Avatar of woody

woody's solution

to Matrix in the Java Track

Published at Nov 14 2019 · 0 comments
Instructions
Test suite
Solution

Given a string representing a matrix of numbers, return the rows and columns of that matrix.

So given a string with embedded newlines like:

9 8 7
5 3 2
6 6 7

representing this matrix:

    1  2  3
  |---------
1 | 9  8  7
2 | 5  3  2
3 | 6  6  7

your code should be able to spit out:

  • A list of the rows, reading each row left-to-right while moving top-to-bottom across the rows,
  • A list of the columns, reading each column top-to-bottom while moving from left-to-right.

The rows for our example matrix:

  • 9, 8, 7
  • 5, 3, 2
  • 6, 6, 7

And its columns:

  • 9, 5, 6
  • 8, 3, 6
  • 7, 2, 7

Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

$ gradle test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore("Remove to run test") annotation.

Source

Warmup to the saddle-points warmup. http://jumpstartlab.com

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

MatrixTest.java

import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertArrayEquals;

public class MatrixTest {

    @Test
    public void extractRowFromOneNumberMatrixTest() {
        String matrixAsString = "1";
        int rowIndex = 1;
        int[] expectedRow = {1};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractRowFromMatrixTest() {
        String matrixAsString = "1 2\n3 4";
        int rowIndex = 2;
        int[] expectedRow = {3, 4};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractRowFromDiffWidthsMatrixTest() {
        String matrixAsString = "1 2\n10 20";
        int rowIndex = 2;
        int[] expectedRow = {10, 20};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractRowFromNonSquareMatrixTest() {
        String matrixAsString = "1 2 3\n4 5 6\n7 8 9\n8 7 6";
        int rowIndex = 4;
        int[] expectedRow = {8, 7, 6};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedRow, matrix.getRow(rowIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractColumnFromOneNumberMatrixTest() {
        String matrixAsString = "1";
        int columnIndex = 1;
        int[] expectedColumn = {1};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractColumnMatrixTest() {
        String matrixAsString = "1 2 3\n4 5 6\n7 8 9";
        int columnIndex = 3;
        int[] expectedColumn = {3, 6, 9};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractColumnFromNonSquareMatrixTest() {
        String matrixAsString = "1 2 3 4\n5 6 7 8\n9 8 7 6";
        int columnIndex = 4;
        int[] expectedColumn = {4, 8, 6};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
    }

    @Ignore("Remove to run test")
    @Test
    public void extractColumnFromDiffWidthsMatrixTest() {
        String matrixAsString = "89 1903 3\n18 3 1\n9 4 800";
        int columnIndex = 2;
        int[] expectedColumn = {1903, 3, 4};

        Matrix matrix = new Matrix(matrixAsString);

        assertArrayEquals(expectedColumn, matrix.getColumn(columnIndex));
    }
}
class Matrix {

    private int[][] matrix;

    Matrix(String matrixAsString) {
        String[] rows = matrixAsString.split("\n");
        for (int i = 0; i < rows.length; i++) {
            String[] columns = rows[i].split(" ");

            if (matrix == null) {
                matrix = new int[rows.length][columns.length];
            }

            for (int j = 0; j < columns.length; j++) {
                matrix[i][j] = Integer.valueOf(columns[j]);
            }
        }
    }

    int[] getRow(int rowNumber) {
        return matrix[rowNumber - 1];
    }

    int[] getColumn(int columnNumber) {
        int[] column = new int[matrix.length];

        for (int i = 0; i < column.length; i++) {
            column[i] = matrix[i][columnNumber - 1];
        }

        return column;
    }
}

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?