ti1220 lecture 2

Challenge the future

DelftUniversity ofTechnology

http://twitter.com/TI1220Concepts of Programming Languages

Eelco Visser

Functional Objects

http://twitter.com/TI1220

http://twitter.com/TI1220

B:6.1

getting started with Scala

TI 1220 - Lecture 2: Functional Objects

Recap

Object-Oriented Programming in Scala

• Expressions

• Immutable and mutable variables (val, var)

• Function definitions

• Classes and objects

• Class members

• instance variables

• methods

• Singleton objects

2

B:6.1

today’s lecture


Overview

Names

• Binding, name spaces

Functional objects

• Objects without mutable state

• Defining operators

Built-in Control Structures

• If, While, For, Exceptions

• Variable scope

3

B:6.1 TI 1220 - Lecture 2: Functional Objects

Concept: Name Binding

I

4

B:6.1

val msg = "Hello, " + "world!"

println(msg)

name binding


Binding Variable Identifiers

5

defining occurrencebinding

val variables cannot be rebound

applied occurrence

B:6.1

var greeting = "Hello, world!"

greeting = "Leave me alone, world!"

name binding


Binding Variable Identifiers

6

var variables can be rebound

rebinding

bindingdefining occurrence

B:6.1

def widthOfLength(s: String) = s.length.toString.length

val maxWidth = widthOfLength(longestLine)

name binding


Binding Function Identifiers

7

bindingdefining occurrence

applied occurrence

name binding


Binding Class Identifiers

8

class ChecksumAccumulator { var sum = 0}

var acc = new ChecksumAccumulatorvar csa = new ChecksumAccumulator

acc.sum = 3csa = acc

binding

applied occurrence

defining occurrence

name binding


Rebinding vs Mutation

9

class ChecksumAccumulator { var sum = 0}

var acc = new ChecksumAccumulatorvar csa = new ChecksumAccumulator

acc.sum = 3csa = acc

mutation

binding

rebinding

B:6.1

object foo { val foo : Int = 0 def foo(x : Int) = x + 1}

object bar { def bar() = foo.foo(foo.foo) }

name binding


Name Spaces

10

variables, functions, objects are in separate name spaces

B:6.1

val msg = "Hello, " + "world!"

println(msg)

name binding


Substitution

11

println("Hello, " + "world!")

?

can we replace applied occurrence with bound expression/value?


Functional Objects

II

12

B:6.1

mathematical laws

TI 1220 - Lecture 2: Functional Objects 13

Rational Numbers

• Rational = Int x Int

• Notation: numerator/denominator

• Addition

• example: 1/2 + 2/3 = 3/6 + 4/6 = (3 + 4)/6 = 7/6

• general: n1/d1 + n2/d2 = (n1*d2 + n2*d1) / (d1*d2)

• Multiplication

• n1/d1 + n2/d2 = (n1 * n2) / (d1 * d2)

• Division

• n1/d1 / n2/d2 = n1/d2 * d2/n2

B:6.2

class Rational(n: Int, d: Int) { println("Created " + n + "/" + d)}

class parameters


Constructing a Rational

14

scala> new Rational(1, 2)Created 1/2res0: Rational = Rational@2d83e895


Immutable Object Trade-offs

Advantages

• easier reasoning

• pass around freely (no risk of undesired mutations)

• cannot be changed concurrently in two threads

• immutable object make safe hashtable keys

Disadvantages

• copying large object graphs vs in-place update

15

B:6.3

class Rational(n: Int, d: Int) { override def toString = n + "/" + d}

overriding methods


Reimplementing toString

16

scala> val half = new Rational(1, 2)half: Rational = 1/2

B:6.4

class Rational(n: Int, d: Int) { require(d != 0) override def toString = n + "/" + d}


Checking Preconditions

17

scala> val half = new Rational(1, 0)java.lang.IllegalArgumentException: requirement failed

B:6.5

class Rational(n: Int, d: Int) { require(d != 0) override def toString = n + "/" + d def add(that: Rational): Rational = new Rational(n * that.d + that.n * d, d * that.d)}


