 10:31

CommunityBot

11 silver badge

answered May 8, 2014 at 21:31

Sitian Liu

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1 Comment

matttm Over a year ago

What are these called? anonymous classes or something?

kpentchev · Accepted Answer · 2014-05-08 21:07:41Z

0

Use a comparator with the following compare method:

public int compare(List<Double> l1, List<Double> l2){
     return l1.get(0).compareTo(l2.get(0));
}

Add to this handling of empty lists as you see fit.

answered May 8, 2014 at 21:07

kpentchev

3,0902 gold badges28 silver badges44 bronze badges

2 Comments

user3618164 Over a year ago

this code is similar to mine. But it sorts the columns by the values of row 0. I want it to sort the rows by the values of column 0.

kpentchev Over a year ago

There is somehing wrong with the second example: row1 is [3, 1] compared to [1, 3] in the previous example. Hence it is ordered as "larger" that [2, 4]. If you want to always sort by the least element in the list, sort the inner lists first.

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