Programmation appliquée en Scala

Why Scala?

• Multi-paradigm language
• Object-oriented and functional
• Statically typed
• Scalable from simple scripts to sophisticated (but still easy-to-use) libraries
• Invented by Martin Odersky at EPFL
• Works with Java VM tools/libraries/infrastructure
• Used for big-iron projects in industry
• Pragmatic choice as successor to Java (whose evolution has slowed to a crawl)
• Expands your mind and makes you think differently about programming in any language

About the Course

• Meets Monday 13:00 - 16:25
• Lecture/Lab/Break/Lecture/Lab
• Short lectures/long labs
• You can ask questions/write reports in English or French
• Course/exams: 40%, labs/homeworks: 30%, project: 30%

What is the Most Important Skill...

...that HEIG-VD computer science graduates wish they had learned better in college?

1. Object-Oriented Programming
2. Functional Programming
3. Programming Projects
4. Giving Presentations in English

Do You Already Know Some Scala?

1. No
2. Just a little
3. I am reasonably fluent
4. I am an expert

Scala Basics

• Types Int, Double, Boolean, String
• Arithmetic like in C++/Java: + - * / %
• Variable type is inferred:

  val luckyNumber = 13 // luckyNumber is an Int

• Function types:

  val square = (x: Int) => x * x // square is an Int => Int

• Semicolons at the end of a line are optional

  val x = 1
  val y = 2 + // end line with operator to indicate that there is more to come
  3

• Everything is an object

  1.to(10) // Apply to method to 1, returns Range(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Map

• 1.to(10) yields a Range
• Collections List, Vector (similar to C++), Range, many more
• map works on any collection

  val square = (x: Int) => x * x 
  1.to(9).map(square)
    // Yields 1 4 9 16 25 ... 81

• Applies function to all elements and yields collection of results
• What is

  1.to(9).map(square).map(x => x % 10).distinct

• No () behind distinct. Rule of thumb: no args and not mutating ⇒ no ()
• How many lines of code would this take in C++?

Functional Programming

• Functional programming: Functions are values
• In C++, values are
  • Primitive types int, double, etc.
  • Structure/class values
  • Pointers
• A function is not a “first class” value in C++
  • Cannot create new functions in a running program
• In a functional programming language, functions are first-class values
  • Can have variables that hold functions
  • Can create new functions

Functional Programming in Scala

• Functions can be values

  val num = 3.14
  val fun = math.ceil _
  fun(num) // prints 4

• Functions can be anonymous...

  (x: Int) => x * x

• ...just like numbers

  3.14

• Of course, can put function values into variables and then use them

  val square = (x: Int) => x * x
  square(10) // prints 100

• ...again, just like numbers

  val pi = 3.14
  pi * 10 // prints 31.4

Why Functional Programming?

• Simpler and clearer programming style
• Often useful to pass functions as parameters

  val numbers = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
  numbers.map((x: Int) => x * x)
     // Prints Vector(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)

• In C++, you can also do this, but there are no anonymous functions.

  vector<int> map(vector<int> values, int (*f)(int)) {
     vector<int> result;
     for (int i = 0; i < size; i++) result.push_back(f(values[i]));
     return result;
  }
  int square(int x) { return x * x; }
   ...
  res = map(numbers, square); // Must pass name of function

• In Scala, but not in C++, a function can produce a new function

Immutability

• Immutable: Cannot change
• In Java, strings are immutable
  • "Hello".toUpper() doesn't change "Hello" but returns a new string "HELLO"
• In Scala, val are immutable

  val num = 3.14
  num = 1.42 // Error

• Pure functional programming: No mutations
• Don't mutate—always return the result as a new value
• Functions that don't mutate state are inherently parallelizable
• Important consideration in light of the end of Moore's Law

Lab

• Format of classes: approx. 20 minutes lecture, 40 minutes lab, 15 minutes wrap-up
• You work with a buddy
• One of you (the coder) writes the code, the other (the scribe) types up answers
• When you get stuck, ask your buddy first!
• Switch coder/scribe roles each lab
• The coder submits worksheet to Fatemeh.Borran@heig-vd.ch. Include the scribe's name in the worksheet!
• The scribe submits answers to Fatemeh.Borran@heig-vd.ch. Include the coder's name in the report!

Part 1: The Scala Worksheet

1. Start the Scala IDE and make a new Scala project: File -> New -> Scala Project. Give a name Worksheets. Right-click on the project in the Package Explorer, then select New -> Scala Worksheet. Call it Day1.
2. Make a new line before the } . Type 6 * 7 and then save (Ctrl+S/⌘+S). What do you get?
3. Edit the line to read val a = 6 * 7 and save. What do you get?
4. Add a. What do you get?
5. Add a = 43. What do you get? Why?
6. Remove that line and add val b; (This time with a semicolon.) What do you get? Why?
7. Now remove that line.

Part 2: Functions are Values

1. Add val triple = (x: Int) => 3 * x. What do you get?
2. Add triple(5). What do you get?

   Tip: These “What do you get” exercises are a lot more enjoyable and effective when you and your buddy first discuss what you think you'll get before you execute the Save command.

3. Add triple. What do you get?
4. What is the type of triple in Scala?
5. What is the type of 5 in Scala?

Part 3: Functions as Parameters

1. Type 1.to(10). What do you get?
2. Type 1.to(10).map(triple). What do you get? Why?
3. How do you get the cubes of the numbers from 1 to 10?
4. How do you get the cubes of the numbers from 1 to 10 without using val? Hint: Anonymous functions

Part 4: Quadratic Residues (Extra Credit)

1. Do you remember this?

   0.to(9).map(square).map(x => x % 10).distinct

   Try it out. What did you get?

2. These are called the “quadratic residues modulo 10”—the values that are squares of some value in { 0...9 }.
3. What are the quadratic residues modulo 11? Modulo 16?
4. Modulo n? Write a function val qres = (n: Int) => ... that produces them. What is your function? What is qres(20)?
5. For n going from 2 to 20, how many quadratic residues are there?

   Hint: 2.to(20).map(...) and map(l -> l.length)

6. Did you get it right? Check against the table in this page. In that table, 0 is not considered a quadratic residue, so your counts should be one higher.
7. What would it have taken to do this in C++?