Visibility of Class Parameters

18

$ fsc Rational.scala Rational.scala:5: error: value d is not a member of Rational new Rational(n * that.d + that.n * d, d * that.d) ^Rational.scala:5: error: value d is not a member of Rational new Rational(n * that.d + that.n * d, d * that.d) ^two errors found

B:6.5

class Rational(n: Int, d: Int) { require(d != 0) val numer: Int = n val denom: Int = d override def toString = numer + "/" + denom def add(that: Rational): Rational = new Rational( numer * that.denom + that.numer * denom, denom * that.denom)}


Adding Fields

19

scala> new Rational(1,2) add new Rational(2,3)res0: Rational = 7/6

B:6.1

class ImperativeRational(n: Int, d: Int) { require(d != 0) var numer: Int = n var denom: Int = d override def toString = numer + "/" + denom def add(that: ImperativeRational) { numer = numer * that.denom + that.numer * denom; denom = denom * that.denom; }}

destructive update


Non-Functional Objects

20

scala> val half = new ImperativeRational(1, 2)half: ImperativeRational = 1/2

scala> val twothirds = new ImperativeRational(2,3)twothirds: ImperativeRational = 2/3

scala> half.add(twothirds)

scala> halfres1: ImperativeRational = 7/6

B:6.6

def lessThan(that: Rational) = this.numer * that.denom < that.numer * this.denom

def max(that: Rational) = if (this.lessThan(that)) that else this

this


Self References

21

B:6.7

class Rational(n: Int, d: Int) { require(d != 0) val numer = n val denom = d def this(n: Int) = this(n, 1) // auxiliary constructor ...}


Auxiliary Constructors

22

scala> new Rational(6)res1: Rational = 6/1

B:6.7

class Rational(n: Int, d: Int) { require(d != 0) private val g = gcd(n.abs, d.abs) val numer = n / g val denom = d / g ... private def gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b)}


Private Fields and Methods

23

scala> new Rational(6,42)res1: Rational = 1/7

B:6.7

def add(that: Rational): Rational = new Rational(numer * that.denom + that.numer * denom, denom * that.denom)

use functions as infix operators


Defining Operators

24

scala> new Rational(1,2).add(new Rational(2,3))res0: Rational = 7/6scala> new Rational(1,2) add new Rational(2,3) res0: Rational = 7/6

B:6.7

operator call is method call


Invoking Operators

25

B:6.7

def +(that: Rational): Rational = new Rational(numer * that.denom + that.numer * denom, denom * that.denom)

def *(that: Rational): Rational = new Rational(numer * that.numer, denom * that.denom)

operator identifiers


Defining Operators

26

scala> val d = a + b * c d: Rational = 11/14scala> val d = a.+(b.*(c))d: Rational = 11/14scala> val d = a * b + c d: Rational = 16/21scala> val d = (a.*(b)).+(c)d: Rational = 16/21

B:6.1

lexical syntax


Identifiers

27

Alphanumeric identifier

• identifier: [$A-Za-z_][$A-Za-z_0-9]* ($ reserved for Scala compiler)

• camel-case convention: toString, HashSet

Operator identifier

• Unicode set of mathematical symbols(Sm) or other symbols(So), or to the 7-bit ASCII characters that are not letters, digits, parentheses, square brackets, curly braces, single or double quote, or an underscore, period,semi-colon, comma, or back tick character.

Literal Identifier

• arbitrary string enclosed in back ticks (` . . . `).

B:6.1

for different argument types


Method Overloading

28

scala> val c = new Rational(3,7)c: Rational = 3/7

scala> c * 2res1: Rational = 6/7


def *(i: Int): Rational = new Rational(numer * i, denom)

B:6.1

does not apply to this


Method Overloading

29



scala> 2 * c <console>:7: error: overloaded method value * with alternatives: (Double)Double <and> (Float)Float <and> (Long)Long <and> (Int)Int <and> (Char)Int <and> (Short)Int <and> (Byte)Int cannot be applied to (Rational) 2 * c ^

B:6.1

implicit def intToRational(x: Int) = new Rational(x)


