// Scala Lecture 1//=================// Value assignments// (their names should be lower case)//====================================val x = 42val y = 3 + 4 val z = x / yval x = 70print(z)// (you cannot reassign values: z = 9 will give an error)//var z = 9//z = 10// Hello World//=============// an example of a stand-alone Scala file// (in the assignments you must submit a plain Scala script)object Hello extends App { println("hello world")}// can then be called with//// $> scalac hello-world.scala// $> scala Hello//// $> java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello// Collections//=============List(1,2,3,1)Set(1,2,3,1)// picking an element in a listval lst = List(1, 2, 3, 1)lst(0)lst(2)// head and taillst.headlst.tail// some alterative syntax for listsNil // empty list1 :: 2 :: 3 :: NilList(1, 2, 3) ::: List(4, 5, 6)// alsoList(1, 2, 3) ++ List(3, 6, 5)Set(1, 2, 3) ++ Set(3, 6, 5)// ranges1 to 10(1 to 10).toList(1 to 10).toList.toString(1 until 10).toList// Equality in Scala is structural//=================================val a = "Dave2"val b = "Dave"if (a == b) println("Equal") else println("Unequal")Set(1,2,3) == Set(3,1,2)List(1,2,3) == List(3,1,2)// this applies to "concrete" values...pretty much // everything; but for example you cannot compare // functions (later), and also not arraysArray(1) == Array(1)// Printing/Strings//==================println("test")val tst = "This is a " ++ "test" print(tst) println(tst) val lst = List(1,2,3,1)println(lst.toString)println(lst.mkString)println(lst.mkString(","))// some methods take more than one argumentprintln(lst.mkString("{", ",", "}"))// (in this case .mkString can take no, one, // or three arguments...this has to do with// default arguments)// Conversion methods//====================List(1,2,3,1).toStringList(1,2,3,1).toSet"hello".toList"hello".toSet1.toDouble1 // an Int1L // a Long1F // a Float1D // a Double// useful list methods on lists//==============================List(1,2,3,4).lengthList(1,2,3,4).reverseList(1,2,3,4).maxList(1,2,3,4).minList(1,2,3,4).sumList(1,2,3,4).take(2).sumList(1,2,3,4).drop(2).sumList(1,2,3,4,3).indexOf(3)"1,2,3,4,5".split(",").mkString("\n")"1,2,3,4,5".split(",").toList"1,2,3,4,5".split(",3,").mkString("\n")"abcdefg".startsWith("abc")// Types (see slide)//===================/* Scala is a strongly typed language * base types Int, Long, BigInt, Float, Double String, Char Boolean... * compound types List[Int] Set[Double] Pairs: (Int, String) List[(BigInt, String)] Option[Int] * user-defined types (later)*/// you can make the type of a value explicitval name = "bob"// type errorsmath.sqrt("64".toDouble)// produces//// error: type mismatch;// found : String("64")// required: Double// math.sqrt("64")// Pairs/Tuples//==============val p = (1, "one")p._1p._2val t = (4,1,2,3)t._4List(("one", 1), ("two", 2), ("three", 3))// Function Definitions//======================def incr(x: Int) : Int = x + 1def double(x: Int) : Int = x + xdef square(x: Int) : Int = x * xdef str(x: Int) : String = x.toStringincr(3)double(4)square(6)str(3)// The general scheme for a function: you have to give a // type to each argument and a return type of the function//// def fname(arg1: ty1, arg2: ty2,..., argn: tyn): rty = {// // }// If-Conditionals//=================// - Scala does not have a then-keyword// - !!both if-else branches need to be present!!def fact(n: Int) : Int = if (n == 0) 1 else n * fact(n - 1)fact(5)fact(150)/* boolean operators == equals != not equals ! not && || and, or*/def fib(n: Int) : Int = { if (n == 0) 1 else if (n == 1) 1 else fib(n - 1) + fib(n - 2)}fib(9)//gcd - Euclid's algorithmdef gcd(a: Int, b: Int) : Int = { if (b == 0) a else gcd(b, a % b)}gcd(48, 18)def power(x: Int, n: Int) : Int = if (n == 0) 1 else x * power(x, n - 1) power(5, 5)// BTW: no returns!!