CODEWITHSUNDEEP
java
S. Inferno
S. Inferno

Reputation: 11

Implementing an array rotating method in Java

This is my first post here so forgive any formatting errors and the like.

So basically I'm having difficulty implementing a method that takes a given array and rotates it 90 degrees clockwise. We're supposed to implement all given methods in terms of an attribute "imp" like so:

class Matrix {
int[][] imp;
/* All methods in Matrix are implemented in terms of ’imp’ */
}

This is what I have so far in my method implementation: 

/**
 * <p> Rotate the matrix by 90 degrees.
 * <p> The old 1st row becomes the new last column, 
 *     the old 2nd row becomes the new 2nd last column, and so on.
 * <p> Hint: attribute 'imp' should be reassigned to a new matrix. 
 * <p> See TestMatrix class for how this method can be used.
 */
public void rotateClockwise() {
    int rows = imp.length;
    int cols = imp[0].length;
    int[][] m2 = new int[cols][rows];
    int i = 0;
    int j = 0;
    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++) {
            m2[i][rows - 1 - i] = imp[i][j];
        }
    }
    imp = m2;
}

This is the test case being used to test my method:

@Test
public void test18_RotateClockwise() {
    Matrix m1 = new Matrix(3, 4); 

    int[] nr0 = {1 , 2 , 3 , 4 };
    int[] nr1 = {5 , 6 , 7 , 8 };
    int[] nr2 = {9 , 10, 11, 12};
    m1.setRow(0, nr0);
    m1.setRow(1, nr1);
    m1.setRow(2, nr2);

    int[] nc0 = {9, 10, 11, 12};
    int[] nc1 = {5, 6 , 7 , 8 };
    int[] nc2 = {1, 2 , 3 , 4 }; 

    int[] nr0_ = {12, 11 , 10 , 9 };
    int[] nr1_ = {8 , 7  , 6  , 5 };
    int[] nr2_ = {4 , 3  , 2  , 1 }; 

    m1.rotateClockwise();
    assertArrayEquals(nc0, m1.getColumn(0));
    assertArrayEquals(nc1, m1.getColumn(1));
    assertArrayEquals(nc2, m1.getColumn(2));

    m1.rotateClockwise();
    assertArrayEquals(nr0_, m1.getRow(0));
    assertArrayEquals(nr1_, m1.getRow(1));
    assertArrayEquals(nr2_, m1.getRow(2));
}

I understand what the test case is trying to do, and my code compiles and doesn't have any exceptions or errors, but I'm getting the wrong output like so:

arrays first differed at element [0]; expected:<9> but was:<0>

I'm assuming this means that I'm basically rotating an empty array. I guess my question is how do I get the array from the test case into my method to rotate it? I tried using getRow and getColumn methods but I get an error telling me I can't call those methods on the attribute imp. Any help is appreciated. Thanks!

Upvotes: 0

Views: 263

Answers (2)

user3738870
user3738870

Reputation: 1632

I think this line is incorrect, as the position into which the value is being written should definitely depend on j as well:

            m2[i][rows - 1 - i] = imp[i][j];

I haven't tested it but this could be a working version:

            m2[j][rows - 1 - i] = imp[i][j];

Edit: Why should j be the first index instead of i? The inner loop goes through the elements of a row inside the original matrix. These elements should be inserted into the same column of the new matrix, but in different rows. The index of the row into which the current element is inserted should increase as the column index of the element in the original matrix increases. j represents the column index in the original matrix, therefore it should represent the row index in the new/rotated matrix.

Upvotes: 2

lkogs
lkogs

Reputation: 63

Do you have a proper constructor for Matrix class?

My hint for you is to check out matrix transpose implementations in java. It's very similar algorithm to yours.

For example: Transpose matrix on java67

Upvotes: 0

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