## What IS Theoretical Computer Science?

(Note: this is a “present-day version” of an article that I published in French around 5 years ago.)

As I mentioned when starting this blog, there’s a few blog posts from my French blog that I kind of want to translate to English. This is part of it: I wrote a series of articles about trying to explain a bit of math and theoretical computer science, and I believe these are pretty good candidates for that.

A few years ago, I was studying at ETH Zürich, and I got asked quite a lot “So, what are you doing these days?” – which was a question that was fairly hard to answer to – typically, I said “Theoretical computer science. Basically maths.” It wasn’t very satisfying, but getting into the details of things when answering a “social” question wasn’t necessarily a good idea either. But maybe there are some people who ARE interested in a bit more detail (if you’re one of the persons to whom I answered that at the time, and you’re still interested, now is your chance 😉 ), and that maybe I could explain things to people who didn’t swim in that area for a few years. To give an idea, I’m aiming at the “I didn’t do maths since high school” level. I’m not sure I managed it – it’s been a while since I’ve quit high school, and I did do a bit of maths since then 🙂

So this is first a very general post to give a few elements, and then I want to give a bit more detail. The idea is that I still love what I studied a few years ago, and I want to explain why I love it. It’s going to ask a bit of work – the first thing I did on the French series was to write a post about “but, but, but… what IS a logarithm?”. At that point I was a bit afraid it’d be an “insult” to the scientific knowledge of my readers, but I still did it because I thought it was fun and because I thought that logarithms were still a very useful notion. It so happened that said logarithm blog post was, by far, my most popular post ever (it still getting hits from search engines these days), so. And then, there’s a few blog posts depending on the mood. A bit of complexity theory, a bit of graph theory (I like graph theory), a bit of probabilities (lol), and the “prerequisites” for all of that (my goal was to be able to explain P vs NP, how to define what NP-complete means, and I eventually got there, and it was a nice exercise).

But still, let’s get to the point. What is theoretical computer science? The first answer I have is that’s it’s the theory of everything you can do with a computer. In the middle of that, you have the notion of algorithm. An algorithm is just a series of instructions. You can think of a cooking recipe as an algorithm:

1. To make béchamel sauce, you need butter, flour and milk.
2. Measure out identical quantities of butter and flour.
3. In a pan over medium heat, melt the butter and add the flour; mix until you get a thick paste.
4. While the béchamel sauce does not have the desired texture, add milk and mix.
5. If you wish, add salt, pepper, nutmeg or cheese.

You have instructions (which are numbered) that can be repeated (while the sauce doesn’t have the desired texture) or that you only execute under certain conditions (you only add nutmeg if you want to).

For another process that is more commonly described as an algorithm, you can think of the long division algorithm you learnt at school.

I’m a bit biased on the categories that you can fit into theoretical computer science, because there are domains that I know better than others, but here’s a strictly non-exhaustive list (if I forgot your favorite domain… that’s because I don’t know what to say about it, don’t hesitate to explain in the comments). For a more complete list, I’ll send you to the Wikipedia article about theoretical computer science, which itself is based on the list from SIGACT (a large theoretical computer science research group). As for the fact that “I do maths” – theoretical computer science is often considered as part of discrete mathematics and, in general, proving that something works often implies writing pretty proofs full of pretty maths (or full of ugly maths, but that’s far less satisfying).

I’m going to allow myself a fair amount of approximations here (in particular, I’m confusing problems and problem instances) – I’m trying to write things simply without writing overlong paragraphs. Those who know will be able to correct on their own; for the others: it’s a first approximation, it’s on some level wrong by definition, but I do intend to make it a bit less wrong in the following posts.

