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j000sh's solution

to Pythagorean Triplet in the Java Track

Published at Jul 27 2020 · 0 comments
Instructions
Test suite
Solution

A Pythagorean triplet is a set of three natural numbers, {a, b, c}, for which,

a**2 + b**2 = c**2

and such that,

a < b < c

For example,

3**2 + 4**2 = 9 + 16 = 25 = 5**2.

Given an input integer N, find all Pythagorean triplets for which a + b + c = N.

For example, with N = 1000, there is exactly one Pythagorean triplet for which a + b + c = 1000: {200, 375, 425}.

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

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

Problem 9 at Project Euler http://projecteuler.net/problem=9

Submitting Incomplete Solutions

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

PythagoreanTripletTest.java

import java.util.Arrays;
import java.util.List;
import java.util.Collections;
import static org.junit.Assert.assertEquals;
import org.junit.Test;
import org.junit.Ignore;

public class PythagoreanTripletTest {

    @Test
    public void tripletsWhoseSumIs12() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(12)
                        .thatSumTo(12)
                        .build();
        List<PythagoreanTriplet> expected
                = Collections.singletonList(new PythagoreanTriplet(3, 4, 5));
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs108() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(108)
                        .thatSumTo(108)
                        .build();
        List<PythagoreanTriplet> expected
                = Collections.singletonList(new PythagoreanTriplet(27, 36, 45));
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs1000() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(1000)
                        .thatSumTo(1000)
                        .build();
        List<PythagoreanTriplet> expected
                = Collections.singletonList(new PythagoreanTriplet(200, 375, 425));
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs1001() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(1001)
                        .thatSumTo(1001)
                        .build();
        List<PythagoreanTriplet> expected = Collections.emptyList();
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs90() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(90)
                        .thatSumTo(90)
                        .build();
        List<PythagoreanTriplet> expected
                = Arrays.asList(
                        new PythagoreanTriplet(9, 40, 41),
                        new PythagoreanTriplet(15, 36, 39));
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs840() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(840)
                        .thatSumTo(840)
                        .build();
        List<PythagoreanTriplet> expected
                = Arrays.asList(
                        new PythagoreanTriplet(40, 399, 401),
                        new PythagoreanTriplet(56, 390, 394),
                        new PythagoreanTriplet(105, 360, 375),
                        new PythagoreanTriplet(120, 350, 370),
                        new PythagoreanTriplet(140, 336, 364),
                        new PythagoreanTriplet(168, 315, 357),
                        new PythagoreanTriplet(210, 280, 350),
                        new PythagoreanTriplet(240, 252, 348));
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void tripletsWhoseSumIs30000() {
        List<PythagoreanTriplet> actual
                = PythagoreanTriplet
                        .makeTripletsList()
                        .withFactorsLessThanOrEqualTo(30000)
                        .thatSumTo(30000)
                        .build();
        List<PythagoreanTriplet> expected
                = Arrays.asList(
                        new PythagoreanTriplet(1200, 14375, 14425),
                        new PythagoreanTriplet(1875, 14000, 14125),
                        new PythagoreanTriplet(5000, 12000, 13000),
                        new PythagoreanTriplet(6000, 11250, 12750),
                        new PythagoreanTriplet(7500, 10000, 12500));
        assertEquals(expected, actual);
    }

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

class PythagoreanTripletBuilder {
    private int factorsLessThan;
    private int sumTo;

    public PythagoreanTripletBuilder withFactorsLessThanOrEqualTo(int n) {
        this.factorsLessThan = n;
        return this;
    }

    public PythagoreanTripletBuilder thatSumTo(int n) {
        this.sumTo = n;
        return this;
    }

    public List<PythagoreanTriplet> build() {
        List<PythagoreanTriplet> triplets = new ArrayList<>();
        for (int a = 1; a < this.factorsLessThan - 2; ++a) {
            for (int b = a + 1; b < this.factorsLessThan - 1; ++b) {
                for (int c = b + 1; c < this.factorsLessThan; ++c) {
                    if ((a + b + c) == this.sumTo && (Math.pow(a, 2) + Math.pow(b, 2) == Math.pow(c, 2))) {
                        triplets.add(new PythagoreanTriplet(a, b, c));
                    }
                }
            }
        }
        return triplets;
    }
}

public class PythagoreanTriplet {
    private final int a;
    private final int b;
    private final int c;
    
    public PythagoreanTriplet(int a, int b, int c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public static PythagoreanTripletBuilder makeTripletsList() {
        return new PythagoreanTripletBuilder();
    }

    public String toString() {
        return "{" + this.a + ", " + this.b + ", " + this.c + "}"; 
    }

    public static void main(String[] args) {
        List<PythagoreanTriplet> triples = PythagoreanTriplet
            .makeTripletsList()
            .withFactorsLessThanOrEqualTo(840)
            .thatSumTo(840)
            .build();
        System.out.println(triples);
    }
}

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.

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