I have a problem that I can't solve. I hope I can make you understand it.

Given the following list of Waypoint objects

List<Waypoint>myWaypoint = new ArrayList<Waypoint>();

I want to calculate the combinations no repetition of groups of 3 (k=3) of the elements in the list and and create a matrix containing only k-group combinations

Example:

List<Waypoint>myWaypoint = new ArrayList<Waypoint>();

Waypoint a = new Waypoint();
Waypoint b = new Waypoint();
Waypoint c = new Waypoint();
Waypoint d = new Waypoint();



myWaypoint.add(a);
myWaypoint.add(b);
myWaypoint.add(c);
myWaypoint.add(d);

• n!/(r!(n-r)!)

  k = 3 n = 4 -> combination: 4

New array of Waypoint object

Expected result of the matrix

The goal is to generate an array containing these objects

CodePudding user response：

If implementing the algorithm is not part of the task, I would recomend a library like combinatoricslib3 which will generate the combinations for you.

Using combinatoricslib3 a simple example using Strings:

Generator.combination("A", "B", "C", "D")
        .simple(3)
        .stream()
        .forEach(System.out::println);

will give you an output

[A, B, C]
[A, B, D]
[A, C, D]
[B, C, D]

You can use the lib to generate combinations of your custom objects by just passing your list and for example collect them to a list of lists. Below an example as a starting point:

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

import org.paukov.combinatorics3.Generator;

public class Example {

    public static void main(String[] args) {
        List<Waypoint> myWaypoint = new ArrayList<>();
        Waypoint a = new Waypoint("a");
        Waypoint b = new Waypoint("b");
        Waypoint c = new Waypoint("c");
        Waypoint d = new Waypoint("d");
        myWaypoint.add(a);
        myWaypoint.add(b);
        myWaypoint.add(c);
        myWaypoint.add(d);

        List<List<Waypoint>> combinations = Generator.combination(myWaypoint)
                .simple(3)
                .stream()
                .collect(Collectors.toList());

        combinations.forEach(System.out::println);
    }

    static class Waypoint {
        String name;
        public Waypoint(String name) {
            this.name = name;
        }
        @Override
        public String toString() {
            return name;
        }
    }
}

You might want to read this post java-combinations-algorithm to find alternatives like Apache Commons or Google Guava