CODEWITHSUNDEEP
scalafunctional-programming
Jade Tang
Jade Tang

Reputation: 319

How to write a compare function in scala?

for example, I want to check whether a is greater than b, here is a my code 

def gt(a:Any,b:Any):Boolean = a match{
  case a:Ordered[Any] => (a compareTo b ) > 0
  case _ => false
}

however, there's more accurate way to implement this? Because the compiler give a warning:

<console>:8: warning: non-variable type argument Any in type pattern Comparable[Any] is unchecked since it is eliminated by erasure
         case a:Comparable[Any] => (a compareTo b ) > 0

update

the implicit view should do the work,but I what I wanna do is have a function with the parameter (a:Any,b:Any), not type parameter. In java, the code maybe like this, but I don't know how to implement it in Scala way

public static boolean gt(Object a, Object b){
    if(a instanceof Comparable && b.getClass() == a.getClass() ){
        return ((Comparable) a).compareTo(b) > 0;
    }else {
        return false;
    }
}

update again,I made a workable solution already, but i know it not the best way,and I will leave this question open

def ls(a: Any, b: Any): Boolean = {
  a match {
    case (a: Comparable[Any]) => (a compareTo b) < 0
    case _ => false
  }
}

Upvotes: 1

Views: 2321

Answers (1)

Ben Reich
Ben Reich

Reputation: 16324

Your method will return incorrectly for some very basic cases (e.g. gt(2,1)). You're probably looking for:

def gt[T <% Ordered[T]](a:T, b:T) = a > b

In your example, you avoid specifying a generic parameter (the parameter T). This guarantees that the arguments to the function are of the same type, thereby attaining some type safety.

You want that type parameter to be ordered. The way this is implemented in Scala is the Ordered trait, which takes a self type as a generic parameter.

The syntax <% means that there needs to be an implicit conversion from T to Ordered[T] (called an view bound type parameter). This is true for all the standard ordered types in Scala (e.g. Int, Char, etc.). Without this view bound, the compiler can't know that the generic parameter is necessary ordered.

Finally, Ordered has a > function on it, so you don't need to use compareTo, since it is already implemented.

You can read more about implicit views in the docs here. You can read more about Ordered in the docs here.

Upvotes: 4

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