| language | filename | contributors | ||||||
|---|---|---|---|---|---|---|---|---|
Scala |
learn.scala |
|
Scala - the scalable language
/*
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. Immutablility 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 = <function1>
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 email(user, domain) = "[email protected]"
"[email protected]" match {
case email(name, domain) => "I know your name, " + name
}
// 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"))
val n = 45
println(s"We have $n apples")
val a = Array(11, 9, 6)
println(s"My second daughter is ${a(2-1)} years old")
// Some characters need to be 'escaped', e.g. a double quote inside a string:
val a = "They stood outside the \"Rose and Crown\""
// Triple double-quotes let strings span multiple rows and contain quotes
val html = """<form id="daform">
<p>Press belo', Joe</p>
| <input type="submit">
</form>"""
// 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
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