This video is free to watch - hope you enjoy it! If you like what you see, there's a ton more where that came from...

# Recursion and Refactoring Our Solar Flares

Most developers know about recursion and have used it once or twice in their careers; perhaps more. It can be quite difficult to get right - this is not the case with Elixir. With a combination of pattern matching and the head/tail functionality of Lists, recursively iterating over lists is straightforward.

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I’m exhausted. I called in sick today… I’ve been up for 28 straight hours which isn’t so bad… I guess but WOW was it cold last night. I think I came down with rickets or something.

Anyway the Science Team reviewed the Solar Flare stuff you did and they said it was “OK”. They didn’t give their feedback to the CTO yet because they wanted some changes made - we get to do that today.

The first thing they want is to understand what the total exposure to all of the flares amounts to. This is a simple summing calculation you should be able to do easily - but the Science Team is being rather micro-manager-y about all of this and they want to evaluate some different ways that we can run the calcs.

Sometimes I like my job… other times…

### Using Recursion

With a combination of pattern matching and the head/tail functionality of Lists, recursively iterating over lists is quite simple:

``````def total_flare_power(list)  do
total_flare_power(list, 0)
end

def total_flare_power([], total),  do: total

def total_flare_power([head | tail], total) do
total_flare_power(tail, new_total)
end
``````

Let your eyes wander on that for a second, and then I’ll explain a bit.

## The Code

Code for this video (and for the entire series) can be found in the production GitHub repo.

• ### Welcome to Red:4

Welcome to Red:4 - Where the Future Starts Yesterday. Blah blah blah we hope you like it here blah blah… We don't have a lot of time so let's just get to it shall we?

• ### Letting Elixir Soak In

Elixir syntax can be familiar and weird at the same time, especially if you're coming from Ruby. To prepare for this section, let's clear our minds - let go of familiar syntax cues and 'the way it's done' in the languages you already know.

• ### Functional Programming Primer

Functional programming has been around forever; it's nothing new. Elixir is a functional language, as is its progenitor, Erlang. If you're coming from an Object-oriented language like Java, Ruby, C#, Python or JavaScript, functional programming will take some getting used to.

• ### I need you to setup a project

You can think of Mix as your Elixir Utility Belt. It builds your projects, runs tasks for you, runs your tests, installs packages - a fascinating tool. That comes later - right now open up your terminal and navigate to a directory where you can save your work.

• ### Calculating Escape Velocity

I hope you're prepared to learn on the job. I'm about to ramp things up on you and it's for the simple reason that I'm under the gun - and to be honest *so are you*. We just got a new CTO, she arrived a few weeks after I got the job. She doesn't like me... which I guess isn't all that surprising.

• ### Pattern Matching Basics

Pattern Matching in Elixir is one of those things that can be vexxing, interesting, or obvious. Ultimately, however, it will be confusing if you have never dealt with it before.

• ### Calculating Orbital Distance

In this part we tidy things up, plugging in Authentication and hooking our download service into Firebase Storage.Nice work so far! The Science Team loves what we're doing and, as you might expect, have given us more work.

• ### Debugging

I'm always amazed when I manage to write Elixir code that compiles correctly the first time. It almost never happens! Let's have a look at a normal sequence you might stumble through, and I'll see if I can save you some time.

• ### Solar Flare Project Setup

We need to get rolling which means we need to setup a project and start cranking out some code! In this video we'll structure our project with tests and get to know the difference between Elixir script files and compilable executables.

• ### List and Enumeration Basics

Functional programming is all about transforming data and if we're going to work with Solar Flares, we need to understand how Lists work - how we query, slice, dice, add and remove. That's what we're going to do now.

• ### Recursion and Refactoring Our Solar Flares

Most developers know about recursion and have used it once or twice in their careers; perhaps more. It can be quite difficult to get right - this is not the case with Elixir. With a combination of pattern matching and the head/tail functionality of Lists, recursively iterating over lists is straightforward.

• ### if, unless, and cond

Here's something weird to think about: in a high-level functional language such as Elixir, writing conditional code is not exactly idiomatic. Using a combination of pattern matching and syntax rules we can write extremely clean code free of if statements and conditionals in general

• ### A Quick Look at OTP

You've seen a lot of Elixir so far and have learned some new concepts, especially if you're new to functional programming. No doubt that along the way you've probably been wondering, however: *how is this all supposed to work together? how can I actually build something?*. I've showed you a few little hints, but it's time to get explicit on this

• ### Working With Ecto and PostgreSQL

You have to store data somewhere and Elixir (and Erlang in general) give you quite a few tools to do so. In this video we'll see how to push data into PostgreSQL using Ecto, the ORM created by the Elixir team.

• ### Troubleshooting OTP Errors

We're in the weeds now! Sorry for the pain, but hey, backing over a cliff is what we do as developers isn't it? You *will find yourself completely stuck* when working with Elixir and I need you to be able to reason your way out of this mess.

• ### Bombs Away!

Developers often confuse *asynchronous* execution with *concurrent* or parallel execution. I do this often! It's something I learn and unlearn, all the time. Let's take a small tangent and discuss the difference.

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