Question

I’m trying to implement the TSP double bridge move which works as follows:
Given a permutation(tour) of cities it splits the permutation in 4 parts and reconnects these parts in different order.
e.g. perm=[a,b,c,d] —> [a,d,c,b].

In the following method i have the temp[] array which contains the pemutation’s cities. i choose 3 random number and split the array in 4 parts

 public void doubleBridge() {
    City[] temp = this.permArray; // it's a tour so the first element equals the last temp[0]=temp[temp.length-1]
    Random  random = new Random();      
    int pos1  = 1+ random.nextInt(temp.length/4);
    int pos2 = pos1 +  1 + random.nextInt(temp.length/4);
    int pos3 = pos2 + 1 + random.nextInt(temp.length/4);
    System.out.println("\nPositions chosen : "+pos1+" "+pos2+" "+pos3);

    City[] part1 = new City[pos1+1];
    part1 = Arrays.copyOfRange(temp, 0, pos1);

    City[] part2 = new City[pos2-pos1-1];
    part2= Arrays.copyOfRange(temp, pos1+1, pos2);

    City[] part3 = new City[pos3-pos2-1];
    part3= Arrays.copyOfRange(temp, pos2+1, pos3);

    City[] part4 = new City[temp.length-1-pos3-1];
    part4= Arrays.copyOfRange(temp, pos3+1, temp.length);

    //City[] newTemp = new City[temp.length];

    System.out.println("\npart1");
    for (City c: part1) {
        System.out.print(c.getId()+" ");
    }


    System.out.println("\npart2");
    for (City c: part2) {
        System.out.print(c.getId()+" ");
    }

    System.out.println("\npart3");
    for (City c: part3) {
        System.out.print(c.getId()+" ");
    }

    System.out.println("\npart4");
    for (City c: part4) {

        System.out.print(c.getId()+" ");
    }
    /*newTemp = concatAll(part1, part2, part3, part4);
    this.permArray = newTemp;
    this.computePermutationLength();*/
}

After running the programm an printing the parts i get.

{38, 18, 27, 2, 20, 35, 1, 42, 50, 22, 52, 36, 44, 31, 19, 33, 3, 25, 29, 49, 12, 4, 7, 30, 43, 24, 48, 45, 26, 39, 11, 15, 21, 34, 28, 8, 13, 51, 41, 17, 10, 37, 46, 32, 16, 23, 14, 5, 9, 6, 47, 40, 38, }LENGTH: 23511950

Positions chosen : 3 12 24

part1
38 18 27

part2
20 35 1 42 50 22 52 36

part3
31 19 33 3 25 29 49 12 4 7 30

part4
24 48 45 26 39 11 15 21 34 28 8 13 51 41 17 10 37 46 32 16 23 14 5 9 6 47 40 38

The problem is that 4 elements are lost. for example: element “2” from the initial permutation doens’t exist either in part1 or part2.

So where’s the problem?

Answer 1

From Arrays.copyOfRange docs, the parameters are :

• original – the array from which a range is to be copied
• from – the initial index of the range to be copied, inclusive
• to – the final index of the range to be copied, exclusive. (This index may lie outside the array.)

So you should have the following four lines : (start/from indexes should not increment)

City[] part1 = new City[pos1+1];
part1 = Arrays.copyOfRange(temp, 0, pos1);

City[] part2 = new City[pos2-pos1-1];
part2= Arrays.copyOfRange(temp, pos1, pos2);

City[] part3 = new City[pos3-pos2-1];
part3= Arrays.copyOfRange(temp, pos2, pos3);

City[] part4 = new City[temp.length-1-pos3-1];
part4= Arrays.copyOfRange(temp, pos3, temp.length);

Regards,
Stéphane