Scala: declaring val within for loop, if condition

Question

I'm a scala beginner and trying to understand how val works in Scala. I read that vals cannot be modified. When I do the following:

for( line <- Source.fromFile(args(0)).getLines() ) {
   val currentLine = line
   println(currentLine)
}

currentLine is updated in each iteration, while I expect it to be initialized with the first line and hold it till the end, or at least give a re-initialization error of some sort. Why is this so? Is the val created and destroyed in each iteration? My second question: I would like to use x outside if in the following code.

if( some condition is satisfied) val x = 2 else val x = 3

As of now, I'm getting an "Illegal start of simple expression" error. Is there a way to use x outside if?

Answer 1

For Loop

You can break it down into a map operation.

for( line <- Source.fromFile(args(0)).getLines() ) {
   val currentLine = line
   println(currentLine)
}

So this code transforms to

Source.fromFile(args(0)).getLines().map( line => block )

block could be any expression. Where in your case block is:

{
   val currentLine = line
   println(currentLine)
}

Here currentLine is local to block and is created for each of the values of line given to map operation.

If-Else

Again following is also wrong:

if( some condition is satisfied) val x = 2 else val x = 3

Essentially if-else in Scala returns a value. So it should be:

if( condition ) expression1 else expression1

In your case you it can be:

if( condition ) { val x = 2 } else { val x = 3 }

However an assignment returns Unit ( or void if you want an analogy with Java / C++ ). So You can simply take the value of if-else like so:

val x = if( condition ) { 2 } else { 3 }
// OR
val x = if( condition ) 2 else 3

Answer 2

1. Yes, the val is created and destroyed on each iteration.

2. val x = if(condition) 2 else 3 would do what you want.

Edit: You could rewrite 2. to if(conditon) {val x = 2} else {val x = 3} (to make it compile) but that would do nothing, since the if does not return anything and the variable can not be used outside the if