learnxinyminutes-docs/scala.html.markdown

language	contributors	filename
Scala	George Petrov http://github.com/petrovg	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!


// Evaluating a command 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 result is the Int with a value 100



// 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 (_+_)


// 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. Research this further  */



// Loops and iteration

1 to 5
val r = 1 to 5
r.foreach( println )

r foreach println     
// NB: Scala is quite lenien 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 )

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

// Tail recursion is an idiomatic way of doing things in Scala. Recursive functions need an 
// explicit return type, the compile 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("nope")

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



// 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          // The suffix .r invokes method r on String, which makes it a Regex

val email(user, domain) = "henry@zkpr.com"

"mrbean@pyahoo.com" match {
  case email(name, domain) => "I know your name, " + name
}



// Strings

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 allow for strings to span multiple rows and contain funny characters
val html = """<form id="daform">
                <p>Press belo', Joe</p>
             |  <input type="submit">
              </form>"""


// Input and output

Further resources

Scala for the impatient

[Twitter Scala school(http://twitter.github.io/scala_school/)

[The scala documentation]

Join the Scala user group