Avatar of MrGarri

MrGarri's solution

to Darts in the Java Track

Published at Apr 16 2019 · 2 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Write a function that returns the earned points in a single toss of a Darts game.

Darts is a game where players throw darts to a target.

In our particular instance of the game, the target rewards with 4 different amounts of points, depending on where the dart lands:

  • If the dart lands outside the target, player earns no points (0 points).
  • If the dart lands in the outer circle of the target, player earns 1 point.
  • If the dart lands in the middle circle of the target, player earns 5 points.
  • If the dart lands in the inner circle of the target, player earns 10 points.

The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are concentric) defined by the coordinates (0, 0).

Write a function that given a point in the target (defined by its real cartesian coordinates x and y), returns the correct amount earned by a dart landing in that point.

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

Inspired by an exercise created by a professor Della Paolera in Argentina

Submitting Incomplete Solutions

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

DartsTest.java

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

import static org.junit.Assert.assertEquals;

public class DartsTest {

    @Test
    public void testDartOutsideTarget() {
        Darts darts = new Darts(-9, 9);
        assertEquals(0, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnBorderOfTarget() {
        Darts darts = new Darts(0, 10);
        assertEquals(1, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInOuterCircle() {
        Darts darts = new Darts(4, 4);
        assertEquals(1, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInBorderBetweenOuterAndMiddleCircles() {
        Darts darts = new Darts(5, 0);
        assertEquals(5, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnMiddleOfCircle() {
        Darts darts = new Darts(0.8, -0.8);
        assertEquals(5, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsOnBorderBetweenMiddleAndInnerCircles() {
        Darts darts = new Darts(0, -1);
        assertEquals(10, darts.score());
    }

    @Ignore("Remove to run test")
    @Test
    public void testDartLandsInTheInnerCircle() {
        Darts darts = new Darts(-0.1, -0.1);
        assertEquals(10, darts.score());
    }

}
import java.awt.geom.Point2D;

class Darts {

    private final double x;
    private final double y;
    private final Point2D CENTER = new Point2D.Double(0, 0);

    Darts(double x, double y) {
        this.x = x;
        this.y = y;
    }

    int score() {
        Point2D hit = new Point2D.Double(x, y);

        if (hit.distance(CENTER) > 10)
            return 0;
        else if (hit.distance(CENTER) > 5)
            return 1;
        else if (hit.distance(CENTER) > 1)
            return 5;
        else
            return 10;
    }

}

Community comments

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Avatar of jlacar

I starred this for the use of Point2D and its distance() method. I also used the name hit in my solution; I think it's a great name that expresses the idea it represents very well. Very good choices, if I say so myself. ;)

I made the same comment on other solutions but will repeat here so you don't have to hunt it down: The API suggests that Dart is an immutable class so I think it's a better choice to calculate the score in the constructor instead and have a final field score rather than keeping the x and y coordinates around longer than they really need to be.

Avatar of jlacar

I've seen this logic in other solutions and I think the aversion to the <= is costing some clarity. I find it a bit disconcerting to see the ring distance boundaries and the corresponding scores in separate sections of the if-else-if statements. At first glance, it appears to be incongruent with the requirements and I had to take a few seconds to go over the logic to make sure I wasn't getting cross-eyed.

On the other hand, I think using <= is more straightforward and the code aligns well with the requirement statement

if (distance <= 1)
    score = 10
else if (distance <= 5)
    score = 5
else if (distance <= 10)
    score = 1
else 
    score = 0

What can you learn from this solution?

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

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