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Adam Bard
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---
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language: Scala
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contributors:
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- ["George Petrov", "http://github.com/petrovg"]
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- ["Dominic Bou-Samra, "http://dbousamra.github.com"]
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filename: learn.scala
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---
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Scala - the scalable language
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```c
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/*
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Set yourself up:
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1) Download Scala - http://www.scala-lang.org/downloads
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2) unzip/untar in your favourite location and put the bin subdir on the path
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3) Start a scala REPL by typing scala. You should see the prompt:
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scala>
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This is the so called REPL. You can run commands in the REPL. Let's do just that:
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*/
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println(10) // prints the integer 10
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println("Boo!") // printlns the string Boo!
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// Some basics
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// Printing, and forcing a new line on the next print
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println("Hello world!")
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// Printing, without forcing a new line on next print
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print("Hello world")
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// Declaring values is done using either var or val
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// val declarations are immutable, whereas var's are mutable. Immutablility is a good thing.
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val x = 10 // x is now 10
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x = 20 // error: reassignment to val
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var x = 10
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x = 20 // x is now 20
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// Single line comments start with two forward slashes
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/*
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Multi line comments look like this.
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*/
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// Boolean values
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true
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false
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// Boolean operations
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!true // false
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!false // true
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true == false // false
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10 > 5 // true
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// Math is as per usual
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1 + 1 // 2
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2 - 1 // 1
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5 * 3 // 15
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6 / 2 // 3
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// Evaluating a command in the REPL gives you the type and value of the result
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1 + 7
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/* The above line results in:
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scala> 1 + 7
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res29: Int = 8
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This means the result of evaluating 1 + 7 is an object of type Int with a value of 8
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1+7 will give you the same result
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*/
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// Everything is an object, including a function. Type these in the REPL:
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7 // results in res30: Int = 7 (res30 is just a generated var name for the result)
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// The next line gives you a function that takes an Int and returns it squared
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(x:Int) => x * x
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// You can assign this function to an identifier, like this:
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val sq = (x:Int) => x * x
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/* The above says this
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sq: Int => Int = <function1>
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Which means that this time we gave an explicit name to the value - sq is a function that take an Int and returns Int.
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sq can be executed as follows:
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*/
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sq(10) // Gives you this: res33: Int = 100. The result is the Int with a value 100
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// Scala allows methods and functions to return, or take as parameters, other functions or methods.
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val add10: Int => Int = _ + 10 // A function taking an Int and returning an Int
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List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element
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// Anonymous functions can be used instead of named functions:
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List(1, 2, 3) map (x => x + 10)
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// And the underscore symbol, can be used if there is just one argument to the anonymous function. It gets bound as the variable
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List(1, 2, 3) map (_ + 10)
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TODO // If the anonymous block AND the function you are applying both take one argument, you can even omit the underscore
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List("Dom", "Bob", "Natalia") foreach println
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// Data structures
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val a = Array(1, 2, 3, 5, 8, 13)
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a(0)
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a(3)
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a(21) // Throws an exception
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val m = Map("fork" -> "tenedor", "spoon" -> "cuchara", "knife" -> "cuchillo")
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m("fork")
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m("spoon")
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m("bottle") // Throws an exception
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val safeM = m.withDefaultValue("no lo se")
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safeM("bottle")
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val s = Set(1, 3, 7)
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s(0)
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s(1)
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/* Look up the documentation of map here - http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map
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* and make sure you can read it
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*/
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// Tuples
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(1, 2)
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(4, 3, 2)
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(1, 2, "three")
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(a, 2, "three")
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// Why have this?
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val divideInts = (x:Int, y:Int) => (x / y, x % y)
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divideInts(10,3) // The function divideInts gives you the result and the remainder
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// To access the elements of a tuple, use _._n where n is the 1-based index of the element
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val d = divideInts(10,3)
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d._1
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d._2
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// Combinators
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s.map(sq)
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val sSquared = s. map(sq)
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sSquared.filter(_ < 10)
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sSquared.reduce (_+_)
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// The filter function takes a predicate (a function from A -> Boolean) and selects all elements which satisfy the predicate
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List(1, 2, 3) filter (_ > 2) // List(3)
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List(
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Person(name = "Dom", age = 23),
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Person(name = "Bob", age = 30)
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).filter(_.age > 25) // List(Person("Bob", 30))
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// Scala a foreach method defined on certain collections that takes a type returning Unit (a void method)
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aListOfNumbers foreach (x => println(x))
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aListOfNumbers foreach println
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// For comprehensions
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for { n <- s } yield sq(n)
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val nSquared2 = for { n <- s } yield sq(n)
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for { n <- nSquared2 if n < 10 } yield n
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for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
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/* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas a for-comprehension
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defines a relationship between two sets of data. Research this further */
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// Loops and iteration
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1 to 5
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val r = 1 to 5
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r.foreach( println )
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r foreach println
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// NB: Scala is quite lenien when it comes to dots and brackets - study the rules separately. This
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// helps write DSLs and APIs that read like English
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(5 to 1 by -1) foreach ( println )
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// A while loops
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var i = 0
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while (i < 10) { println("i " + i); i+=1 }
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while (i < 10) { println("i " + i); i+=1 } // Yes, again. What happened? Why?
