Scala 101 - Val, var, Arrays, Loops and Conditions

Today I’ve started a 3 days course (sponsored by my work place ) in Scala. The prerequisites  are just”Experienced in Java” .

you can find a lot of material on Scala across the web , and you don’t need me for that - So I’ll just give you the highlights and some links

• Compiles to JVM bytecode (like groovy) and integrates with Java seamlessly (But also to compiles to CLR) - So you can start using it right now on your Java bio-sphere (You can call Java code from Scala and vice versa )
• Performance are ~= Java
• Combines both OOP aspects and functional programming - You can use it to do either one , but the real strength is achieved when you combines both approaches
• Statically Typed - but very very strong typing mechanism
• Supports DSLs
• Superb support for concurrency (Using Actors)

Scala is much more flexible then Java , much less verbose (thank god for that) and much better suited for concurrency.

Download Scala from here , unzip it , and run the interpreter (scala.bat)

Lets start coding:

println "Hello, World"

Well , that was fairly easy.

O.K , so we got over the”Hello world” part. Let’s see something more interesting :

val msg = "Hello, World" 
println(msg)

val means”this is a value” . A value , unlike a variable , can not be changed.  You’d also notice that there’s no type to the msg value - Scala just inferred that it’s String.

lets try to change the value :

msg = "Goodbye!"
error: reassignment to val      msg = "Goodbye!"

OK , so vals can not be changed. But we do need variables , mind you ..

var newMsg = "Hello, World"
newMsg = "GoodBye!"

println (newMsg)
>> Goodbye

O.K , so we have variables. What about functions?

def hello ( someone : String ) {
println("Hello, " + someone)
}

hello("World!")
>> Hello, World!

• Declaring a function if done by”def” , like in groovy
• Parameters are declared as paramName : ParamType
• You don’t have to declare a return type.

this would have worked too:

def hello ( someone : String ) : Unit = {
println("Hello, " + someone)
}

hello("World!")
>> Hello, World!

*“Unit” is a fundamental type - slightly like Void in Java. So this function returns nothing.

this would have also worked :

def hello ( someone : String ) {
"Hello, " + someone
}

println hello("World!")
>> Hello, World!

• You don’t have to declare the function return type
• you don’t have to write”return” - the last computed value will be returned

this can also work :

def hello ( someone : String )  = { "Hello, " + someone }

and so will this :

def hello ( someone : String )  =  "Hello, " + someone

So , we get the fundamentals - what about Arrays and conditions?

val arr = Array(1,2)
def arrays(args: Array[Int]) = println (args(0)) 

arrays(arr)
>>1

• You can initialize an array without declaring the type
• you access the index of an array using () (not []!)
• Arrays start at zero

And ifs?

println ( if (1 > 2) " what?! " else "that makes sense" )
>> that makes sense

• If is a computed term (like the ?: operator in Java) - it returns a value

And a for loop?

val nums = Array(1,2)
for ( num <- nums ) println (num)

• The basic use of”for” is more like”foreach” in Java then the standard for

This also works :

nums.foreach( num => println(num))

**
Equality operators: **

== and != are defined for all objects. Unlike in Java  ,  == and != are Value comparisons! (not reference equality!). To check reference equality , use eq.

val l1 = List("A","B")
val l2 = List("A","B")

l1 == List("A", "B")
>> true

l1 == l2
>> true

l1.eq(l2 )
>> false

 

Other side notes

• You can define functions that are called”+” ,”-“ and so forth. This is NOT operator overloading - Scala-wise , these are just functions like any other function. Each function that gets only 1 parameter can be called like this :
  Console println”Something”  - which for Scala is the same as saying “ a + b” .
• Every object has a function called apply(Param) that can be called without the”apply” . So actually , when we used the array like this : arr(1) , we actually called the function arr.apply(1)