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Basics of Scala

How to use scala ?

  • you should have jvm installed.
  • download scala binary form the scala official website
  • it comes with a repl
    • read execute print loop
  • some tools also help you set up scala for the system
  • sbt - scala built tool is one of them
    • it is a build system
    • help to manage dependencies for the scala project

Hello World

object HelloWorld extends App {
  println("Hello, World!")
}
  • singleton object, no need of class like in java
  • compile it with scalac helloworld.scala
  • run it with scala HelloWorld
  • scala -e for interactive mode, and scripting

var and val

  • var for variables whose values will be changed - mutable
  • val for variable whose value will not be changed - immutable

Objects

  • object have states and behavior
  • instance of a class

syntax:

object obj-name{

}

Methods

  • behavior
def functionName ([list of parameters]) : [return type] = {
  //function body
  return [expr]
}

def printHello(){

}

Field

val name = "Shivanshi"
var age = 11

Initialize Variables

var varname = values
var varname:Type = value
varname: Type = value

Conditionals

object demo{
  def isEven(s:Int):Boolean = {
    if(x%2==0) true
    else false
  }
}

Loops

  • while loop is supported
val i = 0;
while(i<10){
  println(i)
  i = i+1
}
  • for loop is supported but with some changes, and diff syntax
for(i <- 1 to 10){
  println(i)
}
  • nested loops in one line
for(i <- 1 to 10; j <- 1 to 100)
  println(i + ' ' + j)

same as

for(i <- 1 to 10){
  for(j <- 1 to 100)
    println(i + ' ' + j)
}
  • do while loop is not supported

Data types in Scala

all java data types are supported

  • Byte, Short
  • Int
  • Long
  • Float
  • Double
  • Char
  • Boolean
  • String

and some additional types

  • Unit - no value, equivalent of void in java

~

  • Null - null or empty reference
  • AnyRef - a supertype of any reference

~

  • Nothing - a subtype of every other types
  • Any - a supertype of any type

for object

Any -> ... -> Nothing

for references

AnyRef -> ... -> Null

Difference b/w null Null Nothing Unit Nil None

  • null - literal, a value
  • Null - a subtype of all reference types

~

  • Nothing
    • It doesn't have any methods or values
    • extends the Any type

~

  • Nil - empty list - List()

~

  • None - subtype of Option type, opposite of Some

~

  • Unit - void empty return type

Types of Functions

first order

  • don't take functions as arguments

higher order

  • take functions as arguments

nested functions

Define function inside another function.

def factorial(x:Int):Int = {
  // making a nested function
  def fact(i:Int, acc:Int):Int = {
    if(i<=1) acc
    else fact(i-1,i * acc)
  }
  fact(x, 1)
}

anonymous

  • Anonymous functions in source code are called function literals
  • and at run time, function literals are instantiated into objects called function values
  • Scala supports first-class functions
    • which means functions can be expressed in function literal syntax,
    • i.e., (x: Int) => x + 1
    • and that functions can be represented by objects
    • which are called function values

e.g.

  • with one parameter var inc = (x:Int) => x+1
  • with two parameter var mul = (x: Int, y: Int) => x*y
  • with zero parameter var userVal = () => { 345 }, println(userVal())

Closures

  • a function, whose return value depends on the value of one or more variables declared outside this function.
  • variable declares outside the function is called - free variable
  • variable in the definition is called bound variable

e.g.

val more = 10 // free variable
var y = (x:Int) => x + more // x-> bound variable
  • the function value (the object) that’s created at runtime from this function literal is called a closure

tail recursion

  • recursion at the end
  • use @tailrec annotation
@tailrec def factorial(x:Int, acc:Int):Int = {
  if(n<=1)
    acc
  else
    factorial(x-1, acc*i)
}

input in scala

var a = scala.io.StdIn.readInt()
var b = scala.io.StdIn.readDouble()
var c = scala.io.StdIn.readLine()

try-catch exceptions

try {
  doSomething()
}
catch {
  case ex: IOException => println("Oops!")
  case ex: NullPointerException => println("Oops!!")
}
finally{
  println("this will execute every time even if code terminates in middle")
  println("so close files here")
}

match

val first = "chips"
first match {
  case "salt" => println("pepper")
  case "chips" => println("salsa")
  case "eggs" => println("bacon")
}

def doChore(chore: String): String = chore match { 
    case "clean dishes" => "scrub, dry"
    case "cook dinner" => "chop, sizzle"
    case _ => "whine, complain"
}

Different Types Of For Loops

  • with single range
for(i <- 1 to 100){
  println(i)
}
  • with multiple range
for(i <- 1 to 10; b <- 1 to 10){
  println(i+j)
}
  • with collections
for(i <- List(2,3,5,6)){
  println(i)
}
  • with filters
for(
  i <- List(1,3,4,6,7)
  if i !=3; if i!=4
) {
    println(i)
}
  • with yield
var a =
  for (
    i <- List(1, 2, 3, 4)
    if i != 3; if i != 4
  ) yield i