CODEWITHSUNDEEP
scalafunctional-programmingpattern-matching
SkyWalker
SkyWalker

Reputation: 14309

Scala: How to cast this if-then-else ladder as pattern matching

I have the following code which works using a if-then-else ladder:

val test: Array[Array[Int]] = ...
val dim = 3 
if (dim < test.length - 1) {
    1   
} else {
    0
}

casting it as pattern matching I attempt:

val dim = 3 
val maxDim = test.length - 1
dim match {
   case _ < maxDim => {
       1        
   }
   case _ => {
       0        
   }
}

but this produces multiple errors ... how can I do it as a pattern matching?

UPDATE: another attempt would be

dim match {
    case maxDim => 0
    case _ => 1
}

but doesn't work either

Upvotes: 0

Views: 201

Answers (2)

Dima
Dima

Reputation: 40500

test.lift(dim).fold(0){ _ => 1}

This is not pattern matching, but the whole point is that you don't need pattern matching in this case.

Explanation: your if-then thingy is basically testing whether the index is valid for an array, and returns 1 if it is, and 0 if it isn't (I am assuming, the lack of test for negative values was an oversight, and not an intentional feature).

Collections in scala are PartialFunctions from Int to the element type, that are defined on the domain of indexes valid for the collection and return the value of the element at that index.

PartialFunction.lift is utility that lets you test whether a partial function is defined and apply it if it is in one go. If it is defined at the argument value, it will return the Option of the result, otherwise, you will get None.

So, test.lift(dim) above returns None if dim was invalid, and Some array otherwise. We then .fold the option: .fold returns first parameter is the option is empty, and executes the function if it is not.

Putting it all together: we get an Option indicating whether the index was valid, then .fold it to return 0 if it was empty or 1 if not.

Upvotes: 4

marstran
marstran

Reputation: 27971

You need an if-clause in your match-case if you need to test a boolean expression. Try this:

dim match {
  case i if i < maxDim => 1
  case _ => 0
}

Upvotes: 2

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