Scala for loop over two lists simultaneously

Question

I have a List[Message] and a List[Author] which have the same number of items, and should be ordered so that at each index, the Message is from the Author.

I also have class that we'll call here SmartMessage, with a constructor taking 2 arguments: a Message and the corresponding Author.

What I want to do, is to create a List[SmartMessage], combining the data of the 2 simple lists.

Extra question: does List preserve insertion order in Scala? Just to make sure I create List[Message] and a List[Author] with same ordering.

Answer 1

You could use zip:

val ms: List[Message] = ???
val as: List[Author] = ???

var sms = for ( (m, a) <- (ms zip as)) yield new SmartMessage(m, a)

If you don't like for-comprehensions you could use map:

var sms = (ms zip as).map{ case (m, a) => new SmartMessage(m, a)}

Method zip creates collection of pairs. In this case List[(Message, Author)].

You could also use zipped method on Tuple2 (and on Tuple3):

var sms = (ms, as).zipped.map{ (m, a) => new SmartMessage(m, a)}

As you can see you don't need pattern matching in map in this case.

Extra

List is Seq and Seq preserves order. See scala collections overview.

There are 3 main branches of collections: Seq, Set and Map.

• Seq preserves order of elements.
• Set contains no duplicate elements.
• Map contains mappings from keys to values.

List in scala is linked list, so you should prepend elements to it, not append. See Performance Characteristics of scala collections.