# SKB – Scala List pattern matching

## Introduction

Periodically, I will publish new exercises so you can slowly build up knowledge about Scala.

It is designed to be done in a very short amount of time and learn a little bit each day, just to create a routine.

This episode will teach you about Scala `List` pattern matching.

Hope you are going to enjoy it! It is designed for anyone to learn Scala from scratch and slowly learn, one Bit at a time.

After this Bit, I would love to hear your feedback in the comments down below.

Feel free to join the Discord server as well if you would like some help and support from the rest of our community.

## What are we learning today?

Today we are going to learn about Scala `List` pattern matching !

Let’s see how `List` is used in the context of pattern matching.

This is a pre-requirement to talk about recursion later on.

Time to try on the exercise on your own and scroll down for more information when you are done or if you are stuck.

## Exercise

Here is an exercise to complete today.

If I did my job well, you should be able to guess by yourself the solution based on what you previously learned and based on the clues.

The goal of the exercise is to replace the `???` by a piece of code so that the exercise compiles and that’s how you win! Good luck!

You can fill the exercise right in here:

Or, if it does not load, go on to Scastie (i7RNOt1qStGNACeNygqEwQ).

## More information about Scala `List`pattern matching

In this exercise you will learn (or have learned, if you have already solved the puzzle) about Scala `List` pattern matching.

First we have to talk about how `List` can be created and modified.

Everything is immutable which is why we always create a new `List`, we don’t update the previous one.

A few new operator today:

• `Nil`: not really an operator, but it describe an empty list, it is a short cut for `List.empty[A]`
• `::`: to add an element at the start of a `List`, which is why to create a `List` using this operator, we have to finish by `Nil`. We actually prepend element to an empty list.
• `+:`: to prepend an element to the `List`. Same as `::`.
• `:+`: to append an element at the end of a `List`

If know that `+:` and `:+` can be confusing but you simply have to remember that `:` is towards the `List`.

Pattern matching now

Using the `::` we can extract head element(s) from the List.

For instance, we can `case head :: tail => ` to extract the first element of the list. We can also do `case a :: b :: tail =>` to extract the two first element of the list. We can continue with as many as we would like.

Similarly to all the other pattern matching construct, we can add conditions to all those patterns. Either by adding `if` at the end or by replacing a value name by a literal value, for instance: `case 1 :: tail =>` will match on any list that start by a `1`. And `case 1 :: Nil => ` will match on a list containing one element being `1`.

Feel free to go back to the exercise, modify the code to try out new things and get a better intuition for Scala `List` pattern matching.

## Conclusion

I hope you have learned something new or had fun during this Scala Knowledge Bit.