Question

I know this question was answered before, but I cant still handle it with my code.
Please can someone pointing out how can I fix it on this particular code. Id like to call trace(); but dont know where to call new trace. I tried different stuff from here but it does not work for me. Thank you!

package matr;

import java.util.Scanner;

final public class Matrix {

    private final int M;
    private final int N;
    private double[][] data;



    public Matrix(int M, int N) {

        this.M = M;
        this.N = N;

        data = new double[M][N];

    }

    public Matrix(double[][] data) {
        M = data.length;
        N = data[0].length;
        this.data = new double[M][N];
        for (int i = 0; i < M; i++)
            for (int j = 0; j < N; j++)
                this.data[i][j] = data[i][j];
    }

    private Matrix(Matrix A) {
        this(A.data);
    }

    public static Matrix random(int M, int N, int r) {

        Matrix A = new Matrix(M, N);
        for (int i = 0; i < M; i++) {
            for (int j = 0; j < N; j++) {
                A.data[i][j] = (Math.random() * r);
            }
        }
        return A;
    }

    public double trace() {
        // trace a = new trace();

        double t = 0;
        for (int i = 0; i < Math.min(M, N); i++) {
            t += data[i][i];
        }
        return t;
    }

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        System.out.println("rows: ");
        try {
            int x = Math.abs(scan.nextInt());

            System.out.println("columns: ");
            int y = Math.abs(scan.nextInt());
            System.out
                    .println("generate: ");
            int r = scan.nextInt();

            Matrix A = Matrix.random(x, y, r);
            System.out.println("random A");

            trace();

        } catch (java.util.InputMismatchException e) {
            System.out.println("Please enter a valid int");
        }
    }
}

Answer 1

call A.trace()

1. use lower-case names for variables and fields
2. your main method is static. and your trace() method is a non-static method of the Matrix class. That means you have to call trace() on an instance of Matrix.