如何基于其他集合的所有元素的笛卡尔积构建集合？

Question

Can You share any good solution for creating immutable collection in Scala based on full iteration of items in several arrays/another collections?

Eg in Java you can use:

List<String> signals = ...;
List<SignalState> states = ...;

List<SignalAndState> result = new ArrayList<~>(signals.size() * states.size());

for (String signal: signals) {
  for (SignalState state: states) {
    // some if() condition or process() function can be here 
    result.add(new SignalAndState(signal, state))
  }
}

What are the best practices of building smth like this using Scala? The same approach (using for() in for()) is bad idea, I think, and is not compatible with object-functional nature of Scala language at all.

Answer 1

I am not sure about the best practice, but one way you could accomplish this is to us a use a for comprehension to create the collection you are looking for:

val signals = List[String](...)
val states = List[SignalState](...)

for(signal <- signals; state <- states) yield new SignalAndState(signal, state)

That should yield a List[SignalAndState] with all the elements

Alternately, you could use a flatMap and map to accomplish the same result, like:

signals flatMap ( signal => states map ( state => new SignalAndState(signal, state)))