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rootulp's solution

to Difference Of Squares in the Java Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.

The square of the sum of the first ten natural numbers is (1 + 2 + ... + 10)² = 55² = 3025.

The sum of the squares of the first ten natural numbers is 1² + 2² + ... + 10² = 385.

Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.

Running the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

Source

Problem 6 at Project Euler http://projecteuler.net/problem=6

Submitting Incomplete Solutions

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

DifferenceOfSquaresCalculatorTest.java

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

import static org.junit.Assert.assertEquals;

public class DifferenceOfSquaresCalculatorTest {

    private DifferenceOfSquaresCalculator calculator;

    @Before
    public void setUp() {
        calculator = new DifferenceOfSquaresCalculator();
    }

    @Test
    public void testSquareOfSumUpToOne() {
        int expected = 1;
        int actual = calculator.computeSquareOfSumTo(1);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testSquareOfSumUpToFive() {
        int expected = 225;
        int actual = calculator.computeSquareOfSumTo(5);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testSquareOfSumUpToHundred() {
        int expected = 25502500;
        int actual = calculator.computeSquareOfSumTo(100);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testSumOfSquaresUpToOne() {
        int expected = 1;
        int actual = calculator.computeSumOfSquaresTo(1);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testSumOfSquaresUpToFive() {
        int expected = 55;
        int actual = calculator.computeSumOfSquaresTo(5);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testSumOfSquaresUpToHundred() {
        int expected = 338350;
        int actual = calculator.computeSumOfSquaresTo(100);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testDifferenceOfSquaresUpToOne() {
        int expected = 0;
        int actual = calculator.computeDifferenceOfSquares(1);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testDifferenceOfSquaresUpToFive() {
        int expected = 170;
        int actual = calculator.computeDifferenceOfSquares(5);
        assertEquals(expected, actual);
    }

    @Ignore("Remove to run test")
    @Test
    public void testDifferenceOfSquaresUpToHundred() {
        int expected = 25164150;
        int actual = calculator.computeDifferenceOfSquares(100);
        assertEquals(expected, actual);
    }

}
import java.util.stream.IntStream;

public final class DifferenceOfSquaresCalculator {

  public int computeSquareOfSumTo(int num) {
    return (int) Math.pow(IntStream.rangeClosed(0, num).sum(), 2);
  }

  public int computeSumOfSquaresTo(int num) {
    return IntStream.rangeClosed(0, num).map(x -> (int) Math.pow(x, 2)).sum();
  }

  public int computeDifferenceOfSquares(int num) {
    return Math.abs(computeSquareOfSumTo(num) - computeSumOfSquaresTo(num));
  }

}

Community comments

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Avatar of jtigger

Hey @rootulp!

For some addition fun around this exercise, check out a small collection of styles that have arisen: http://exercism.io/submissions/85467bc28d41452f92f21640d0946e90

Would be interesting to hear your thoughts on the trade-offs of each style!

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