# Kotlin Tidbit: Loops and Recursion

Given a `LinkedList` data structure that is defined as:

```kotlin
data class LinkedList<T>(val value: T, val next: LinkedList<T>? = null)
```

Each `LinkedList` entry is a node of the current `value` and a reference to `next` which points to another `LinkedList` with the rest of the elements in the collection. The end of the list is denoted by `next` being `null`.

Here are example instances of LinkedList with Int nodes.

```kotlin
// 1 -> 4 -> 3 -> 1
val linkedListOne = LinkedList(1, LinkedList(4, LinkedList(3, LinkedList(1))))

// 6 -> 8 -> 3 -> 1
val linkedListTwo = LinkedList(6, LinkedList(8, LinkedList(3, LinkedList(1))))
    
```

To illustrate ways of iterating through these elements, let's write a function that returns the number of elements in the LinkedList.

## Using `while` Loop.

Using this *imperative approach*, we keep track of the number of elements in the collection via a variable `count`; as long as there is a `next` node we keep counting the elements.

Here we are making use of statements, and relying on side-effects to achieve our end goal by specifying step by step what needs to be done.

```kotlin
fun LinkedList<*>?.size(): Int {
    var count = 0
    var current: LinkedList<*>? = this
    while (current != null) {
        count++
        current = current.next
    }
    return count
}
```

## Using Recursion.

Using this *functional* *approach*, we keep track of the total number of elements in the collection by repeatedly evaluating an expression to count the total number of elements in the collection until the last element.

Here we are making use of a [recursive](https://kotlinlang.org/docs/functions.html#tail-recursive-functions) function, that repeatedly evaluates if the current node is `null` and if it is it returns the total number of elements `c`, otherwise, it calls itself again for evaluation passing the total number of elements increased by 1, and a reference to the `next` node in the collection.

```kotlin

fun LinkedList<*>?.length(): Int {
    tailrec fun count(c: Int, l: LinkedList<*>?): Int = 
        if (l == null) c
        else count(c + 1, l.next)
    return count(0, this)
}
```

## Conclusion.

Both approaches yield the same result in terms of figuring out the total number of elements in a particular collection. This Kotlin Tidbit serves to illustrate how looping can be achieved using either an imperative or a functional approach.