function change variable in java

Question

I want to check if a variable ("matrix") is changed when sent to a specific function ("moveAround"), so I created another variable ("matrixB") and I gave it the value that comes from the function given the first variable, and now I compare the two variables.

The problem is that for some reason the first variable is changed as well as the second.

The code looks like this:

matrixB = moveAround(matrix, userDir);
cmpMatrices(matrix, matrixB);

The function "moveAround" is meant to return a different matrix then given (most of the time). the function "cmpMatrices" returns true if the two matrices are alike, and false there is at least one component that does not equal.

For some reason when I get "matrixB" from the "moveAround" function after giving it "matrix" it changes both "matrix" and "matrixB".

The function "moveAround" calls another function name "moveUp"/"moveDown"/"moveRight"/"moveLeft" in respect to "userDir".

The function "moveUp" looks like this:

public static int[][] moveUp(int[][] matrix) {

    int i, j, m;

    for (j = 0; j < matrix.length; j++) {
        m = 1;
        for (i = 0; i + m < matrix.length;) {
            if (matrix[i + m][j] == 0) {
                m++;
            } else {
                if (matrix[i + m][j] == matrix[i][j]) {
                    matrix[i][j] = 2 * matrix[i][j];
                    matrix[i + m][j] = 0;
                    i++;
                } else {
                    if (matrix[i][j] == 0) {
                        matrix[i][j] = matrix[i + m][j];
                        matrix[i + m][j] = 0;
                        m++;
                    } else {
                        if (m != 1) {
                            matrix[i + 1][j] = matrix[i + m][j];
                            matrix[i + m][j] = 0;
                            i++;
                        } else {
                            i++;
                        }
                    }
                }
            }
        }
    }
    return matrix;
}

The other move Down/Right/Left are very similar just different direction.

The function "cmpMatrices" looks like this:

public static boolean cmpMatrices(int[][] matrixA, int[][] matrixB){
    for(int i = 0; i < matrixA.length; i++){
        for(int j = 0; j < matrixA[i].length; j++){
            if(matrixA[i][j] != matrixB[i][j]){
                return false;
            }
        }
    }
    return true;
}

can you add definition of moveAround method.It could help to understand your issue. — Deep, Jan 17 '20 at 11:09
If you did something along the lines of `matrixB = matrix` they're pointing to the same object, so if you change one you'll change the other as well. — Federico klez Culloca, Jan 17 '20 at 11:12
Just create a boolean variable eg `changed` and set it to `true` every time you update the matrix... — dan1st, Jan 17 '20 at 11:28
The `moveUp` method modifies the matrix in place instead of returning a new one, so the one that you pass as a parameter and the one returned are the same matrix, so when you modify one you modify the other as well. — Federico klez Culloca, Jan 17 '20 at 11:35

score 0 · Answer 1 · answered Jan 17 '20 at 11:36

moveAround alters the passed matrix and returns it. A bit redundant: either return void, or make a new matrix object.

Variables in java typically hold a value, the matrix object. Assigning to an other variable will share that matrix object.

    int[][] a = new int[][]{{1, 2},{3, 4}};
    int[][] b = deepCopy(a);
    a[0][0] = 10;
    System.out.println(Arrays.deepToString(b));

static int[][] deepCopy(int[][] a) {
    int[][] clone = a.clone();
    for (int i = 0; i < a.length; ++i) {
        clone[i] = clone[i].clone();
    }
    return clone;
}

score 0 · Answer 2 · answered Jan 17 '20 at 11:44

0

You need to copy the object before modifying it.

This might help you How do I do a deep copy of a 2d array in Java?

answered Jan 17 '20 at 11:44

Mike

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function change variable in java

2 Answers2