Lecture 1: Introduction

# CS-3110: Formal Languages and Automata

### Spring 2024

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# Instructor and Schedule

* Instructor: Dr. **Markus** Eger (he/him)
 
 * Email: <a href="mailto:meger@cpp.edu">meger@cpp.edu</a>
 
 * Office hours: Tuesday, Thursday: 1pm-2.30pm, Building 8, Room 43, Wednesday 1.30-2.30pm, online (Zoom or Discord)
 
 * Class: Tuesday, Thursday, 4.00-5.15pm, Building 8, Room 345

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# About Me

* Originally from Austria
 
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# About Me

<img src="/CI-2700/assets/img/austria.png" width="70%"/>
 
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* Originally from Austria
 
 * BSc and MSc in Computer Science from University of Technology Graz, Austria 
 
 * PhD in Computer Science from NC State University, USA

* Visiting professor at the University of Costa Rica 2019-2020

* I work on AI for games involving communication and/or cooperation (Hanabi, Werewolf, Pandemic, etc.)
 
 * Games I played recently: The Talos Principle 2, Factorio, Star Wars: Jedi Knight II, Dominion
 
 * I also like board games
 
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# About Me

<img src="/pageassets/personal/cats3.png" width="100%"/>
 
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* What are automata/languages/why are we here?
  
  * Math review: sets and languages 
  
  * Regular languages, finite automata 
  
  * Context-Free Grammars and Languages, pushdown automata 
  
  * Non-Context Free Grammars and Languages 
  
  * General Grammars
  
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# Class Resources

* Canvas
  
  * [Discord](https://discord.gg/Y4vuuzPCgy)
  
  * Submission: Canvas 
  
  * Textbook
  
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class: small
  
# Textbook (Free!)
 
<center>
<img src="/CS3110/assets/img/textbook.png" width="40%"/>
</center>
  
<a href="http://math.hws.edu/FoundationsOfComputation/">Foundations of Computation</a>
    
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# Grading

* Homework: 4*15%

* Presentation: 15%
   
* Midterm: 10%

* Final exam (cumulative!): 15%

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# Class structure
  
  * Thursdays: Lecture about theory with smaller examples throughout
  
  * Tuesdays: Student presentations of two larger examples that need the theory
  
  * Every student needs to present once per semester
  
  * I will also discuss the homework assignments and any potential pitfalls in class
  
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# Student Presentations

* Organize in groups of 2 (**one** group of three, if there is an odd number of students)

* Each group will be assigned one example to work out and present **step-by-step**

* There will be three presentations per week, with a time limit of 20 minutes per presentation

* Student presentations start in week 6 (February 27); before that I will *demonstrate* the structure these presentations should have during the Tuesday lectures

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# Student Presentations: Bonus Points

* You can earn up to 5 bonus points for excellent presentations

* Excellent can mean:

- Outstanding visualizations
   
   - An interactive tool (online or offline)
   
   - A clever alternative solution
   
* Basically, something that goes beyond just explaining the steps of the solution

* If it is truly great (i.e. 4-5 bonus points), I may want to use it in the future **with your permission**, but points are **not** contingent on giving me this permission
  
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# Homework

* There will be four homework assignments

* You have to work on these assignments **in groups of up to two students**

* Homework problems will be based on the problems shown in class and the presentations

* Homework assignment will usually have a bonus problem you can solve for extra points! These bonus problems are intentionally a bit trickier, but you might get partial credit even if you can't figure it out completely.

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# Homework

* Homework 1: Languages and Regular Expressions, due Thursday 2/29

* Homework 2: Finite Automata, due **Tuesday** 3/19

* Homework 3: Context-Free Languages, due Thursday 4/18

* Homework 4: PDAs, Non-Context Free Languages, due Thursday, 5/9

All due dates are given as ["Anywhere on Earth"](https://en.wikipedia.org/wiki/Anywhere_on_Earth)

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# Plagiarism

* Please don't cheat

* Homework assignments are done in groups of up to two students: You are free to discuss them with others, but the written submission has to be **your own** (this means no ChatGPT either!)

* Plagiarism results in 0 points on the entire homework for **everyone** involved

* I also report every incident to the office of student conduct

* It's not worth it! Ask questions if you struggle, the goal is for you to learn :)

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# Why Formal Languages?

Which of these are valid email addresses? (in form; they likely won't reach anyone)

* jon@snow.north

* R+L@J

* "cersei lannister"@casterly.rock

* -\`-\`-{@roses.com

**All of them!**

Neat trick: If you have "name@gmail.com", "name+website@gmail.com" also goes to the same mail box, but you can see who sells your email address.

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# In practice ...

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# Invalid Email?

- I **know** that I can have a "+" in my email address

- How do I know that?

- Someone actually specified what an email address can look like!

- Let's take a look ...

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# RFC 5122

```
addr-spec       =   local-part "@" domain

local-part      =   dot-atom / quoted-string

domain          =   dot-atom 
    
dot-atom        =   1*atext *("." 1*atext)

atom            =   1*atext

atext           =   ALPHA / DIGIT / "!" / "#" / "$" / "%" / 
                       "&" / "'" / "*" / "+" / "-" / "/" /
                       "=" / "?" / "^" / "_" / "`" / "{" /
                       "|" / "}" / "~"
                       
qtext           =   %d33 / %d35-91 / %d93-126
                       
quoted-string   =   DQUOTE *([FWS] qtext) [FWS] DQUOTE
```

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# So what?

- I actually simplified the email address format

- Parsing *actually valid* email addresses can be quite tricky

- On the other hand, we just saw a glimpse of how one might specify something like that

- Formal languages are exactly that: A way to formally specify syntax

- We will talk about two parts: Specification and recognition

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# Specification

* In order to talk about what is "valid", we need to be able to describe it unambiguously

* Say you build a website with a login, and your boss requests that you ensure "strong passwords"

* You will want to specify what "strong" means for passwords (e.g. "at least 8 characters, at least one digit, one upper-case and one lower-case letter")

* There are many ways to write such a specification, we will look at a few of them

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# Recognition

* Once you know what is "valid" and what is not, you often need to verify that a given input is indeed "valid"

* This will be the "core" of the class: How do you "recognize" if a given string/word follows the rules

* Depending on the complexity of the rules, there are different approaches, which are the automata in the title of this class

* We will start with the most restricted formalism, and explore what it can and can not do

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# Math

Cady: *I like math.*

Damian: *Eww. Why?*

Cady: *Because it's the same in every country.*

Mean Girls (2004)

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# Math

* Before we start with actually defining what it means for "input" to be "valid" (or "words" to be in a "language", as we will call it), we will review some discrete math

* Our "languages" will be sets, so we will want to have a firm grasp on some concepts from set theory

* To investigate what our different automata can and can not do, we will also do some proofs

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# To Do

* Organize in groups of two for the presentations (use the [class Discord](https://discord.gg/Y4vuuzPCgy), if needed)
  
  * Submit your topic preferences and group on Canvas by February 7, AoE (It's a "quiz" called "Presentation Topic Selection")
  
  * We will start with an introduction to languages, and why this class exists and is important 
  
  * We will also review some concepts from discrete math; you may know some/most of it, but you'll probably learn something new as well