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artemkorsakov's solution

to Saddle Points in the Java Track

Published at Jun 16 2019 · 0 comments
Instructions
Test suite
Solution

Detect saddle points in a matrix.

So say you have a matrix like so:

    1  2  3
  |---------
1 | 9  8  7
2 | 5  3  2     <--- saddle point at column 1, row 2, with value 5
3 | 6  6  7

It has a saddle point at column 1, row 2.

It's called a "saddle point" because it is greater than or equal to every element in its row and less than or equal to every element in its column.

A matrix may have zero or more saddle points.

Your code should be able to provide the (possibly empty) list of all the saddle points for any given matrix.

The matrix can have a different number of rows and columns (Non square).

Note that you may find other definitions of matrix saddle points online, but the tests for this exercise follow the above unambiguous definition.

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

J Dalbey's Programming Practice problems http://users.csc.calpoly.edu/~jdalbey/103/Projects/ProgrammingPractice.html

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 java.util.*;

import static org.junit.Assert.assertEquals;

public class MatrixTest {

    @Test
    public void testCanIdentifySingleSaddlePoint() {
        Matrix matrix = new Matrix(Arrays.asList(
            Arrays.asList(9, 8, 7),
            Arrays.asList(5, 3, 2),
            Arrays.asList(6, 6, 7)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = Collections.singleton(new MatrixCoordinate(2, 1));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyThatEmptyMatrixHasNoSaddlePoints() {
        Matrix matrix = new Matrix(new ArrayList<>());

        Set<MatrixCoordinate> expectedSaddlePoints = Collections.emptySet();

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyLackOfSaddlePointsWhenThereAreNone() {
        Matrix matrix = new Matrix(Arrays.asList(
            Arrays.asList(1, 2, 3),
            Arrays.asList(3, 1, 2),
            Arrays.asList(2, 3, 1)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = Collections.emptySet();

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyMultipleSaddlePointsInAColumn() {
        Matrix matrix = new Matrix(Arrays.asList(
            Arrays.asList(4, 5, 4),
            Arrays.asList(3, 5, 5),
            Arrays.asList(1, 5, 4)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
            new MatrixCoordinate(1, 2),
            new MatrixCoordinate(2, 2),
            new MatrixCoordinate(3, 2)
        ));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyMultipleSaddlePointsInARow() {
        Matrix matrix = new Matrix(Arrays.asList(
                Arrays.asList(6, 7, 8),
                Arrays.asList(5, 5, 5),
                Arrays.asList(7, 5, 6)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
                new MatrixCoordinate(2, 1),
                new MatrixCoordinate(2, 2),
                new MatrixCoordinate(2, 3)
        ));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifySaddlePointInBottomRightCorner() {
        Matrix matrix = new Matrix(Arrays.asList(
            Arrays.asList(8, 7, 9),
            Arrays.asList(6, 7, 6),
            Arrays.asList(3, 2, 5)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = Collections.singleton(new MatrixCoordinate(3, 3));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifySaddlePointsInANonSquareMatrix() {
        Matrix matrix = new Matrix(Arrays.asList(
                Arrays.asList(3, 1, 3),
                Arrays.asList(3, 2, 4)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
                new MatrixCoordinate(1, 3),
                new MatrixCoordinate(1, 1)
        ));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyThatSaddlePointsInASingleColumnMatrixAreThoseWithMinimumValue() {
        Matrix matrix = new Matrix(Arrays.asList(
                Collections.singletonList(2),
                Collections.singletonList(1),
                Collections.singletonList(4),
                Collections.singletonList(1)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
                new MatrixCoordinate(2, 1),
                new MatrixCoordinate(4, 1)
        ));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

    @Ignore("Remove to run test")
    @Test
    public void testCanIdentifyThatSaddlePointsInASingleRowMatrixAreThoseWithMaximumValue() {
        Matrix matrix = new Matrix(Arrays.asList(
                Arrays.asList(2, 5, 3, 5)
        ));

        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
                new MatrixCoordinate(1, 2),
                new MatrixCoordinate(1, 4)
        ));

        assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
    }

}
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

class Matrix {
    private List<List<Integer>> rows;
    private List<Integer> maxesPerRow;
    private List<Integer> minsPerColumn;

    Matrix(List<List<Integer>> values) {
        rows = values;
    }

    Set<MatrixCoordinate> getSaddlePoints() {
        if (rows.isEmpty()) {
            return new HashSet<>();
        }

        Set<MatrixCoordinate> result = new HashSet<>();
        maxesPerRow = getMaxesPerRow();
        minsPerColumn = getMinsPerColumn();

        for (int i = 0; i < rows.size(); i++) {
            for (int j = 0; j < rows.get(0).size(); j++) {
                if (isSaddlePoint(i, j)) {
                    result.add(new MatrixCoordinate(i, j));
                }
            }
        }

        return result;
    }

    private List<Integer> getMaxesPerRow() {
        List<Integer> maxesPerRow = new ArrayList<>();
        for (List<Integer> row : rows) {
            int max = row.stream().mapToInt(it -> it).max().orElse(Integer.MIN_VALUE);
            maxesPerRow.add(max);
        }

        return maxesPerRow;
    }

    private List<Integer> getMinsPerColumn() {
        List<Integer> minsPerColumn = new ArrayList<>();
        for (int i = 0; i < rows.get(0).size(); i++) {
            int min = Integer.MAX_VALUE;
            for (List<Integer> row : rows) {
                if (row.get(i) < min) {
                    min = row.get(i);
                }
            }
            minsPerColumn.add(min);
        }

        return minsPerColumn;
    }

    private boolean isSaddlePoint(int row, int column) {
        return rows.get(row).get(column) == maxesPerRow.get(row)
                && rows.get(row).get(column) == minsPerColumn.get(column);
    }
}

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