Implicit Conversions

30

scala> 2 * c res4: Rational = 6/7



B:6.1

functional objects


Summary

31

Immutable objects

• class parameters

• immutable fields (val)

• methods don’t change object, but return value

Natural, concise notation

• methods as infix operators, operator identifiers

• method overloading

• implicit conversion

TI1220 - Introduction

Exam Question

32

How many Rational objects are created while executing:

class Rational(n: Int, d: Int) { require(d != 0) val numer: Int = n val denom: Int = d override def toString = numer + "/" + denom def +(that: Rational): Rational = new Rational( numer * that.denom + that.numer * denom, denom * that.denom)}var half = new Rational(1,2)half = half + half + half

(a) 1(b) 2(c) 3(d) 4

B:6.1 TI 1220 - Lecture 2: Functional Objects 33

coffee break

B:6 TI 1220 - Lecture 2: Functional Objects

II

34

Built-in Control Structures


Control-Flow Structures

Control-Flow Structures

• ordering execution

• choice, iteration, exception

Functional Control Structures

• can be used as expression

• return a value

Minimal set of built-in control structures

• control abstraction using function literals (next week)

35

B:7.1

val filename = if (!args.isEmpty) args(0) else "default.txt"

conditional choice


If Expressions

36

var filename = "default.txt"if (!args.isEmpty) filename = args(0)

imperative style

functional style

B:7.1

replace equals by equals


Equational Reasoning

37

val filename = if (!args.isEmpty) args(0) else "default.txt"println(filename)

println(if (!args.isEmpty) args(0) else "default.txt")

B:7.1

replace equals by equals


Equational Reasoning

38

val x = e; f(x) <=> f(e)

if e has no side effects

B:7.2

def gcdLoop(x: Long, y: Long): Long = { var a = x var b = y while (a != 0) { val temp = a a = b % a b = temp } b}

iteration


While Loops

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B:7.2

var line = ""do { line = readLine() println("Read: " + line)} while (line != "")

iteration


Do-While Loops

40

B:7.2

var line = ""while ((line = readLine()) != "") // This doesn’t work! println("Read: " + line)


Assignment has Type Unit

41

B:7.2

def gcd(x: Long, y: Long): Long = if (y == 0) x else gcd(y, x % y)


Iteration vs Recursion

42

def gcdLoop(x: Long, y: Long): Long = { var a = x var b = y while (a != 0) { val temp = a a = b % a b = temp } b}

B:7.3

val filesHere = (new java.io.File(".")).listFilesfor (file <- filesHere) println(file)

functional iteration


For Expressions

43

B:7.3

scala> for (i <- 1 to 4) | println("Iteration " + i)Iteration 1Iteration 2Iteration 3Iteration 4

iterating over sequence of numbers


Ranges

44

// Not common in Scala...for (i <- 0 to filesHere.length - 1) println(filesHere(i))

B:6.1

for (file <- filesHere if file.getName.endsWith(".scala")) println(file)

iterating over subset of a collection


Filtering

45

for (file <- filesHere) if (file.getName.endsWith(".scala")) println(file)

B:7.3

for ( file <- filesHere if file.isFile; if file.getName.endsWith(".scala")) println(file)


Multiple Filters

46

B:7.3

def fileLines(file: java.io.File) = scala.io.Source.fromFile(file).getLines.toList

def grep(pattern: String) = for ( file <- filesHere if file.getName.endsWith(".scala"); line <- fileLines(file) if line.trim.matches(pattern) ) println(file + ": " + line.trim)

grep(".*gcd.*")


Nested Iteration

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B:7.3

def grep(pattern: String) = for { file <- filesHere if file.getName.endsWith(".scala") line <- fileLines(file) trimmed = line.trim if trimmed.matches(pattern) } println(file + ": " + trimmed)grep(".*gcd.*")


Mid-Stream Variable Binding

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B:7.3

def scalaFiles = for { file <- filesHere if file.getName.endsWith(".scala") } yield file

yield


Producing a New Collection

49

B:7.3

val forLineLengths = for { file <- filesHere if file.getName.endsWith(".scala") line <- fileLines(file) trimmed = line.trim if trimmed.matches(".*for.*") } yield trimmed.length


