Question

I am working on a lab and need some assistance if possible.

I have created a multidimensional array which is populated with random integers >= 0 up to 100 (inclusive) and I am attempting to apply Prim’s algorithm (via a method that I have in another class) to this multidimensional array, but it keeps giving me unwanted results (either zeroes, or the value that I put in for ‘n’).

Please be aware that I have applied Prim’s algorithm (via the method in another class) to two other arrays and it has worked perfectly well; however, now that I have created a multidimensional array populated entirely by random natural integers between 0 and 100 inclusive, it ceases to work. Here is the code from the classes (I have left the code in for the aforementioned two arrays that I have worked on in case something can be derived from there):

import java.util.Arrays;
import java.util.Random;

public class Lab6 {

static double[][] g = new double[][] {{0, 1, 2} , {1, 0, 3} , {2, 3, 0}};
static double mst[][] = MST.PrimsMST(g);

static double[][] lecExample = new double[][] {{0, 1, 2, 3, 0} , {1, 0, 6, 0, 5} , {2, 6, 0 ,4, 1} , {3, 0, 4, 0, 2} , {0, 5, 1, 2, 0}};
static double mst2[][] = MST.PrimsMST(lecExample);


public static void printArray(){

    System.out.println("Matrix (g):");
    for (int i = 0; i < g.length; i++) {
             for (int c = 0; c < g[i].length; c++) {
                 System.out.print(" " + g[i][c]);
             }
         System.out.println("");
    }

    System.out.println();

    System.out.println("MST:");
    for (int i = 0; i < mst.length; i++){
            for (int c = 0; c < mst[i].length; c++){
                System.out.print(" " + mst[i][c]);
            }
        System.out.println("");
    }

    System.out.println("Matrix (lecExample):");
    for (int i = 0; i < lecExample.length; i++) {
             for (int c = 0; c < lecExample[i].length; c++) {
                 System.out.print(" " + lecExample[i][c]);
             }
         System.out.println("");
    }

    System.out.println();

    System.out.println("MST2:");
    for (int i = 0; i < mst2.length; i++){
            for (int c = 0; c < mst2[i].length; c++){
                System.out.print(" " + mst2[i][c]);
            }
        System.out.println("");
    }

}


static Random random = new Random();
static int r = random.nextInt() & 100;

public static void randomArray(int n){

    double[][] array = new double[][] {{n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}};
    double mst3[][] = MST.PrimsMST(array);

    System.out.println("Matrix (Random Number Array):");
    for(int i = 0 ; i < array.length ; i++ ) { 
       for (int c = 0 ; c < array[i].length; c++ ) { 
          array[i][c] = random.nextInt(101);
       }
    }

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

    System.out.println("MST3:");
    for (int i = 0; i < mst3.length; i++){
            for (int c = 0; c < mst3[i].length; c++){
                System.out.print(" " + mst3[i][c]);
            }
        System.out.println("");
    }

}

public static void main(String[] args){

    printArray();
    System.out.println("\n");
    randomArray(50);

}

}

MST.java:

import java.util.*;

public class MST
{
//Search for the next applicable edge
static private Edge LocateEdge(ArrayList<Integer> v,ArrayList<Edge> edges)
{
    for (Iterator<Edge> it = edges.iterator(); it.hasNext();)
    {
        Edge e = it.next();
        int x = e.i;
        int y = e.j;
        int xv = v.indexOf(x);
        int yv = v.indexOf(y);
        if (xv > -1 && yv == -1)
        {
            return(e);
        }
        if (xv == -1 && yv > -1)
        {
            return(e);
        }
    }
    //Error condition
    return(new Edge(-1,-1,0.0));
}
@SuppressWarnings("unchecked")
//d is a distance matrix, high value edges are more costly
//Assume that d is symmetric and square
public static double[][] PrimsMST(double[][] d)
{
    int i,j,n = d.length;
    double res[][] = new double[n][n];
    //Store edges as an ArrayList
    ArrayList<Edge> edges = new ArrayList<Edge>();
    for(i=0;i<n-1;++i)
    {
        for(j=i+1;j<n;++j)
        {
            //Only non zero edges
            if (d[i][j] != 0.0) edges.add(new Edge(i,j,d[i][j]));
        }
    }
    //Sort the edges by weight
    Collections.sort(edges,new CompareEdge());
    //Don't do anything more if all the edges are zero
    if (edges.size() == 0) return(res);
    //List of variables that have been allocated
    ArrayList<Integer> v = new ArrayList<Integer>();
    //Pick cheapest edge
    v.add(edges.get(0).i);
    //Loop while there are still nodes to connect
    while(v.size() != n)
    {
        Edge e = LocateEdge(v,edges);
        if (v.indexOf(e.i) == -1) v.add(e.i);
        if (v.indexOf(e.j) == -1) v.add(e.j);
        res[e.i][e.j] = e.w;
        res[e.j][e.i] = e.w;
    }
    return(res);
}

