// Scala Lecture 1

//=================

// Value assignments

// (their names should be lower case)

//====================================

val x = 42

val y = 3 + 4

val 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)

// ranges

1 to 10

(1 to 10).toList

(1 until 10).toList

// picking an element in a list

val lst = List(1, 2, 3, 1)

lst(0)

lst(2)

// some alterative syntax for lists

1 :: 2 :: 3 :: Nil

List(1, 2, 3) ::: List(4, 5, 6)

// Equality in Scala 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 + 7

val n2 = 5 + 5

n1 == n2

// 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(","))

println(lst.mkString(", "))

// some methods take more than one argument

println(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).toString

List(1,2,3,1).toSet

"hello".toList.tail

1.toDouble

// useful list methods

List(1,2,3,4).length

List(1,2,3,4).reverse

List(1,2,3,4).max

List(1,2,3,4).min

List(1,2,3,4).sum

List(1,2,3,4).take(2).sum

List(1,2,3,4).drop(2).sum

List(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 explicit

val name: String = "leo"

// type errors

math.sqrt("64")

// produces

//

// error: type mismatch;

// found   : String("64")

// required: Double

// math.sqrt("64")

// Pairs/Tuples

//==============

val p = (1, "one")

p._1

p._2

val t = (4,1,2,3)

t._4

List(("one", 1), ("two", 2), ("three", 3))

// Function Definitions

//======================

def incr(x: Int) : Int = x + 1

def double(x: Int) : Int = x + x

def square(x: Int) : Int = x * x

def str(x: Int) : String = x.toString

incr(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 = {

//    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 present!

def 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 algorithm

def 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)

// Option type

//=============

//in Java if something unusually happens, you return null or something

//

//in Scala you use Options instead

//   - if the value is present, you use Some(value)

//   - if no value is present, you use None

List(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.Source

val my_url = "https://nms.kcl.ac.uk/christian.urban/"

Source.fromURL(my_url).mkString

Try(Source.fromURL(my_url).mkString).getOrElse("")

Try(Some(Source.fromURL(my_url).mkString)).getOrElse(None)

// the same for files

Try(Some(Source.fromFile("text.txt").mkString)).getOrElse(None)

def get_contents(name: String) : List[String] = 

  Source.fromFile(name).getLines.toList

get_contents("test.txt")

// slightly better - return Nil

def 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")

get_contents("test.txt")

// String Interpolations

//=======================

val n = 3

println("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 * n

// you can assign the result of a for-comprehension

// to a value

val mult_table = 

  for (n <- (1 to 10).toList; 

       m <- (1 to 10).toList) yield m * n

println(mult_table.mkString)

mult_table.sliding(10,10).mkString("\n")

// the list/set/... can also be constructed in any 

// other way

for (n <- Set(10,12,4,5,7,8,10)) yield n * n

// with if-predicates / filters

for (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 patterns

val 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 

for (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 pairs

def 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, say

val lst = ('a' to 'm').toList

// Why not just? Why making your life so complicated?

for (c <- lst) println(c)

// Aside: concurrency 

for (n <- (1 to 10)) println(n)

import scala.collection.parallel.CollectionConverters._

for (n <- (1 to 10).par) println(n)

// for measuring time

def 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).toList

time_needed(10, for (n <- list) yield n + 42)

time_needed(10, for (n <- list.par) yield n + 42)

list.sum

list.par.sum

time_needed(10, list.sum)

time_needed(10, list.par.sum)