Composing filters

50

B:7.4

val half = if (n % 2 == 0) n / 2 else throw new RuntimeException("n must be even")

exception handling


Throwing Exceptions

51

B:7.4

import java.io.FileReaderimport java.io.FileNotFoundExceptionimport java.io.IOExceptiontry { val f = new FileReader("input.txt") // Use and close file} catch { case ex: FileNotFoundException => // Handle missing file case ex: IOException => // Handle other I/O error}

exception handling


Catching Exceptions

52

B:7.4

import java.io.FileReaderval file = new FileReader("input.txt")try { // Use the file} finally { file.close() // Be sure to close the file}

exception handling


Finally

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B:7.4

import java.net.URLimport java.net.MalformedURLExceptiondef urlFor(path: String) = try { new URL(path) } catch { case e: MalformedURLException => new URL("http://www.scalalang.org") }

exception handling


Yielding a Value

54

http://www.scalalang.org

http://www.scalalang.org

B:7.5

val firstArg = if (args.length > 0) args(0) else ""firstArg match { case "salt" => println("pepper") case "chips" => println("salsa") case "eggs" => println("bacon") case _ => println("huh?")}

choosing between actions


Match Expressions

55

B:7.5

val firstArg = if (!args.isEmpty) args(0) else ""val friend = firstArg match { case "salt" => "pepper" case "chips" => "salsa" case "eggs" => "bacon" case _ => "huh?" }println(friend)

choosing between values


Match Expressions

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B:7.6

int i = 0; // This is Javaboolean foundIt = false;while (i < args.length) { if (args[i].startsWith("")) { i = i + 1; continue; } if (args[i].endsWith(".scala")) { foundIt = true; break; } i = i + 1;}

not in Scala


Break and Continue in Java

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B:7.6

var i = 0var foundIt = falsewhile (i < args.length && !foundIt) { if (!args(i).startsWith("")) { if (args(i).endsWith(".scala")) foundIt = true } i = i + 1}


Replace Break/Continue with If ...

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B:7.6

def searchFrom(i: Int): Int = if (i >= args.length) 1 else if (args(i).startsWith("")) searchFrom(i + 1) else if (args(i).endsWith(".scala")) i else searchFrom(i + 1)val i = searchFrom(0)


... or with Recursion

59

B:7.7

almost identical to Java


Variable Scope

60

Variables are declared in a scope

• { var j = 1; ... }

Inner scopes shadow variables in outer scopes

• { var j = 1; { var j = 2; ... } }

B:6.1

$ scala printmultitable.scala 1 2 3 4 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20 3 6 9 12 15 18 21 24 27 30 4 8 12 16 20 24 28 32 36 40 5 10 15 20 25 30 35 40 45 50 6 12 18 24 30 36 42 48 54 60 7 14 21 28 35 42 49 56 63 70 8 16 24 32 40 48 56 64 72 80 9 18 27 36 45 54 63 72 81 90 10 20 30 40 50 60 70 80 90 100


Multiplication Table

B:7.7 TI 1220 - Lecture 2: Functional Objects 62

def printMultiTable() { var i = 1 // only i in scope here while (i <= 10) { var j = 1 // both i and j in scope here while (j <= 10) { val prod = (i * j).toString // i, j, and prod in scope here var k = prod.length // i, j, prod, and k in scope here while (k < 4) { print(" ") k += 1 } print(prod) j += 1 } // i and j still in scope; prod and k out of scope println() i += 1 } // i still in scope; j, prod, and k out of scope}

B:6.1

// Returns a row as a sequencedef makeRowSeq(row: Int) = for (col <- 1 to 10) yield { val prod = (row * col).toString val padding = " " * (4 - prod.length) padding + prod }

// Returns a row as a stringdef makeRow(row: Int) = makeRowSeq(row).mkString

// Returns table as a string with one row per linedef multiTable() = { val tableSeq = // a sequence of row strings for (row <- 1 to 10) yield makeRow(row) tableSeq.mkString("\n")}


