Published at Feb 20 2019
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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.

Make sure you have read D page on exercism.io. This covers the basic information on setting up the development environment expected by the exercises.

Get the first test compiling, linking and passing by following the three rules of test-driven development. Create just enough structure by declaring namespaces, functions, classes, etc., to satisfy any compiler errors and get the test to fail. Then write just enough code to get the test to pass. Once you've done that, uncomment the next test by moving the following line past the next test.

```
static if (all_tests_enabled)
```

This may result in compile errors as new constructs may be invoked that you haven't yet declared or defined. Again, fix the compile errors minimally to get a failing test, then change the code minimally to pass the test, refactor your implementation for readability and expressiveness and then go on to the next test.

Try to use standard D facilities in preference to writing your own low-level algorithms or facilities by hand. DRefLanguage and DReference are references to the D language and D standard library.

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

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

```
module difference_of_squares;
unittest {
const int allTestsEnabled = 0;
assert(squares(5).squareOfSum == 225);
static if (allTestsEnabled) {
assert(squares(10).squareOfSum == 3_025);
assert(squares(100).squareOfSum == 25_502_500);
assert(squares(5).sumOfSquares == 55);
assert(squares(10).sumOfSquares == 385);
assert(squares(100).sumOfSquares == 338_350);
assert(squares(0).difference == 0);
assert(squares(5).difference == 170);
assert(squares(10).difference == 2_640);
assert(squares(100).difference == 25_164_150);
}
}
```

```
module difference_of_squares;
unittest {
const int allTestsEnabled = 0;
assert(squares(5).squareOfSum == 225);
static if (allTestsEnabled) {
assert(squares(10).squareOfSum == 3_025);
assert(squares(100).squareOfSum == 25_502_500);
assert(squares(5).sumOfSquares == 55);
assert(squares(10).sumOfSquares == 385);
assert(squares(100).sumOfSquares == 338_350);
assert(squares(0).difference == 0);
assert(squares(5).difference == 170);
assert(squares(10).difference == 2_640);
assert(squares(100).difference == 25_164_150);
}
}
class Squares {
int difference;
int squareOfSum;
int sumOfSquares;
}
int squareOf(int n) {
return n * n;
}
Squares squares(int n) {
Squares result = new Squares();
result.squareOfSum = squareOf((n * (n + 1)) / 2);
result.sumOfSquares = (n * (n + 1) * ((2 * n) + 1)) / 6;
result.difference = result.squareOfSum - result.sumOfSquares;
return result;
}
```

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?

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