learnxinyminutes-docs/scala.html.markdown at ade01b4ad8258c9e12f86e2bb9aa5426f60bc500


/*
  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 (Read-Eval-Print Loop). You may type any valid
  Scala expression into it, and the result will be printed. We will explain what
  Scala files look like further into this tutorial, but for now, let's start
  with some basics.
*/


/////////////////////////////////////////////////
// 1. Basics
/////////////////////////////////////////////////

// Single line comments start with two forward slashes

/*
  Multi line comments, as you can already see from above, look like this.
*/

// Printing, and forcing a new line on the next print
println("Hello world!")
println(10)

// 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 y = 10
y = 20  // y is now 20

/*
  Scala is a statically typed language, yet note that in the above declarations, we did not specify
  a type. This is due to a language feature called type inference. In most cases, Scala compiler can
  guess what the type of a variable is, so you don't have to type it every time. We can explicitly
  declare the type of a variable like so:
*/
val z: Int = 10
val a: Double = 1.0
val b: Double = 10  // Notice automatic conversion from Int to Double, result is 10.0, not 10

// 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
6 / 4 // 1
6.0 / 4 // 1.5


// Evaluating an expression 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

  Note that "res29" is a sequentially generated variable name to store the results of the
  expressions you typed, your output may differ.
*/

"Scala strings are surrounded by double quotes"
'a' // A Scala Char
// 'Single quote strings don't exist' <= This causes an error

// Strings have the usual Java methods defined on them
"hello world".length
"hello world".substring(2, 6)
"hello world".replace("C", "3")

// They also have some extra Scala methods. See also: scala.collection.immutable.StringOps
"hello world".take(5)
"hello world".drop(5)

// String interpolation: notice the prefix "s"
val n = 45
s"We have $n apples" // => "We have 45 apples"

// Expressions inside interpolated strings are also possible
val a = Array(11, 9, 6)
s"My second daughter is ${a(0) - a(2)} years old." // => "My second daughter is 5 years old."
s"We have double the amount of ${n / 2.0} in apples." // => "We have double the amount of 22.5 in apples."
s"Power of 2: ${math.pow(2, 2)}" // => "Power of 2: 4"

// Formatting with interpolated strings with the prefix "f"
f"Power of 5: ${math.pow(5, 2)}%1.0f" // "Power of 5: 25"
f"Square root of 122: ${math.sqrt(122)}%1.4f" // "Square root of 122: 11.0454"

// Raw strings, ignoring special characters.
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:
"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 = """<form id="daform">
                <p>Press belo', Joe</p>
                <input type="submit">
              </form>"""


/////////////////////////////////////////////////
// 2. Functions
/////////////////////////////////////////////////

// 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


/////////////////////////////////////////////////
// 3. Flow Control
/////////////////////////////////////////////////

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 than 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)
}
showNumbersInRange(1,14)


// 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"


/////////////////////////////////////////////////
// 4. 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


/////////////////////////////////////////////////
// 5. Object Oriented Programming
/////////////////////////////////////////////////

/*
  Aside: Everything we've done so far in this tutorial has been simple
  expressions (values, functions, etc). These expressions are fine to type into
  the command-line interpreter for quick tests, but they cannot exist by
  themselves in a Scala file. For example, you cannot have just "val x = 5" in
  a Scala file. Instead, the only top-level constructs allowed in Scala are:

  - objects
  - classes
  - case classes
  - traits

  And now we will explain what these are.
*/

class Dog(br: String) {
  var breed: String = br
  //A method called bark, returning a String
  def bark: String = {
    // the body of the method
    "Woof, woof!"
  }
}

val mydog = new Dog("greyhound")
println(mydog.breed) // => "greyhound"
println(mydog.bark) // => "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")


// Objects and traits coming soon!


/////////////////////////////////////////////////
// 6. 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"


/////////////////////////////////////////////////
// 7. Functional Programming
/////////////////////////////////////////////////

// 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


// 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)
case class Person(name:String, phoneNumber:String)
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)
val aListOfNumbers = List(1, 2, 3, 4, 10, 20, 100)
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. */


/////////////////////////////////////////////////
// 8. Implicits
/////////////////////////////////////////////////

Coming soon!


/////////////////////////////////////////////////
// 9. Misc
/////////////////////////////////////////////////

// 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

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