Refactoring Imperative-Style Code

Term Rewriting

Summary

VII

64

lessons learned


Summary

Functional Objects

• immutable objects

• operations create new objects

Functional Control Structures

• return a value

• have a type

• can be used as expressions

65

required reading


Literature

Programming in Scala

• Chapter 6: Functional Objects

• Chapter 7: Built-in Control Structures

66

B:6.1

complex numbers


Exercises Week 2

Scala Test

• using unit testing framework to define executable tests

Complex numbers

• define class to represent complex numbers

• define tests

Note: all assignments should be done individually!


http://www.scala-lang.org/api/current/index.html



B:6.1

formatting rules


Scala Style Guide

http://www.codecommit.com/scala-style-guide.pdf

indentation: 2 spaces



B:6.1

coming next


Outlook

Lecture 3: Functions & Closures

• Chapters 8, 9

Lecture 4: List Programming

• Chapters 15, 16, 17

Lecture 5: Trees

• Chapters 26, 22, 23

Lab Week 2

• Complex numbers in Scala

70


Exam Question (Week 1)

71

What happens when we execute the following code:

val greetStrings = new Array[String](3)greetStrings(0) = "Hello"greetStrings(1) = ", "greetStrings(2) = "world!\n"

(a) Error: greetStrings is an immutable variable that cannot be assigned to

(b) Error: Array is a immutable data structure that cannot be assigned to

(c) No error: greetStrings is a mutable variable that can be assigned to

(d) No error: Array is a mutable data structure that can be assigned to



72

What happens when we execute the following code:

val greetStrings = new Array[String](3)greetStrings(0) = "Hello"greetStrings(1) = ", "greetStrings(2) = "world!\n"

(a) Error: greetStrings is an immutable variable that cannot be assigned to

(b) Error: Array is a immutable data structure that cannot be assigned to

(c) No error: greetStrings is a mutable variable that can be assigned to

(d) No error: Array is a mutable data structure that can be assigned to



73

What is the return type of makeRowSeq:

def makeRowSeq(row: Int) = for (col <- 1 to 10) yield { val prod = (row * col).toString val padding = " " * (4 - prod.length) padding + prod }

(a) String(b) Int(c) IndexedSeq[String](d) IndexedSeq[Int]

B:6.1

copyrights


Pictures

Slide 1: A Plumpish Proportion by SSG Robert Stewart some rights reservedSlide 3: McPhillips’ Map of the City of Winnipeg by Manitoba Historical Maps, some rights reserved

Slide 19: Envelopes by benchilada, some rights reservedSlide 20: Report card by Carosaurus, some rights reservedSlide 30: Sun is Shining by el patojo, some rights reserved

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http://www.flickr.com/people/carosaurus

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http://www.flickr.com/photos/elpatojo/301428983/

http://www.flickr.com/photos/elpatojo/301428983/

B:6.1

copyrights


Pictures

Slide: Dinner at Roussillon (Martin Odersky) by Miles Sabin, some rights reservedSlide: Portrait: Alex Payne by Dave Fayram, some rights reservedSlide: “Plumbing Nightmare” by Natalie Wilkie, some rights reservedSlide: HIV: The Moleskine Summary by Niels OlsonSlide: remember to thank all the books you haven’t read over the past three years by Natalia Osiatynska, Some rights reservedSlide: Stupid Exam by Remi Carreiro, Some rights reservedSlide: Practice makes perfect by Simon Probert

http://www.flickr.com/photos/garrettc/3747802654/ Bombe detail by Garret Coakley, Some rights reserved

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http://www.flickr.com/photos/montpelier/3363005591/

http://www.flickr.com/photos/montpelier/3363005591/

http://www.flickr.com/photos/davefayram/4283737907/

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http://www.flickr.com/photos/tesla314/226928724/

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http://www.flickr.com/photos/niels_olson/3045784983/

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http://www.flickr.com/photos/purplepick/2554134479/

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http://www.piano-lessons.net/news_item.php?id=9

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http://www.flickr.com/photos/garrettc/3747802654/

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ti1220 lecture 2

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