How to perform an operation in scala's foreach?

Question

I am trying to understand how to perform an operation when using foreach. For e.g. how can I print element+1 of alist using foreach

Answer 1

For e.g. how can I add +1 to each element of a list using foreach

Edit:

To answer your updated question, you can definitely use foreach:

alist.foreach(x => println(x + 1))

You can't use placeholder syntax inside println since the compler infers it as:

alist.foreach(x => println(x => x + 1))

According to it's expansion laws. println takes a parameter of type Any, so it can't bind it to a concrete type with a plus method.

If you're interested in the rules of placeholder syntax, see Hidden features of Scala

---

You cannot mutate a List[+A] using foreach in Scala, since lists are immutable and foreach return type is Unit. What you can do is project a new list from an existing one using the map transformation:

scala> val list = List(1,2,3,4)
list: List[Int] = List(1, 2, 3, 4)

scala> val plusOneList = list.map(_ + 1)
plusOneList: List[Int] = List(2, 3, 4, 5)

If you look at the signature for foreach, you see that it takes a function: A => Unit, which takes an element of type A and projects back a Unit. This signature is a sign for a side effecting method and since list is immutable, that doesn't help us.

If you used a mutable list, such as ListBuffer, then you could use side effects for population:

scala> val listBuffer = mutable.ListBuffer[Int]()
listBuffer: scala.collection.mutable.ListBuffer[Int] = ListBuffer()

scala> (0 to 4).foreach(x => listBuffer += x)

scala> listBuffer
res10: scala.collection.mutable.ListBuffer[Int] = ListBuffer(0, 1, 2, 3, 4)