Assignment expression in for/yield cause of Scala?

Question

My codes looks like this:

  type Occurrences = List[(Char, Int)]
  def subtract(x: Occurrences, y: Occurrences): Occurrences = for {
    (theChar, theInt) <- x
    yMap = y.toMap
    finalInt = theInt - yMap.getOrElse(theChar,0)
    if finalInt != 0
  } yield (theChar, finalInt)

I was wondering whether yMap= y.toMap is evaluated only once or many times.. If it was evaluated many times, what would be the correct syntax to make it to be evaluated only once?

Answer 1

Short Answer

Just bring the ymap = y.toMap part out of the for comprehension.

def subtract(x: Occurrences, y: Occurrences): Occurrences = {
    val yMap = y.toMap
    for {
      (theChar, theInt) <- x
      finalInt = theInt - yMap.getOrElse(theChar, 0)
      if finalInt != 0
    } yield (theChar, finalInt)
  }

Detailed Answer

Scala For Comprehensions is just a syntactic sugar.

For instance your code will be translated to the following code by compiler (not exactly the following code but the concept is the same):

def subtract(x: Occurrences, y: Occurrences): Occurrences = x map {
    case (theChar, theInt) =>
      def yMap = y.toMap
      def finalInt = theInt - yMap.getOrElse(theChar, 0)
      (theChar, finalInt)
  } filter {
    case (_, theInt) =>
      theInt != 0
  }

So any expression inside the map part will be executed for every item of the collection (x in this case). By moving the y = y.toMap part out of the for block the code will be translated into :

  def subtract(x: Occurrences, y: Occurrences): Occurrences = {
    def yMap = y.toMap
    x map {
      case (theChar, theInt) =>
        def finalInt = theInt - yMap.getOrElse(theChar, 0)
        (theChar, finalInt)
    } filter {
      case (_, theInt) =>
        theInt != 0
    }
  }

which is most probably what you really want.