So, to me, theoretical computer science contains among others:

• Algorithmics. Given a problem, how do you solve it with an algorithm, how do you prove that your solving method (the algorithm) is correct, and how much time do you need to get a result?
• Complexity theory. It includes, but does not restrict itself to, the “how much time do you need to get a result?” from the previous point. We’re also interested in the question “given a problem, what are the minimum, average and worst case time and memory consumption that will allow me to solve it?”.
• Graph theory. A graph is a mathematical structure made of points (the “vertices” of the graph) and lines (the “edges” of the graph). One example could be a subway map: the stops are the vertices, the edges are the connexions from one stop to another with a subway line. In graph theory, we look at graph properties (for example: can I go from one point to another; what is the shortest path from one point to another; can I still go from point A to point B if I remove this vertex – or, in my previous example, if I shut down this subway station), and at algorithms to decide these properties (how do I find the shortest path from one point to another in the graph).
• Data structures. How do I store the data of the problem I’m interested in in the most efficient way possible (from a time and space perspective) for the problem I want to solve? Or, to go back to the previous example, how do I store my subway map so that I can compute stuff about it? Data structures and algorithms go hand in hand – you need data on which to run your algorithm, and your data without algorithms is quite often just storage.
• Computational geometry. How to represent and solve with a computer problems that are related to geometry? A common example is the post office problem: given the location of all post offices in a city, how do I efficiently find the one closest to my home? If I’m only interested in the one that’s closest to my home, I can compute the distance between my home and all post offices, but how can I improve on that if I’m interested in that information for all houses in the city? And what do I do if I shut down or open a new post office?
• Randomness. What is the impact of randomness for all this? For some algorithms, it’s interesting to add some randomness in the computation (I flip a coin, if I get heads the algorithm does operation X, if I get tails the algorithm does operation Y). What impact does it have on algorithms – in particular their correctness, and how to assess the time that a random algorithm will need to (correctly) solve a problem? Random data structures can also be interesting. For instance, you can define random graphs: you take a set of points, and you add edges between them with some probability. Depending on how you define your random graph, it can be a fairly good model of social networks (who’s friend with whom)… or of the human brain.
• Cryptography. If Alice wants to send a message to Bob without Eve being able to read it, and if Bob wants to be sure that the message he just read does come from Alice, how can they do that? And how to prove that Eve cannot read the message, and that Bob can be sure of where the message comes from.
• Computability. It turns out you can’t solve everything with an algorithm, and that you can even prove that some things cannot be computed with an algorithm. The most famous example is the “halting problem”, which is a bit meta, but I hope you’ll forgive me. You can prove that there is no program that can decide if a given problem eventually stops and give a result or if it just loops for ever. The proof is very pretty, but it kind of hurts the brain the first time you see it; maybe I’ll eventually write a blog post about it. (Note: I said that “maybe” five years ago, maybe I should do that for real 😉 )

There, those are all things for which I have at least some notions. Theoretical computer science includes some more stuff for which my knowledge is mostly zero: information theory (I’m somewhat ashamed of that), machine learning (or how do you make a computer recognize pictures of cats), formal methods (or how do I prove that my code does not have bugs), quantum computing (would a quantum computer be better at recognizing pictures of cat than a normal computer), computational biology (how do I check whether two DNA sequences are close of one another or not), and I probably still forget a few.

Also note that different people may have different opinions about what is or is not theoretical computer science. I’m not necessarily convinced of the fact that most machine learning as it is applied today is theoretical computer science, because it still looks very experimental, as in “we tweaked that parameter and consequently the computer is 2.2% better at recognizing cats. (Note: I’d probably be less “critical” now than I was five years ago on that topic. Maybe. Probably.) Wikipedia also gives distributed computing as theoretical computer science. It most definitely intersects, but I wouldn’t say that distributed computing is a subset of theoretical computer science. Same for computational biology: some elements are part of it (the example of the DNA strings), but I’m pretty sure that not all computational biologists would define themselves as theoretical computer scientists 🙂

I thought this would be a pretty short blog post: I think that’s a failure. I hope it was still somewhat interesting; if there are things you think I can talk about in a future blog post in this category… well, don’t hesitate. As I mentioned, I actually have some amount of content that I aim to translate from my archives; I am open to suggestions still, my goal is to Blog More 🙂