How to add to a list? Scala

Question

I'm completely new to scala and don't understand why this list isn't coming out right. When I run the program I just get List() as output when I should be getting a list of all the elements of the parameter squared. This should be very simple but the :+ operation is what I've found online and it isn't working.

def Squareall (l: List[Int]): List[Int] ={
  var r : List[Int]=List()
  for (i<-0 to l.length-1){
    r:+ l(i)*l(i)
  }
  return r
}

Answer 1

The imperative coding style you have in your example is usually avoided in Scala. The for in Scala is an expression, meaning it results in a value, and thus you can transform your input list directly. To use a for to transform your input List you could do something like this:

def squareAll (l: List[Int]): List[Int] = {
  for (i <- l) yield i * i
}

If you don't supply the yield statement a for results in the Unit type, which is like void in Java. This flavor of for loop is generally for producing side effects , like printing to the screen or writing to a file. If you really just want to transform the data, then there is no need to create and manipulate the resulting List . (Also method names in Scala are generally "camel cased".)

Most people would probably use the List.map method instead, though:

def squareAll (l: List[Int]) = l.map((x: Int) => x * x)

or even...

def squareAll (l: List[Int]) = l.map(Math.pow(_, 2.0))

Answer 2

You have to assign the newly created list to r like this:

r = r:+ l(i)*l(i)

This is because by default List in Scala is immutable and :+ returns a new list, doesn't update the old one.

Of course there's also a mutable variation of a list scala.collection.mutable.MutableList . You can use the .++= method on it to grow the collection.

val mutableList = scala.collection.mutable.MutableList(1, 2)
mutableList.++=(List(3, 4))