Question

Write a method static int [][] pr (int [] v) that returns a 2D array consisting of all the pairwise products
of the elements of v. E.g., for a = {{2, 3, 1} the method returns 2D array b = {{4, 6, 2}, {6, 9, 3}, {2, 3, 1}}. Please note: This is an 2010 exam question, taken from Software and Programming 1 module.

public class ETenBTwoD
{
    public static int [] [] pr (int [] v)
    {
        int [] a = {2, 3, 1};
        int [][] b = {{0, 0, 0},{0, 0, 0}, {0, 0, 0}};
        for (int i = 0; i<a.length; i++)
        {
            a[i] = a[i] * a[i];
        }
        return a;
    }
}

I am not sure how to do this. Help will be appreciated. Thanks

Answer 1

The key is to use a nested loop. As you can see v[i] takes the first number (choosen by the first loop) and multiplies it by all the numbers in v (parameter array) since j has to increment the full length of v. The first loop then picks out the second number and the second loop does it again. In b[i][j] the i represents the current number we are calculating pairwise products for.

public static void main(String[] args){
    int[] v = {5,4,3,2,1};
    pr(v);}
    public static int [] [] pr (int [] v)
    {
        int [][] b =new int[v.length][v.length] ;
        for (int i = 0; i<v.length; i++)
        {
            for(int j=0; j<v.length;j++){
                b[i][j] = v[i] * v[j];
            }
        }

        System.out.println(b[0][0]+" "+b[0][1]+" "+b[0][2]);
        System.out.println(b[1][0]+" "+b[1][1]+" "+b[1][2]);
        System.out.println(b[2][0]+" "+b[2][1]+" "+b[2][2]);
        return b;
    }