--- language: Scala filename: learnscala.scala contributors: - ["George Petrov", "http://github.com/petrovg"] - ["Dominic Bou-Samra", "http://dbousamra.github.com"] filename: learn.scala --- Scala - the scalable language ```cpp /* Set yourself up: 1) Download Scala - http://www.scala-lang.org/downloads 2) unzip/untar in your favourite location and put the bin subdir on the path 3) Start a scala REPL by typing scala. You should see the prompt: scala> This is the so called REPL. You can run commands in the REPL. Let's do just that: */ println(10) // prints the integer 10 println("Boo!") // printlns the string Boo! // Some basics // Printing, and forcing a new line on the next print println("Hello world!") // Printing, without forcing a new line on next print print("Hello world") // Declaring values is done using either var or val // val declarations are immutable, whereas var's are mutable. Immutability is // a good thing. val x = 10 // x is now 10 x = 20 // error: reassignment to val var x = 10 x = 20 // x is now 20 // Single line comments start with two forward slashes /* Multi line comments look like this. */ // Boolean values true false // Boolean operations !true // false !false // true true == false // false 10 > 5 // true // Math is as per usual 1 + 1 // 2 2 - 1 // 1 5 * 3 // 15 6 / 2 // 3 // Evaluating a command in the REPL gives you the type and value of the result 1 + 7 /* The above line results in: scala> 1 + 7 res29: Int = 8 This means the result of evaluating 1 + 7 is an object of type Int with a value of 8 1+7 will give you the same result */ // Everything is an object, including a function. Type these in the REPL: 7 // results in res30: Int = 7 (res30 is just a generated var name for the result) // The next line gives you a function that takes an Int and returns it squared (x:Int) => x * x // You can assign this function to an identifier, like this: val sq = (x:Int) => x * x /* The above says this sq: Int => Int = Which means that this time we gave an explicit name to the value - sq is a function that take an Int and returns Int. sq can be executed as follows: */ sq(10) // Gives you this: res33: Int = 100. // The colon explicitly defines the type of a value, in this case a function // taking an Int and returning an Int. val add10: Int => Int = _ + 10 // Scala allows methods and functions to return, or take as parameters, other // functions or methods. List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element // Anonymous functions can be used instead of named functions: List(1, 2, 3) map (x => x + 10) // And the underscore symbol, can be used if there is just one argument to the // anonymous function. It gets bound as the variable List(1, 2, 3) map (_ + 10) // If the anonymous block AND the function you are applying both take one // argument, you can even omit the underscore List("Dom", "Bob", "Natalia") foreach println // Data structures val a = Array(1, 2, 3, 5, 8, 13) a(0) a(3) a(21) // Throws an exception val m = Map("fork" -> "tenedor", "spoon" -> "cuchara", "knife" -> "cuchillo") m("fork") m("spoon") m("bottle") // Throws an exception val safeM = m.withDefaultValue("no lo se") safeM("bottle") val s = Set(1, 3, 7) s(0) s(1) /* Look up the documentation of map here - * http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map * and make sure you can read it */ // Tuples (1, 2) (4, 3, 2) (1, 2, "three") (a, 2, "three") // Why have this? val divideInts = (x:Int, y:Int) => (x / y, x % y) divideInts(10,3) // The function divideInts gives you the result and the remainder // To access the elements of a tuple, use _._n where n is the 1-based index of // the element val d = divideInts(10,3) d._1 d._2 // Combinators s.map(sq) val sSquared = s. map(sq) sSquared.filter(_ < 10) sSquared.reduce (_+_) // The filter function takes a predicate (a function from A -> Boolean) and // selects all elements which satisfy the predicate List(1, 2, 3) filter (_ > 2) // List(3) List( Person(name = "Dom", age = 23), Person(name = "Bob", age = 30) ).filter(_.age > 25) // List(Person("Bob", 30)) // Scala a foreach method defined on certain collections that takes a type // returning Unit (a void method) aListOfNumbers foreach (x => println(x)) aListOfNumbers foreach println // For comprehensions for { n <- s } yield sq(n) val nSquared2 = for { n <- s } yield sq(n) for { n <- nSquared2 if n < 10 } yield n for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared /* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas a for-comprehension defines a relationship between two sets of data. */ // Loops and iteration 1 to 5 val r = 1 to 5 r.foreach( println ) r foreach println // NB: Scala is quite lenient when it comes to dots and brackets - study the // rules separately. This helps write DSLs and APIs that read like English (5 to 1 by -1) foreach ( println ) // A while loops var i = 0 while (i < 10) { println("i " + i); i+=1 } while (i < 10) { println("i " + i); i+=1 } // Yes, again. What happened? Why? i // Show the value of i. Note that while is a loop in the classical sense - // it executes sequentially while changing the loop variable. while is very // fast, faster that Java // loops, but using the combinators and // comprehensions above