2-dimensional Array - Print Black Image Depending On Number Of Rows And Columns

Nov 21, 2014

I wrote this code which print a black image depends on the number of rows and columns you give it

public class BlackImg {
private Mycolor[][] colorArr; //Mycolor is a class i wrote that represents colors.
// no need for showing the class here.
// so i created here an array of type Mycolor, for example:
// { {(255,255,255), {(127,127,0)} }

[Code] .....

my problem is that my output comes good except the last line ,

Output:
(0,0,0) (0,0,0) (0,0,0) (0,0,0)
(0,0,0) (0,0,0) (0,0,0) (0,0,0)
(0,0,0) (0,0,0) (0,0,0) (0,0,0)
BlackImg@1db9742 //what is this line , why does it showing up ?

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Matrix Algebra - Number Of Rows And Columns

Mar 19, 2015

I am continuing on in trying to build up the basics of matrix algebra from scratch.

I have created an object class called Matrix

import java.util.ArrayList;
public class Matrix {
public int NumRows;
public int NumColumns;

// This array contains the entries of our matrix.
ArrayList<Double> entry = new ArrayList<Double>();

[Code] ......

Bottom line: a matrix has a number of rows and a number of columns, and for each pair of row and column, we have a number in our matrix. The DisplayMatrix method prints my matrix to the screen, and the GetEntry method returns a particular value. This all works fine and dandy as far as I can tell.

A fundamental operation done to matrices to obtain a special matrix called the RREF is the process of switching 2 rows. Here is a method I have written that switches two rows of a matrix, and outputs the result as a new matrix. Note that I want the original matrix to be unchanged.

// Switch two rows
public static Matrix SwapRows(Matrix A, int r1, int r2){
if(r1<1 || r1>A.NumRows || r2<1 || r2>A.NumRows)
PRINTLN("illegally switching rows");
Matrix C = A;
double dummy[] = new double[A.NumColumns];

[Code] ....

How I call this, inside a public static void main(String[] args), is as follows:

// Declares that A is a 2 by 2 matrix.
Matrix A = new Matrix(2,2);

// We now add values in. The top left entry of A is 4, the top right entry of A is 1, the bottom left entry of A is 2, and the bottom right entry of A is 6.

double pony[]= new double[4];
pony[0]=4;
pony[1]=1;
pony[2]=2;
pony[3]=6;
A.AddEntries(pony);

// We can display the matrix in the output, and it looks exactly as expected!

A.DisplayMatrix();

// I am now going to create a new matrix called B. It is going to be obtained by flipping the first and second rows of A.

//Note that I want A is stay as I initialized it.

//I dont want A to have it's 2 rows switched. I want B to the matrix obtained by switching two rows of A.

Matrix B=SwapRows(A,1,2);
B.DisplayMatrix();

// Displaying B gives me the desired result. However, if I now display A again, it is the same as B.

A.DisplayMatrix();

Why is my matrix A being modified? Of course, I am more than capable of providing more details/comments if requested. I suspect that this is a super silly mistake.