// Scala Lecture 1
//=================
// Value assignments
// (their names should be lower case)
//====================================
//var z = 9
//z = 10
val x = 42
val y = 3 + 4
val z = x / y
val x = 70
// (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 to 10).toList.toString
(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)
// also
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)
// this applies to "concrete" values...pretty much
// everything; but for example you cannot compare
// functions (later), and also not arrays
Array(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(","))
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
"hello".toList.tail
1.toDouble
1L // a long
1F // a float
// 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".toDouble)
// 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 foo(s: String) : String = {
val tmp = s ++ s ++ s
val res = s ++ s
res
}
foo("test")
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 = {
//
// }
// 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)
// For-Comprehensions (not For-Loops)
//====================================
(1 to 10).toList
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
for (n <- (1 to 10)) yield {
n * n
}
if (1 == 2) "a" else "b"
// with if-predicates / filters
for (n <- (1 to 3).toList;
m <- (1 to 3).toList;
if (n + m) % 2 == 0) 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
// general pattern of for-yield
// (yield can be several lines)
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)
(1 to 10).toList
(1 until 10).toList
// BTW: a roundabout way of printing out a list, say
val lst = ('a' to 'm').toList
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
// 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 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)
// ...but par does not make everything faster
list.sum
list.par.sum
time_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 parallel
var cnt = 0
for(i <- (1 to 1000000).par) cnt += 1
println(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).toSet
val B = (0 to 999 by 4).toSet
count_intersection(A, B)
// but do not try to add .par to the for-loop above
//propper parallel version
def count_intersection2(A: Set[Int], B: Set[Int]) : Int =
A.par.count(x => B contains x)
count_intersection2(A, B)
//another bad example
def test() = {
var cnt = 0
for(i <- (1 to 1000000).par) cnt += 1
println(cnt)
}
test()
// 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.