# Using Recursion to Loop in Elm

This post is centered on the following problem:

```
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.
```

Credit for the problem goes to exercism.io.

The plan is:

```
1) solve it with a `for` loop in Javascript,
2) solve it with recursion in Javascript,
3) translate the recursive solution to Elm
```

### With a `for`

Loop

The `for`

loop solution goes like this:

```
-- get the square of the sum of n by:
-- going from 1 to n
-- and adding each number to a total
-- return the total after the loop is done
-- get the sum of the squares of n by:
-- going from 1 to n
-- and adding the square of each number to a total
-- return the total after the loop is done
-- subtract the latter from the former
```

In code, it looks something like the following.

```
function squareOfSum(number){
let sum = 0;
for (let i = 1; i <= number; i++) {
sum += i;
}
return Math.pow(sum, 2);
}
function sumOfSquares(number) {
let sum = 0;
for (let i = 1; i <= number; i++) {
sum += Math.pow(i, 2);
}
return sum;
}
function difference(number) {
return squareOfSum(number) - sumOfSquares(number);
}
console.log(difference(10) === 2640); // true
```

Thanks to my extensive test suite, I can confidently refactor and use recursion instead.

### In Order to Understand Recursion…

The recursive equivalent of the above solution goes like this:

```
-- get the square of the sum of n by:
-- getting the triangular number for n by:
-- returning 0 if n is 0
-- adding n to the triangular number of n - 1
-- get the sum of the squares of n by:
-- returning 0 if n is 0
-- adding the square of n to the sum of the squares of n - 1
-- subtract the latter from the former
```

So, recursion is acting as a different way of looping by defining an action for each number `n`

down to 1 and a final action to end the loop when `n`

gets to 0.

I googled “factorial with adding instead of multiplying” and found “triangular numbers”, so the function for calculating the sum of positive integers from 1 to `N`

is called `triangulate`

🤷🏻♂️.

Let’s write that function first:

```
function triangulate(n) {
if (n === 0) {
return 0;
} else {
return n + triangulate(n - 1);
}
}
// which can be simplified to:
function triangulate(n) {
return n === 0 ? 0 : n + triangulate(n - 1);
}
```

Using the triangulate function, we can get the `squareOfSum`

function:

```
function squareOfSum(n) {
const sum = triangulate(n);
return Math.pow(sum, 2);
}
```

The `sumOfSquares`

function can also use recursion:

```
function sumOfSquares(n) {
if (n === 0) {
return 0;
} else {
return Math.pow(n, 2) + sumOfSquares(n - 1);
}
}
// again, can be reduced to..
function sumOfSquares(n) {
return n === 0 ? Math.pow(n, 2) + sumOfSquares(n - 1);
}
```

One final thought on the Javascript solution is to make `triangulate`

a bit more generic and add a second parameter for an exponent.

```
const triangulate = (n, exp = 1) =>
n === 0
? 0
: Math.pow(n, exp) + triangulate(n - 1, exp);
```

Then `sumOfSquares`

can be written as the following:

```
function sumOfSquares(n) {
return triangulate(n, 2);
}
```

### How about some Elm?

Elm doesn’t have `for`

loops. Whaaaaa

Yea, for real.

Luckily, we already know this problem can be solved without a `for`

loop. So what’s the Elm equivalent of the recursive solution above? Well, let’s refactor `sumOfSquares`

just *one* more time in Javascript, using a switch statement with only two cases this time.

```
function sumOfSquares(n) {
switch (n) {
case 0:
return 0;
default:
return Math.pow(n, 2) + sumOfSquares(n - 1);
}
}
```

Elm has a `case`

statement, so a nearly equivalent function will work:

```
sumOfSquares : Int -> Int
sumOfSquares n =
case n of
0 -> 0
_ -> (n ^ 2) + sumOfSquares (n - 1)
```

We can apply a similar approach to `squareOfSum`

:

```
squareOfSum : Int -> Int
squareOfSum n =
let
triangulate x =
case x of
0 -> 0
_ -> x + triangulate (x - 1)
in
(triangulate n) ^ 2
```

Then the final function `difference`

is just:

```
difference : Int -> Int
difference n =
(squareOfSum n) - (sumOfSquares n)
```

And voila, we have solved a `for`

-loop-friendly problem in Elm, a language with no `for`

loop.

### A Better Way?

While we *can* use recursion to loop through the numbers between `0`

and `N`

, we can also make use of other utilities exposed in Elm Core.

For example, `List.range`

and `List.sum`

make this problem much easier.

```
import List exposing (map, range, sum)
square : Int -> Int
square n =
n ^ 2
squareOfSum : Int -> Int
squareOfSum n =
range 1 n |> sum |> square
sumOfSquares : Int -> Int
sumOfSquares n =
range 1 n |> map square |> sum
difference : Int -> Int
difference n =
squareOfSum n - sumOfSquares n
```

Since `for`

loops are one of the first things we learn as programmers, it’s easy to fall back on `for`

loops in solutions to everyday problems. Using Elm has taught me that `for`

loops aren’t necessary most of the time and seeking a different solution can lead to more declarative and readable code.

Thanks for reading :)