// "last" line (expression) in a function determines the // resultdef average(xs: List[Int]) : Int = { if (xs.length == 0) 0 else xs.sum / xs.length}average(List())// For-Comprehensions (not For-Loops)//====================================val lst = (1 to 10).toListfor (n <- lst) yield n * n for (n <- lst) yield { square(n) + double(n)}for (n <- (1 to 10).toList; m <- (1 to 5).toList) yield (n, m, n * m)// you can assign the result of a for-comprehension// to a valueval mult_table = for (n <- (1 to 10).toList; m <- (1 to 10).toList) yield n * mprintln(mult_table.mkString)mult_table.sliding(10,10).mkString("\n")// for-comprehensions also work for other// collectionsfor (n <- Set(10,12,4,5,7,8,10)) yield n * nfor (n <- (1 to 10)) yield { n * n }// with if-predicates / filtersif (1 == 2) "a" else "b"for (n <- (1 to 3).toList; m <- (1 to 3).toList; if (n + m) % 2 == 0) yield (n, m)// with patternsval lst = List((1, 4), (2, 3), (3, 2), (4, 1))for ((m, n) <- lst) yield m + n for (p <- lst) yield p._1 + p._2 // general pattern of for-yield // (yield can be several lines)for (pat <- ...) yield { // potentially complicated // calculation of a result}// For without yield//===================// with only a side-effect (no list is produced),// has no "yield"for (n <- (1 to 10).toList) println(n * n)for (n <- (1 to 10).toList) yield n * n// BTW: a roundabout way of printing out a list, sayval lst = ('a' to 'm').toListfor (i <- (0 until lst.length)) println(lst(i))// Why not just? Why making your life so complicated?for (c <- lst) println(c)// Functions producing multiple outputs//======================================def get_ascii(c: Char) : (Char, Int) = (c, c.toInt)get_ascii('a')// .maxBy, sortBy with pairsdef get_length(s: String) : (String, Int) = (s, s.length) val lst = List("zero", "one", "two", "three", "four", "ten")val strs = for (s <- lst) yield get_length(s)strs.sortBy(_._2)strs.sortBy(_._1)strs.maxBy(_._2)strs.maxBy(_._1)// Aside: concurrency // scala -Yrepl-class-based -cp scala-parallel-collections_2.13-0.2.0.jar for (n <- (1 to 10)) println(n)import scala.collection.parallel.CollectionConverters._for (n <- (1 to 10).par) println(n)// for measuring timedef time_needed[T](n: Int, code: => T) = { val start = System.nanoTime() for (i <- (0 to n)) code val end = System.nanoTime() (end - start) / 1.0e9}val list = (1 to 1000000).toListtime_needed(10, for (n <- list) yield n + 42)time_needed(10, for (n <- list.par) yield n + 42)// ...but par does not make everything fasterlist.sumlist.par.sumtime_needed(10, list.sum)time_needed(10, list.par.sum)// Mutable vs Immutable//======================//// Remember:// - no vars, no ++i, no +=// - no mutable data-structures (no Arrays, no ListBuffers)// But what the heck....lets try to count to 1 Mio in parallelvar cnt = 0for(i <- (1 to 1000000).par) cnt += 1println(s"Should be 1 Mio: $cnt")// Or// Q: Count how many elements are in the intersections of // two sets?// A; IMPROPER WAY (mutable counter)def count_intersection(A: Set[Int], B: Set[Int]) : Int = { var count = 0 for (x <- A.par; if (B contains x)) count += 1 count}val A = (0 to 999).toSetval B = (0 to 999 by 4).toSetcount_intersection(A, B)// but do not try to add .par to the for-loop above//propper parallel versiondef count_intersection2(A: Set[Int], B: Set[Int]) : Int = A.par.count(x => B contains x)count_intersection2(A, B)//another bad exampledef test() = { var cnt = 0 for(i <- (1 to 1000000).par) cnt += 1 println(cnt)}test()// Regular Expressions (the built in ones)val s = """Any so-called "politician" should respect a vote."""print(s)print("""foo""")val reg = """\d+""".rreg.findAllIn("bbbbaaabbbaaaccc").toListreg.replaceAllIn("bbbbaaabbbaaaccc", "*")reg.replaceAllIn("bbbb0aaa1bbba2aac3cc", "_")reg.replaceAllIn("bbbb00aaa11bbba232aac33cc", "_")// Further Information//=====================// The Scala homepage and general information is at//// http://www.scala-lang.org// http://docs.scala-lang.org////// It should be fairly easy to install the Scala binary and// run Scala on the commandline. People also use Scala with // Vim and Jedit. I currently settled on VS Code//// https://code.visualstudio.com//// There are also plugins for Eclipse and IntelliJ - YMMV.// Finally there are online editors specifically designed for // running Scala applications (but do not blame me if you lose // all what you typed in)://// https://scalafiddle.io // https://scastie.scala-lang.org//////// Scala Library Docs//====================//// http://www.scala-lang.org/api/current///// Scala Tutorials//// http://docs.scala-lang.org/tutorials///// There are also a massive number of Scala tutorials on youtube// and there are tons of books and free material. Google is your // friend.