都什麼時代了，你還在寫 while loop 嗎？

Slide 1

Slide 1 text

){}w ile(){}w {}while(){}w le(){}while(){}w }while(){}while(){}w e(){}while(){}while(){}w while(){}while(){}while(){}w (){}while(){}while(){}while(){}w hile(){}while(){}while(){}while(){}w ){}while(){}while(){}while(){}while(){}w ile(){}while(){}while(){}while(){}while(){}w {}while(){}while(){}while(){}while(){}while(){}w 都什麼時代了你還在寫 while loop 嗎？ INFECTED PROGRAM WHILE LOOP @pishen

Slide 2

Slide 2 text

Pishen Vpon 5 years Scala Scala Taiwan Open Source VocaRadio sbt-lighter annoy4s AkkaUI Gitter Facebook Scala Kitchen 日本 Scala 旅遊團 https://gitter.im/ScalaTaiwan/ScalaTaiwan https://www.facebook.com/groups/ScalaTW/ https://www.meetup.com/Scala-Taiwan-Meetup/ Meetup (each month) Since 2015 冬

Slide 3

Slide 3 text

2018-10-19 2018-12-31 ?

Slide 4

Slide 4 text

2018-10-19 2018-12-31 2018-10-20 2018-10-21 2018-10-22 • • • 2018-12-28 2018-12-30 2018-12-29

Slide 5

Slide 5 text

Solution 1: While Loop

Slide 6

Slide 6 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates) java.time.LocalDate

Slide 7

Slide 7 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates)

Slide 8

Slide 8 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates) append List(1,2,3) :+ 4 List(1,2,3,4)

Slide 9

Slide 9 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates)

Slide 10

Slide 10 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates) dates.length == x dates.length == x + 1

Slide 11

Slide 11 text

var dates = List(startDate) //while (dates.last.isBefore(endDate)) { // // // dates = dates :+ dates.last.plusDays(1) // // //} println(dates) ✔ Compiler still pass! dates.length == x dates.length == x + 1

Slide 12

Slide 12 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates) dates.length == x dates.length == x + 1

Slide 13

Slide 13 text

var dates = List(startDate) println(dates) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } dates.length == x dates.length == x + 1 ✔ Compiler still pass!

Slide 14

Slide 14 text

var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates) dates.length == x dates.length == x + 1 An optional part of code that *will* change the program's behavior. ?

Slide 15

Slide 15 text

1970s Don't use GOTO 1995 25 years GOTO is prohibited! https://en.wikipedia.org/wiki/Goto 2018 23 years Don't use while

Slide 16

Slide 16 text

Solution 2: Tail Recursion

Slide 17

Slide 17 text

Slide 18

Slide 18 text

Slide 19

Slide 19 text

Slide 20

Slide 20 text

Slide 21

Slide 21 text

def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } val dates = iterate(List(startDate)) println(dates) iterate [d1,d2,d3,d4,d5] iterate [d1] iterate [d1,d2] iterate [d1,d2,d3] iterate [d1,d2,d3,d4] [d1,d2,d3,d4,d5] [d1,d2,d3,d4,d5] [d1,d2,d3,d4,d5] [d1,d2,d3,d4,d5] [d1,d2,d3,d4,d5]

Slide 22

Slide 22 text

Slide 23

Slide 23 text

Slide 24

Slide 24 text

Slide 25

Slide 25 text

//def iterate(dates: List[LocalDate]): List[LocalDate] = { // if (dates.last.isBefore(endDate)) { // iterate(dates :+ dates.last.plusDays(1)) // } else { // dates // } //} val dates = iterate(List(startDate)) println(dates) ✗ Compile Error!

Slide 26

Slide 26 text

Slide 27

Slide 27 text

val dates = iterate(List(startDate)) def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } println(dates) ✗ Compile Error!

Slide 28

Slide 28 text

Slide 29

Slide 29 text

def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } val dates = iterate(List(startDate)) println(dates) var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates)

Slide 30

Slide 30 text

def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } val dates = iterate(List(startDate)) println(dates) var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates)

Slide 31

Slide 31 text

def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } val dates = iterate(List(startDate)) println(dates) var dates = List(startDate) while (dates.last.isBefore(endDate)) { dates = dates :+ dates.last.plusDays(1) } println(dates)

Slide 32

Slide 32 text

def iterate(dates: List[LocalDate]): List[LocalDate] = { if (dates.last.isBefore(endDate)) { iterate(dates :+ dates.last.plusDays(1)) } else { dates } } Tail Recursion Only call yourself at the last position. Tail Call Optimization Prevent memory from stack overflow. https://en.wikipedia.org/wiki/Tail_call https://www.artima.com/pins1ed/functions-and-closures.html#8.9 Not available in Java ⚠

Slide 33

Slide 33 text

Solution 3: Stream

Slide 34

Slide 34 text

1 2 3 4 5 6 7 val numbers = Stream.range(1, 8) Not evaluated (Take no memory)

Slide 35

Slide 35 text

1 2 3 4 5 6 7 Not evaluated (Take no memory) val numbers = Stream.range(1, 8) val n2 = numbers(2)

Slide 36

Slide 36 text

1 2 3 4 5 6 7 val numbers = Stream.range(1, 8) val n2 = numbers(2) Memoization

Slide 37

Slide 37 text

1 2 3 4 5 6 7 val numbers = Stream.range(1, 8) val n2 = numbers(2) val n4 = numbers(4) Memoization

Slide 38

Slide 38 text

1 2 3 4 5 6 7 Memoization val numbers = Stream.range(1, 8) val n2 = numbers(2) val n4 = numbers(4)

