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A recursive function is a function that calls itself. It’s an intriguing approach to solve specific coding challenges. In this article, you’ll learn how to work with recursion in Swift.

Here’s what we’ll get into:

- What is recursion and how does it work?
- Useful applications of recursion
- Why it’s smart to stop the recursive function
- How recursion is especially suitable for certain problems

Ready? Let’s go.

The word *recursion* means “a repeated application of a recursive procedure or definition” – and that’s a recursion itself! In essence, a recursion happens when a thing is defined in terms of itself or its type. And in Swift, recursion means that a function calls *itself*!

Let’s take a look at an example:

func sum(_ numbers: [Int]) -> Int

{

guard !numbers.isEmpty else {

return 0

}

return numbers.first! + sum(Array(numbers.dropFirst()))

}

let result = sum([1, 2, 3])

print(result)

{

guard !numbers.isEmpty else {

return 0

}

return numbers.first! + sum(Array(numbers.dropFirst()))

}

let result = sum([1, 2, 3])

print(result)

The above function `sum(_:)`

calculates the sum of an array of integers by using recursion. The function takes an array of integers as input, and returns an integer as output. The key aspect of the function is that **it calls itself**! Can you spot where?

How does the `sum(_:)`

function work? Here’s how:

- First, at the top of the function, we’re using the guard statement to check that the
`numbers`

array is not empty. When the array is empty, the function returns`0`

. As you’ll soon see, this is a crucial aspect of recursion! - Then, the function returns a value. It returns the first number in the array
*plus*the sum of the remaining numbers in the array (the “tail”). This is where the recursion happens!

The *head* of an array is its first item. The *tail* of a sequence, is the array minus its head – so the remaining items. Think of it like a snake!

Let’s dive into the syntax some more. First, the `return`

statement:

```
return numbers.first! + sum(Array(numbers.dropFirst()))
```

The `first`

property on the `numbers`

array contains the first value in the array. We’re force-unwrapping this value, because it’s an *optional*. We’re certain that the array is not empty, so it’s OK to use force-unwrapping here.

Then, the `numbers.dropFirst()`

call will return a subsequence of a the `numbers`

array *without* the first item, hence “drop first”. We’re providing that subsequence to the `Array()`

initializer, so we’ll turn the subsequence into an actual array again. This array is then fed into the `sum(_:)`

function again.

In short, we’re adding the first item of the array and the sum of the tail of the array. The recursion happens in calling `sum(_:)`

for the tail of the array repeatedly, while repeatedly dropping the first item of the array. This effectively keeps adding the first array item to the rest of the array. And you ultimately end up with `1 + 2 + 3`

!

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If you draw a comparison between recursion and loops, you’ll see that a recursive function typically iterates or “loops over” values by calling itself. The function calls itself, calls itself, calls itself, etcetera, and in every call, it returns a value to the caller. The loop nests deeper and deeper, until… yeah, until what!?

If you don’t terminate a recursive loop, you’ll easily create what’s known as an *infinite loop*. The recursion never stops, and keeps looping forever. And that isn’t very useful, right?

In `sum(_:)`

function, the following block of Swift code will stop the recursion:

```
guard !numbers.isEmpty else {
return 0
}
```

When the input array is empty, the `guard`

statement invokes its else clause and returns `0`

. This early return will prevent the function from calling itself in the subsequent line of code (see example), and terminates the recursion as a result.

Every recursive function needs an escape. In the `sum(_:)`

function, it’s logical to terminate the recursion when the `numbers`

array is empty. After all, when it’s empty, there aren’t any more numbers to add, so we return `0`

.

The last recursive function call to `sum(_:)`

will be an empty array, because the before last array has only one item, and we’re calling `dropFirst()`

. That last function call needs to return something, because it’s added to the sum, so we return `0`

. But most importantly, we prevent `sum(_:)`

from being called *again*, and break the recursion.

You might wonder: *“Why use recursion at all?”* After all, we could have easily calculated the sum of an array of integers like this:

```
let numbers = [1, 2, 3]
var sum = 0
for n in numbers {
sum += n
}
print(numbers)
```

This way of getting the sum of an array is simpler, easier to read, and about as elegant. What do you need recursion for?

As it turns out, there are specific problems in mathematics and programming that are easier to solve with recursion. Consider, for example, that you need to measure *all* branches of a tree, but you don’t know how many branches each branch has. You could use recursion to dive into each branch, and its sub-branches, and its sub-branches, etcetera, to add up the sum of every branch.

Likewise, a search algorithm called *binary search* uses a simple approach to find an item in a sorted array. With each recursive pass, the array is cut in half, because the algorithm has figured out in which of the two halves the to-be-found item is.

And of course, you can use recursion for the sheer fun and beauty of it. Here, check out this recursive function to calculate any *n*-th Fibonacci number.

func fibonacci(_ i: Int) -> Int

{

if i <= 2 { return 1 }

return fibonacci(i - 1) + fibonacci(i - 2)

}

print(fibonacci(8))

{

if i <= 2 { return 1 }

return fibonacci(i - 1) + fibonacci(i - 2)

}

print(fibonacci(8))

The Fibonacci numbers are a sequence in which any number is the sum of the two preceding numbers. The sequence starts at `1, 1`

(or `0, 1`

), so the first few numbers are:

```
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144
```

You can clearly see that the fifth number `5`

is the result of `2 + 3`

, and `13`

is the result of `8 + 5`

. We could create an intricate `for in`

loop with several temporary values, but it’s much easier to comprehensively describe the Fibonacci sequence by using recursion.

The `fibonacci(_:)`

function will return the Fibonacci number for the given sequence number, or index, so `fibonacci(8)`

returns `21`

. The function considers `1, 1`

as the first two Fibonacci numbers.

First, the recursion itself happens like this:

```
return fibonacci(i - 1) + fibonacci(i - 2)
```

Differently said, add the previous Fibonacci number and the Fibonacci number before that, and return the result. You can see how this spirals into more recursion, because the subsequent `fibonacci(_:)`

calls will start looking for *earlier* Fibonacci numbers to get to the end result.

Speaking of the end, that happens when we reach this bit of code:

```
if i <= 2 {
return 1
}
```

We’re ending the recursion by not making a subsequent call to the recursive function, just as with `sum(_:)`

. By returning early, the `fibonacci(_:)`

functions aren’t called again. And when does that happen? For the first two Fibonacci sequence numbers, 1 and 2, for which the result is `1`

. Neat!

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Recursion in Swift. A function that calls itself. Now you know! It’s a useful, fun approach to looping over data, and to solve challenges. Why don’t you see if you can put it to use in your next iOS project or Swift challenge?

Want to learn more? Check out these resources:

**Hi, I'm Reinder.**

I help developers play with code.

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