Question

I can’t seem to get this toString() method to work? The deepToString method works perfectly, except that I have to print them out in an organized way, like a matrix would look like with aligned rows and columns. I had it working a while ago, but I changed something and now god knows what I did, I have no idea how to get it back. Anyhow, does anyone know how to output a multidimensional array to matrix like string form?

Also, another issue I am having is figuring out how to check if the numbers >= 0, because they can’t be negative. Not sure how to do that? Thought I could store each value as it loops and check if it’s negative, but I just keep getting confused and/or running into errors. Any help regarding these issues would be greatly appreciated, I have been working on this for like 5hrs and have not gotten anywhere! _

Here is the code I have so far:

import java.util.Arrays;

public class MatrixOperations {
public static void main(String[] args) {
    double[][] matrix1 = { { 0.0, 1.0, 2.0 }, { 3.0, 4.0, 5.0 },
            { 6.0, 7.0, 0.8 }, };

    double[][] matrix2 = { { 1.0, 1.0, 1.0 }, { 0.0, 0.0, 0.0 },
            { 2.0, 2.0, 2.0 } };

    System.out.println(toString(matrix1));
    System.out.println(Arrays.deepToString(add(matrix1, matrix2)));
}

// Throws an IllegalArgumentException unless A and B contain n arrays of
// doubles, each of
// which contains m doubles, where both n and m are positive. (In other
// words, both A
// and B are n-by-m arrays.)
//
// Otherwise, returns the n-by-m array that represents the matrix sum of A
// and B.

public static double[][] add(double[][] A, double[][] B) {
    if (A.length != B.length || A[1].length != B[1].length) {
        throw new IllegalArgumentException("Rows and Columns Must Be Equal");
    }
    double[][] S = new double[A.length][A[1].length];
    for (int i = 0; i < A.length; i++) {
        // double valueAt = ;
        for (int j = 0; j < A[1].length; j++) {
            S[i][j] = A[i][j] + B[i][j];
        }
    }
    return S;
}

// Throws an IllegalArgumentException unless A contains n arrays of doubles,
// each of
// which contains k doubles, and B contains k arrays of doubles, each of
// which contains
// m doubles, where n, k, and m are all positive. (In other words, A is an
// n-by-k array and B is a k-by-m array.)

// Otherwise, returns the n-by-m array that represents the matrix product of
// A and B.

// public static double[][] mul (double[][] A, double[][] B) {
// if (A[1].length != B.length){
// throw new IllegalArgumentException("Column-A Must Equal Row-B");
// }
// }

// Throws an IllegalArgumentException unless M contains n arrays of doubles,
// each of
// which contains m doubles, where both n and m are positive. (In other
// words, M
// is a n-by-m array.

// Otherwise, returns a String which, when printed, will be M displayed as a
// nicely
// formatted n-by-m table of doubles.

public static String toString(double[][] M) {
    String separator = ", ";
    StringBuffer result = new StringBuffer();
    if (M.length > 0) {
        result.append(M[0]);
        for (int i = 0; i < M.length; i++) {
            result.append(separator);
            result.append(M[i]);
        }
    }
    return result.toString();
}
}

Thank you for all your help! 🙂

Answer 1

Your toString method will not print the second dimension, you need two for loops:

public static String toString(double[][] M) {
    String separator = ", ";
    StringBuffer result = new StringBuffer();

    // iterate over the first dimension
    for (int i = 0; i < M.length; i++) {
        // iterate over the second dimension
        for(int j = 0; j < M[i].length; j++){
            result.append(M[i][j]);
            result.append(separator);
        }
        // remove the last separator
        result.setLength(result.length() - separator.length());
        // add a line break.
        result.append("\n");
    }
    return result.toString();
}

To check all the numbers in a given matrix are non-negative, you again need to iterate through every element in the matrix. In the case where any number is negative, you can immediately return, if you get through the whole matrix without finding any negatives, then there are none:

public static boolean isNonNegative(double[][] m){
    // iterate over first dimension
    for(int i = 0; i < m.length; i++){
        // iterate over second dimension
        for(int j = 0; j < m[i].length; j++){
            if(m[i][j] < 0){
                // found a negative element, return immediately
                // since this means the whole matrix cannot be
                // called non-negative
                return false;
            }
        }
    }
    // if we get here then no elements have been found to be negative
    // thus the matrix is non-negative.
    return true;
}

On a side note, in your add function on the line where you say for (int j = 0; j < A[1].length; j++) {, you might have a problem if your matrix is 1xN. In Java, arrays are 0-indexed, the first row is 0, not 1. A generally safer approach for you would be to do what I’ve shown in these two code samples and use A[i].length as you’re assured based on your for loop indexed on i that the i-th row of A exists.