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i // Show the value of i. Note that while is a loop in the classical sense - it executes
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// sequentially while changing the loop variable. while is very fast, faster that Java
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// loops, but using the combinators and comprehensions above is easier to understand
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// and parallelize
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// A do while loop
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do {
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println("x is still less then 10");
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x += 1
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} while (x < 10)
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// Tail recursion is an idiomatic way of doing recurring things in Scala. Recursive functions need an
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// explicit return type, the compiler can't infer it. Here it's Unit.
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def showNumbersInRange(a:Int, b:Int):Unit = { print(a); if (a < b) showNumbersInRange(a+1, b) }
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// Conditionals
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val x = 10
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if (x == 1) println("yeah")
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if (x == 10) println("yeah")
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if (x == 11) println("yeah")
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if (x == 11) println ("yeah") else println("nay")
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println(if (x == 10) "yeah" else "nope")
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val text = if (x == 10) "yeah" else "nope"
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var i = 0
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while (i < 10) { println("i " + i); i+=1 }
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// Object oriented features
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// Classname is Dog
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class Dog {
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//A method called bark, returning a String
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def bark: String = {
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// the body of the method
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"Woof, woof!"
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}
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}
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// Classes can contain nearly any other construct, including other classes, functions, methods, objects, case classes, traits etc.
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// Case classes
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case class Person(name:String, phoneNumber:String)
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Person("George", "1234") == Person("Kate", "1236")
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// Pattern matching
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val me = Person("George", "1234")
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me match { case Person(name, number) => "We matched someone : " + name + ", phone : " + number }
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me match { case Person(name, number) => "Match : " + name; case _ => "Hm..." }
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me match { case Person("George", number) => "Match"; case _ => "Hm..." }
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me match { case Person("Kate", number) => "Match"; case _ => "Hm..." }
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me match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
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val kate = Person("Kate", "1234")
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kate match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
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// Regular expressions
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val email = "(.*)@(.*)".r // The suffix .r invokes method r on String, which makes it a Regex
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val email(user, domain) = "henry@zkpr.com"
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"mrbean@pyahoo.com" match {
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case email(name, domain) => "I know your name, " + name
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}
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// Strings
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"Scala strings are surrounded by double quotes" //
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'a' // A Scala Char
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'Single quote strings don't exist' // Error
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"Strings have the usual Java methods defined on them".length
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"They also have some extra Scala methods.".reverse // See scala.collection.immutable.StringOps
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println("ABCDEF".length)
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println("ABCDEF".substring(2, 6))
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println("ABCDEF".replace("C", "3"))
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val n = 45
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println(s"We have $n apples")
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val a = Array(11, 9, 6)
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println(s"My second daughter is ${a(2-1)} years old")
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// Some characters need to be 'escaped', e.g. a double quote inside a string:
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val a = "They stood outside the \"Rose and Crown\""
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// Triple double-quotes allow for strings to span multiple rows and contain funny characters
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val html = """<form id="daform">
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<p>Press belo', Joe</p>
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</form>"""
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// Application structure and organization
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// Importing things
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import scala.collection.immutable.List
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// Import all "sub packages"
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import scala.collection.immutable._
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// Import multiple classes in one statement
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import scala.collection.immutable.{List, Map}
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// Rename an import using '=>'
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import scala.collection.immutable{ List => ImmutableList }
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// Import all classes, except some. The following excludes Map and Set:
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import scala.collection.immutable.{Map => _, Set => _, _}
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// Your programs entry point is defined in an scala file using an object, with a single method, main:
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object Application {
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def main(args: Array[String]): Unit = {
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// stuff goes here.
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}
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}
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// Files can contain multiple classes and objects. Compile with scalac
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// Input and output
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// To read a file line by line
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import scala.io.Source
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for(line <- Source.fromPath("myfile.txt").getLines())
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println(line)
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// To write a file use Java's PrintWriter
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```
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## Further resources
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[Scala for the impatient](http://horstmann.com/scala/)
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[Twitter Scala school(http://twitter.github.io/scala_school/)
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[The scala documentation]
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Join the [Scala user group](https://groups.google.com/forum/#!forum/scala-user)
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