exercism/java/matrix

Matrix.java

class Matrix {

    private int[][] matrixAsArray;

    Matrix(String matrixAsString) {
        this.matrixAsArray = convertToArray(matrixAsString);
    }

    int[] getRow(int rowNumber) {
       return matrixAsArray[rowNumber];
        }

    int[] getColumn(int columnNumber) {
      int[] column = new int[matrixAsArray.length];
        for (int i = 0; i < column.length; i++) {
          column[i] = getRow(i)[columnNumber];
        }
        return column;
    }

    private int[][] convertToArray(String matrixAsString) {
      String[] rows = matrixAsString.split("[\\n]+");
      int[][] matrixArray = new int[rows.length][];

      for ( int i = 0; i < rows.length; i++) {
        String[] rowChars = rows[i].split("\\s");
        int[] row = new int[rowChars.length];
        for (int c = 0; c < row.length; c++) {
          row[c] = Integer.valueOf(rowChars[c]);
        }
        matrixArray[i] = row;
      }
      return matrixArray;
    }
}

MatrixTest.java

import org.junit.Test;

import static org.junit.Assert.assertArrayEquals;

public class MatrixTest {
	
	@Test
	public void extractRowFromOneNumberMatrixTest() {
		String matrixAsString = "1";
		int rowIndex = 0;
		int[] expectedRow = {1};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
	}
	
	@Test
	public void extractRowFromMatrixTest() {
		String matrixAsString = "1 2\n3 4";
		int rowIndex = 1;
		int[] expectedRow = {3, 4};
		
		Matrix matrix = new Matrix(matrixAsString);
		assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
	}
	
	@Test
	public void extractRowFromDiffWidthsMatrixTest() {
		String matrixAsString = "1 2\n10 20";
		int rowIndex = 1;
		int[] expectedRow = {10, 20};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
	}
	
	@Test
	public void extractRowFromNonSquareMatrixTest() {
		String matrixAsString = "1 2 3\n4 5 6\n7 8 9\n8 7 6";
		int rowIndex = 2;
		int[] expectedRow = {7, 8, 9};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
	}
	
	@Test
	public void extractColumnFromOneNumberMatrixTest() {
		String matrixAsString = "1";
		int columnIndex = 0;
		int[] expectedColumn = {1};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
	}
	
	@Test
	public void extractColumnMatrixTest() {
		String matrixAsString = "1 2 3\n4 5 6\n7 8 9";
		int columnIndex = 2;
		int[] expectedColumn = {3, 6, 9};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
	}
	
	@Test
	public void extractColumnFromNonSquareMatrixTest() {
		String matrixAsString = "1 2 3\n4 5 6\n7 8 9\n8 7 6";
		int columnIndex = 2;
		int[] expectedColumn = {3, 6, 9, 6};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
	}
	
	@Test
	public void extractColumnFromDiffWidthsMatrixTest() {
		String matrixAsString = "89 1903 3\n18 3 1\n9 4 800";
		int columnIndex = 1;
		int[] expectedColumn = {1903, 3, 4};
		
		Matrix matrix = new Matrix(matrixAsString);
		
		assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
	}
}

Mentor discussion

zantetsuken88
Hi, my first attempt at 2d arrays so maybe there's a better way of doing it, but all the tests pass

dusan-rychnovsky
Much better :)

I think it would be better to internally keep the data in the parsed array form, rather than as string. This way you don't need to parse the matrix again every time. Especially in the getColumn method you parse the string number of rows + 1 times!

zantetsuken88
Hi, I don't really understand the feedback. Would you perhaps be able to explain in more detail please?

Thanks

dusan-rychnovsky
Yes, I mean, what if you store int[][] matrix instead of String matrixAsString? Then in getRow you can just return matrix[rowNumber]. This is a much more efficient solution, considering that your current solution needs to parse the matrix string (row + 1) times when you call getColumn.