is easier to understand and parallelize // A do while loop do { println("x is still less then 10"); x += 1 } while (x < 10) // Tail recursion is an idiomatic way of doing recurring things in Scala. // Recursive functions need an explicit return type, the compiler can't infer it. // Here it's Unit. def showNumbersInRange(a:Int, b:Int):Unit = { print(a) if (a < b) showNumbersInRange(a + 1, b) } // Conditionals val x = 10 if (x == 1) println("yeah") if (x == 10) println("yeah") if (x == 11) println("yeah") if (x == 11) println ("yeah") else println("nay") println(if (x == 10) "yeah" else "nope") val text = if (x == 10) "yeah" else "nope" var i = 0 while (i < 10) { println("i " + i); i+=1 } // Object oriented features // Classname is Dog class Dog { //A method called bark, returning a String def bark: String = { // the body of the method "Woof, woof!" } } // Classes can contain nearly any other construct, including other classes, // functions, methods, objects, case classes, traits etc. // Case classes case class Person(name:String, phoneNumber:String) Person("George", "1234") == Person("Kate", "1236") // Pattern matching val me = Person("George", "1234") me match { case Person(name, number) => { "We matched someone : " + name + ", phone : " + number }} me match { case Person(name, number) => "Match : " + name; case _ => "Hm..." } me match { case Person("George", number) => "Match"; case _ => "Hm..." } me match { case Person("Kate", number) => "Match"; case _ => "Hm..." } me match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" } val kate = Person("Kate", "1234") kate match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" } // Regular expressions val email = "(.*)@(.*)".r // Invoking r on String makes it a Regex val serialKey = """(\d{5})-(\d{5})-(\d{5})-(\d{5})""".r // Using verbatim (multiline) syntax val matcher = (value: String) => { println(value match { case email(name, domain) => s"It was an email: $name" case serialKey(p1, p2, p3, p4) => s"Serial key: $p1, $p2, $p3, $p4" case _ => s"No match on '$value'" // default if no match found }) } matcher("mrbean@pyahoo.com") // => "It was an email: mrbean" matcher("nope..") // => "No match on 'nope..'" matcher("52917") // => "No match on '52917'" matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917" // Strings "Scala strings are surrounded by double quotes" // 'a' // A Scala Char 'Single quote strings don't exist' // Error "Strings have the usual Java methods defined on them".length "They also have some extra Scala methods.".reverse // Seealso: scala.collection.immutable.StringOps println("ABCDEF".length) println("ABCDEF".substring(2, 6)) println("ABCDEF".replace("C", "3")) // String interpolation val n = 45 println(s"We have $n apples") // => "We have 45 apples" // Expressions inside interpolated strings are also possible val a = Array(11, 9, 6) println(s"My second daughter is ${a(0) - a(2)} years old.") // => "My second daughter is 5 years old." println(s"We have double the amount of ${n / 2.0} in apples.") // => "We have double the amount of 22.5 in apples." println(s"Power of 2: ${math.pow(2, 2)}") // => "Power of 2: 4" // Formatting with interpolated strings (note the prefixed f) println(f"Power of 5: ${math.pow(5, 2)}%1.0f") // "Power of 5: 25" println(f"Square root of 122: ${math.sqrt(122)}%1.4f") // "Square root of 122" // Ignoring special characters. println(raw"New line feed: \n. Carriage return: \r.") // => "New line feed: \n. Carriage return: \r." // Some characters need to be 'escaped', e.g. a double quote inside a string: val a = "They stood outside the \"Rose and Crown\"" // => "They stood outside the "Rose and Crown"" // Triple double-quotes let strings span multiple rows and contain quotes val html = """

Press belo', Joe

|
""" // Application structure and organization // Importing things import scala.collection.immutable.List // Import all "sub packages" import scala.collection.immutable._ // Import multiple classes in one statement import scala.collection.immutable.{List, Map} // Rename an import using '=>' import scala.collection.immutable.{ List => ImmutableList } // Import all classes, except some. The following excludes Map and Set: import scala.collection.immutable.{Map => _, Set => _, _} // Your programs entry point is defined in an scala file using an object, with a // single method, main: object Application { def main(args: Array[String]): Unit = { // stuff goes here. } } // Files can contain multiple classes and objects. Compile with scalac // Input and output // To read a file line by line import scala.io.Source for(line <- Source.fromFile("myfile.txt").getLines()) println(line) // To write a file use Java's PrintWriter val writer = new PrintWriter("myfile.txt") writer.write("Writing line for line" + util.Properties.lineSeparator) writer.write("Another line here" + util.Properties.lineSeparator) writer.close() ``` ## Further resources [Scala for the impatient](http://horstmann.com/scala/) [Twitter Scala school](http://twitter.github.io/scala_school/) [The scala documentation](http://docs.scala-lang.org/) [Try Scala in your browser](http://scalatutorials.com/tour/) Join the [Scala user group](https://groups.google.com/forum/#!forum/scala-user)