Error: Not found: Value S (Scala)

Question

val Array(k,s) = readLine.split(" ").map(_.toInt)

This code works fine. But not this:

val Array(k,S) = readLine.split(" ").map(_.toInt)

Capitalizing "s" here gives me an error: error: not found: value S

What is going on?

Answer 1

When you are creating the k and s identifiers with val Array(k,s) = ..., you are using pattern matching to define them.

From the Scala Specifications (1.1 Identifiers):

The rules for pattern matching further distinguish between variable identifiers, which start with a lower case letter, and constant identifiers, which do not.

That is, when you say val Array(k,S) = ..., you're actually matching S against a constant. Since you have no S defined, Scala reports error: not found: value S.

---

Note that Scala will throw a MatchError if the constant is defined but it still cannot find a match :

scala> val S = 3
S: Int = 3

scala> val Array(k, S) = Array(1, 3)
k: Int = 1

scala> val Array(k, S) = Array(1, 4)
scala.MatchError: [I@813ab53 (of class [I)
  ... 33 elided

Answer 2

When using extractors, symbols that begin with a lower case character will be interpreted as a variable to hold an extracted value. On the other hand, symbols that begin with an upper case character are used to refer to variables/values declared in an outer scope.

Another example:

val X = 2

something match {
  case (X, y) => // matches if `something` is a pair whose first member is 2, and assigns the second member to `y`
  case (x, y) => // matches if `something` is a pair, and extracts both `x` and `y`   
}