Slide 39

Slide 39 text

1 2 3 4 5 6 7 val numbers = Stream.range(1, 8) val n2 = numbers(2) val n4 = numbers(4) val list = numbers.toList

Slide 40

Slide 40 text

val numbers = Stream.range(1, 8) val n2 = numbers(2) val n4 = numbers(4) val list = numbers.toList 1 2 3 4 5 6 7 1 2 3 4 5 6 7 list

Slide 41

Slide 41 text

Stream.continually(1) Stream.iterate(0)(x => x + 2) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 2 4 6 8 10 12 14 16 18 20 2 +2 +2 +2 Stream.continually(List(1,2,3)).flatten 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 lambda function

Slide 42

Slide 42 text

stream.map(x => x * 2) 1 2 3 4 5 6 7 8 9 10 11 1 2 4 6 8 10 12 14 16 18 20 22 2

Slide 43

Slide 43 text

stream.filter(x => x % 2 == 1) 1 2 3 4 5 6 7 8 9 10 11 1 1 3 5 7 9 11 13 15 17 19 21 2

Slide 44

Slide 44 text

stream.flatMap(x => List(x, x)) 1 2 3 4 5 6 7 8 9 10 11 1 1 1 2 2 3 3 4 4 5 5 6 1 1 2 2 3 3 4 4 5 5 6 6

Slide 45

Slide 45 text

stream.take(5) 1 2 3 4 5 6 7 8 9 10 11 1 1 2 3 4 5

Slide 46

Slide 46 text

stream.takeWhile(x => x < 6) 1 2 3 4 5 6 7 8 9 10 11 1 1 2 3 4 5

Slide 47

Slide 47 text

val dates = Stream.iterate(startDate)(_.plusDays(1)) x => x.plusDays(1) plusDays(1) d1 d2 d3 d4 d5 d6 d7 d8 d9 d10 d11

Slide 48

Slide 48 text

val dates = Stream.iterate(startDate)(_.plusDays(1)) .takeWhile(_.isBefore(endDate.plusDays(1))) d1 d2 d3 d4 d5 d6 d7 d8 d9 d10 d11 endDate d1 d2 d3 d4 d5 d6 d7 d8 d9 dates

Slide 49

Slide 49 text

val dates = Stream.iterate(startDate)(_.plusDays(1)) .takeWhile(_.isBefore(endDate.plusDays(1))) println(dates.toList)

Slide 50

Slide 50 text

val numbers = Stream.iterate(0)(_ + 1) val n3 = numbers(3) 1 2 3 4 5 6 7 8 9 10 11 1 val numbers = Iterator.iterate(0)(_ + 1) val n3 = numbers(3) 1 2 3 4 5 6 7 8 9 10 11 1 Works like Java's Stream Memoization

Slide 51

Slide 51 text

1 2 3 4 5 6 7 Stream.range(1, 8) LazyList.range(1, 8) 1 2 3 4 5 6 7 Scala 2.13.0 (fully lazy)

Slide 52

Slide 52 text

Example 1: Auto Pager

Slide 53

Slide 53 text

val page: Page = getFirstPage() val items: List[Item] = page.getItems() val hasNext: Boolean = page.hasNextPage() val nextPage: Page = page.getNextPage() May return null if Page doesn't exist. Given a number n, please return first n items to me?

Slide 54

Slide 54 text

def take(n: Int): List[Item] = { var page = getFirstPage() if (page != null) { var items = page.getItems() while(page.hasNextPage() && items.size < n) { page = page.getNextPage() items = items ++ page.getItems() } items.take(n) } else { List.empty[Item] } }

Slide 55

Slide 55 text

val items = Stream .iterate(getFirstPage())(_.getNextPage) .takeWhile(_ != null) .flatMap(_.getItems) p1 p2 p3 null getNextPage() Exceptions p1 p2 p3 takeWhile() i1 i2 i3 i4 i5 i6 i7 i8 i9 flatMap() items.take(5)

Slide 56

Slide 56 text

Slide 57

Slide 57 text

Slide 58

Slide 58 text

Example 2: Fibonacci Sequence

Slide 59

Slide 59 text

stream.tail 1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 7 8 9 10 11 1

Slide 60

Slide 60 text

streamA.zip(streamB) 1 2 3 4 5 6 7 8 9 10 11 streamA 1 2 3 1 2 3 1 2 3 1 2 streamB

Slide 61

Slide 61 text

streamA.zip(streamB).map { case (a, b) => a + b } 1 2 3 4 5 6 7 8 9 10 11 streamA 1 2 3 1 2 3 1 2 3 1 2 streamB 2 4 6 5 7 9 8 10 12 11 13

Slide 62

Slide 62 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 fibs 0 1 fibs 1 fibs.tail

Slide 63

Slide 63 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 0 1 1 fibs fibs fibs.tail

Slide 64

Slide 64 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 0 1 1 1 1 fibs fibs fibs.tail

Slide 65

Slide 65 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 2 0 1 1 1 1 fibs fibs fibs.tail

Slide 66

Slide 66 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 2 0 1 1 2 1 1 2 fibs fibs fibs.tail

Slide 67

Slide 67 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 2 3 0 1 1 2 1 1 2 fibs fibs fibs.tail

Slide 68

Slide 68 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 2 3 0 1 1 2 3 1 1 2 3 fibs fibs fibs.tail

Slide 69

Slide 69 text

val fibs: Stream[Int] = 0 #:: 1 #:: fibs.zip(fibs.tail).map { case (a, b) => a + b } 0 1 1 2 3 5 8 13 21 34 55 0 1 1 2 3 5 8 13 21 1 1 2 3 5 8 13 21 34 fibs fibs fibs.tail

Slide 70

Slide 70 text

T h a n k Y o u