// Scala Lecture 1//=================// Value assignments// (their names should be lower case)//====================================val x = 42val y = 3 + 4val z = x / y// (you cannot reassign values: z = 9 will give an error)// 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)1 to 10(1 to 10).toList(1 until 10).toList// an element in a listval lst = List(1, 2, 3, 1)lst(0)lst(2)// some alterative syntax for lists1 :: 2 :: 3 :: NilList(1, 2, 3) ::: List(4, 5, 6)// Equality is structural//========================val a = "Dave"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)val n1 = 3 + 7val n2 = 5 + 5n1 == n2// this applies to "concrete" values;// you cannot compare functions// Printing/Strings//==================println("test")val tst = "This is a " + "test\n" 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("{", ",", "}"))// Conversion methods//====================List(1,2,3,1).toStringList(1,2,3,1).toSet"hello".toList.tail1.toDouble// useful list methodsList(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 (slide)//===============/* Scala is a strongly typed language * some base types Int, Long, BigInt, Float, Double String, Char Boolean * some compound types List[Int], Set[Double] Pairs: (Int, String) List[(BigInt, String)] Option[Int]*/// 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.toStringsquare(6)// 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 = {// body // }//// BTW: no returns!!// "last" line (expression) in a function determines the result//def silly(n: Int) : Int = { if (n < 10) n * n else n + n}// If-Conditionals//=================// - Scala does not have a then-keyword// - both if-else branches need to be presentdef fact(n: Int) : Int = if (n == 0) 1 else n * fact(n - 1)fact(5)fact(150)/* boolean operators == equals ! not && || and, or*/def fact2(n: BigInt) : BigInt = if (n == 0) 1 else n * fact2(n - 1)fact2(150)def fib(n: Int) : Int = if (n == 0) 1 else if (n == 1) 1 else fib(n - 1) + fib(n - 2)//gcd - Euclid's algorithmdef gcd(a: Int, b: Int) : Int = { if (b == 0) a else { val foo = 42 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)// Option type//=============//in Java if something unusually happens, you return null////in Scala you use Options instead// - if the value is present, you use Some(value)// - if no value is present, you use NoneList(7,2,3,4,5,6).find(_ < 4)List(5,6,7,8,9).find(_ < 4)// error handling with Options (no exceptions)//// Try(something).getOrElse(what_to_do_in_case_of_an_exception)//import scala.util._import io.Sourceval my_url = "https://nms.imperial.ac.uk/christian.urban/"Source.fromURL(my_url).mkStringTry(Source.fromURL(my_url).mkString).getOrElse("")Try(Some(Source.fromURL(my_url).mkString)).getOrElse(None)// the same for filesTry(Some(Source.fromFile("text.txt").mkString)).getOrElse(None)// function reading something from files...def get_contents(name: String) : List[String] = Source.fromFile(name).getLines.toListget_contents("test.txt")// slightly better - return Nildef get_contents(name: String) : List[String] = Try(Source.fromFile(name).getLines.toList).getOrElse(List())get_contents("text.txt")// much better - you record in the type that things can go wrong def get_contents(name: String) : Option[List[String]] = Try(Some(Source.fromFile(name).getLines.toList)).getOrElse(None)get_contents("text.txt")// String Interpolations//=======================val n = 3println("The square of " + n + " is " + square(n) + ".")println(s"The square of ${n} is ${square(n)}.")// helpful for debugging purposes//// "The most effective debugging tool is still careful thought, // coupled with judiciously placed print statements."// — Brian W. Kernighan, in Unix for Beginners (1979)def gcd_db(a: Int, b: Int) : Int = { println(s"Function called with ${a} and ${b}.") if (b == 0) a else gcd_db(b, a % b)}gcd_db(48, 18)// Asserts/Testing//=================assert(gcd(48, 18) == 6)assert(gcd(48, 18) == 5, "The gcd test failed")// For-Comprehensions (not For-Loops)//====================================for (n <- (1 to 10).toList) yield { square(n) + 1}for (n <- (1 to 10).toList; m <- (1 to 10).toList) yield m * nval mult_table = for (n <- (1 to 10).toList; m <- (1 to 10).toList) yield m * nprintln(mult_table.mkString)mult_table.sliding(10,10).mkString("\n")// the list/set/... can also be constructed in any // other wayfor (n <- Set(10,12,4,5,7,8,10)) yield n * n// with if-predicates / filtersfor (n <- (1 to 3).toList; m <- (1 to 3).toList; if (n + m) % 2 == 0; if (n * m) < 2) yield (n, m)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-yieldfor (p <- ...) yield { // potentially complicated // calculation of a result}// 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)// For without yield//===================// with only a side-effect (no list is produced),// has no "yield"for (n <- (1 to 10)) println(n)// BTW: a roundabout way of printing out a list, sayval lst = ('a' to 'm').toListfor (n <- lst) println(n)for (i <- (0 until lst.length)) println(lst(i))// Why not just? Why making your life so complicated?for (c <- lst) println(c)// Aside: concurrency // (ONLY WORKS OUT-OF-THE-BOX IN SCALA 2.11.8, not in SCALA 2.12)// (would need to have this wrapped into a function, or// REPL called with scala -Yrepl-class-based)for (n <- (1 to 10)) println(n)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)// Just for "Fun": Mutable vs Immutable//=======================================//// - no vars, no ++i, no +=// - no mutable data-structures (no Arrays, no ListBuffers)// Q: Count how many elements are in the intersections of two sets?def count_intersection(A: Set[Int], B: Set[Int]) : Int = { var count = 0 for (x <- A; if (B contains x)) count += 1 count}val A = (1 to 1000).toSetval B = (1 to 1000 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)//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 A = (1 to 1000000).toSetval B = (1 to 1000000 by 4).toSettime_needed(10, count_intersection(A, B))time_needed(10, count_intersection2(A, B))// 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.