01. Basics - RobertMakyla/scalaWiki GitHub Wiki
Console uses read-eval-print loop (REPL) Scala is 'the REPL' working on JVM
Ctrl + Enter : evaluates scala console
Value (constant)
val two = 1 + 1 // In scala you are ENCOURAGED to use VALUES
val four = two * 2
four = 0 // error : reassignment to val
val four = 0 // OK : it's a new value
Variable (can be reassigned)
var i = 0 // In scala you are discouraged to use VARIABLES
i = i + 1
Semicolons (;) are required only if you have many statement in one line There's no need to specify type of var/val. But when re-assigning var, the type must be the same (otherwise: i="no more 1" --> error: type mismatch)
It's an error to declare var/val without initializing it ( var/val myV --> error: '=' expected) If I want to, I can specify type:
val greeting: String = ???
val greeting: String = null
val greeting: Any = "Hello"
val myStrOne, myStrTwo: String = "Hello"
val myValOne, myValTwo: Any = "Hello"
There is no distinction between primitives and classes
1.toString --> res0: java.lang.String = 1
Scala wraps it automatically (programmer doesn't have to do it)
Types
- Byte - 8 bits
- Short - 16 bytes ( - 2^15 -> 2^15 - 1 )
- Char - 16 bytes ( 0 to 2^16 -1 )
- Int - 32 bytes ( - 2^31 to 2^31 - 1 )
- Long - 64 bytes
Converters
Scala converts all Strings or "anything" into StringOps objects :
"123".intersect("345") // "4"
Scala converts all Int/Double/Char into RichInt/RichDouble/RichChar objects.
E.g. RichInt class :
1.to(10) // Range (1,2,3,4,5,6,7,8,9,10)
1 to 10 // Range (1,2,3,4,5,6,7,8,9,10)
Digital/String converters - in Scala we use methods, not casts
99.44.toInt --> 99
99.toChar --> 'c'
"99.44".toDouble --> 99.44
"99.44".toInt --> java.lang.NumberFormatException
##Operators - in scala they are methods
1 + 2 - is a shorthand for method 1.+(2)
2 * 2 - is a shorthand for method 2.*(2)
(any symbol in method's name is accepted)
1 toString - it's 1.toString or 1.toString // "1"
1 toString() // "1"
1.toString // "1"
1.toString() // "1"
(The API dictates that StringOps.toString has parentheses, but since we don't provide any argument, we can skip it)
"hello" distinct // "helo"
"hello".distinct // "helo"
"hello" distinct() // error: The API says that StringOps.distinct
"hello".distinct() // has no parentheses
###RULE OF THUMB : Parameterless methods that don't modify the objects, have no parentheses
"123" intersect "345" // "123".intersect("345") (result: "3")
"" + 123 intersect "345" // (""+123).intersect("345") (result: "3")
counter += 1 - instead of counter++, which does not exist in scala
counter -= 1 - instead of counter--, which does not exist in scala
##Scala over Java
val x: BigInt = 1234567890
x * x * x - no need to x.multiply(x).multiply(x)
x pow 3 - no need to x.pow(3)
Functions
pow(2,2) // 4 - no need for: BigInteger.pow(2,2)
min(3, Pi) // 3
"0123"(2) // char '2'
All these functions are in scala.math package (import scala.math._ or import math._)
Without such import I can still call: scala.math.power(2,2) or just math.pow(2,2)
##Singleton Objects
Scala does not not have static methods, but it has Singleton objects - discussed later
##Methods
The apply method - common Scala idiom for constructing objects. No need to use "new" keyword in Scala
"012345"(4) - it's "012345".apply(4)
"012345" apply(4)
"012345" apply 4
- StringOps API: def apply(index: Int): Char
Return element at index n
- BigInt API: def apply(x: String): BigInt
Translates the decimal String into a BigInt.
Exercises:
res - results are values, not variables, because if I try to reassign other value, I get error
"hi"*3 --> "hihihi" because StringOps API: def *(n: Int): String
Return the current string concatenated n times.
scala.util.Random.nextInt(100) --> random 0-99
12345 : BigInt --> res: BigInt = 12345 (In Java: (BigInteger) 12345)
val myb : BigInt = 12345 --> myb: BigInt = 12345 (In Java: BigInteger myb = ...)
"012345"(0) - it's like "012345" head or "012345".head
"012345"("012345".length -1) - it's like "012345" last or "012345".last
"0123456789" head --> 0
"0123456789" tail --> 123456789
"0123456789" take 2 --> 01
"0123456789" drop 2 --> 23456789
"0123456789" takeRight 2 --> 89
"0123456789" dropRight 2 --> 01234567