blue-sky - 6 months ago 51

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`

`map`

Answer

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