}

Whenever I run the program, this is the outcome:

Matrix (Random Number Array):
[85.0, 11.0, 79.0, 25.0, 30.0]
[62.0, 55.0, 39.0, 21.0, 92.0]
[31.0, 76.0, 3.0, 74.0, 43.0]
[59.0, 97.0, 91.0, 60.0, 7.0]
[96.0, 44.0, 26.0, 66.0, 31.0]

MST3:
0.0 50.0 50.0 50.0 50.0
50.0 0.0 0.0 0.0 0.0
50.0 0.0 0.0 0.0 0.0
50.0 0.0 0.0 0.0 0.0
50.0 0.0 0.0 0.0 0.0

There are two other classes which handle storing the edge weights (Edge.java) and also comparing the edge weights (CompareEdges.java), but they are not relevant to this particular question.

I hope that somebody is able to help as I have spent a number of hours attempting to solve this.

Many thanks.

Mick

Answer 1

Here is the problem:

public static void randomArray(int n){

    n = 0;

    double[][] array = new double[][] {{n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}, {n, n, n, n, n}};
    double mst3[][] = MST.PrimsMST(array);

You create an array of 0’s, and you apply MST to it. And then you override your array with random numbers, but the MST method was called on the array of 0’s, not on the array of random numbers.

Also, on a design level, I think you should take some time to restructure and factorize your code a little, otherwise you will have a lot of problems when building more complicated projects:

• You should call the MST methods from your main method(), or from a method, not from the top-level of the class.
• You should also initialize your random generator in a method
• You don’t have to initialize an array with 0’s, you can just specify the size. (the = {} initialization should only be used when you want to initialize your array with specific values)
• You wrote 5 times an array displaying code, which is exactly the same, this is the sign that you should have a method doing this.
• Also, you use arrays of double to store int, so I think you probably want to switch to int

So I’m thinking your class should look more like this.

public class Lab6{
    static int[][] g= new int[][] {{0, 1, 2} , {1, 0, 3} , {2, 3, 0}};
    static int[][] lecExample = new int[][] {{0, 1, 2, 3, 0} , {1, 0, 6, 0, 5} , {2, 6, 0 ,4, 1} , {3, 0, 4, 0, 2} , {0, 5, 1, 2, 0}};


    public static void main(String[] args){
        displayArray(g);
        displayArray(MST.PrimMST(g));
        displayArray(lecExample);
        displayArray(MST.PrimMST(lecExample));

        int[][] randomArray = getRandomArray(50);
        displayArray(randomArray);
        displayArray(MST.PrimMST(randomArray));
    }

    public static int[][] getRandomArray(int n){
        int[][] a = new int[n][n];
        Random r = new Random();

        for(int i = 0; i < a.length; i++){
            for(int j = 0; j < a[i].length; j++){
                a[i][j] = r.nextInt();
            }
        }

        return a;
    }

    public static void displayArray(int[] a){
        for(int i = 0; i < a.length; i++){
            for(int j = 0; j < a[i].length; j++){
                System.out.print(" " + a[i][j]);
            }
            System.out.println("");
        }
    }
}