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I have a simple task:

Given a sequence of real numbers, replace all of its members larger than Z by Z and count the number of replacements.

I solved this task using Scala, but my code looks like imperative-style code. I want to make my code more functional. How can I rewrite my program so it becomes functional-style code?

def main(args : Array[String]) {
    println("Enter Z number: ")
    val z = readDouble();

    println("Enter the length of sequence: ")
    val lenght = readInt();

    var seq: List[Double] = List();

    // Filling sequence
    for (i <- 1 to lenght) {
      println("Enter sequence item № " + i)
      val item = readDouble();
      seq = item :: seq;
    }

    // number of replacements
    var substituteCount = 0;
    //replacing all numbers larger Z by Z
    seq = seq.map(x => {if (x > z){substituteCount+= 1; z} else x})

    println("Replacement count: " + substituteCount)
    println("New sequence: " + seq.reverse.mkString(" "))
}
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Let's split this up a bit; firstly, let's put the sequence filling code in its own function:

def populateSequence(length: Int): IndexedSeq[Double] = {
  for {
    i <- 1 to length
    _ = println("Enter sequence item ? " + i)
    item = readDouble()
  } yield item
}

This utilizes the a combination of for and yield - in this case, this returns an IndexedSeq[Double], but don't worry, you can treat this like a List (or simply convert it to one with a .toList if you really want to.

Our code structure then looks like this:

def main(args: Array[String]) {
  println("Enter Z number: ")
  val z = readDouble();

  println("Enter the length of sequence: ")
  val length = readInt();

  val seq = populateSequence(length) 

Ok, where to from here? Well, in this case, instead of modifying substituteCount as we go, I'd rather just calculate it up front:

val substituteCount = seq count (_ > z)

To get our modified List (or IndexedSequence), we can use another for expression. Again, one thing to remember is that both for and if are expressions in Scala, so we can do this:

val modified = for {
  v <- seq
  item = if (v > z) z else v
} yield item

We can then print everything as usual:

println("Replacement count: " + substituteCount)
println("New sequence: " + modified.mkString(" "))
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  • \$\begingroup\$ Thank you, very nice, especially this statement: val substituteCount = seq count (_ > z) :) But does val modified =... is better than using map method as in my code? I thought that the only one place in my code written in true functional style is usage of map method :) \$\endgroup\$ – MyTitle May 20 '13 at 14:05
  • 1
    \$\begingroup\$ map and for-expressions are actually exactly the same - under the covers, for gets converted into a map. But yeah, whichever way you prefer really - using map there is totally fine too. \$\endgroup\$ – Yuushi May 20 '13 at 14:47

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