Remove mutable sum var - scala

Question

Here is an entropy calculation based on an answer by Jeff Atwood : How to calculate the entropy of a file? which is based on http://en.wikipedia.org/wiki/Entropy_(information_theory) :
object MeasureEntropy extends App {
val s = "measure measure here measure measure measure"
def entropyValue(s: String) = {
val m = s.split(" ").toList.groupBy((word: String) => word).mapValues(_.length.toDouble)
var result: Double = 0.0;
val len = s.split(" ").length;
m map {
case (key, value: Double) =>
{
var frequency: Double = value / len;
result -= frequency * (scala.math.log(frequency) / scala.math.log(2));
}
}
result;
}
println(entropyValue(s))
}
I'd like to improve this by removing the mutable state relating to :
var result: Double = 0.0;
How to combine the result into a single calculation over the map function ?

Answer 1

Using foldLeft, or in this case /: which is a syntactic sugar for it:
(0d /: m) {case (result, (key,value)) =>
val frequency = value / len
result - frequency * (scala.math.log(frequency) / scala.math.log(2))
}
Docs: http://www.scala-lang.org/files/archive/api/current/index.html#scala.collection.immutable.Map#/:B(op:(B,A)=>B):B

Answer 2

A simple sum will do the trick:
m.map {
case (key, value: Double) =>
val frequency: Double = value / len;
- frequency * (scala.math.log(frequency) / scala.math.log(2));
}.sum

Answer 3

It can be written using foldLeft like below.
def entropyValue(s: String) = {
val m = s.split(" ").toList.groupBy((word: String) => word).mapValues(_.length.toDouble)
val len = s.split(" ").length
m.foldLeft(0.0)((r, t) => r - ((t._2 / len) * (scala.math.log(t._2 / len) / scala.math.log(2))))
}