Shows 2-Die Histogram

DiceHistogrammer.scala

import util.Random.nextInt

/**
Dice
Write a program that simulates throwing two six-sided dice 200 times, and creates a histogram of the results, like this:
 
 2: XXXX
 3: XXXXXXXXXX
 4: XXXXXXXXXXXXXXXXXXXXXX
 5: XXXXXXXXXXXXXXXXXXXXX
 6: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 7: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 8: XXXXXXXXXXXXXXXXXXXXXXXXXX
 9: XXXXXXXXXXXXXXXXXXXXX
10: XXXXXXXXXXXXXX
11: XXXXXXXXXXXX
12: XXXX
**/

object DiceHistogrammer extends App {
  println(DiceHist.roll(200))
}

trait DiceHist {
  def roll(): DiceHist
}

object DiceHist {
  private val IndOffset = 2
  private val NRows = 11
  
  private def apply(rolls: IndexedSeq[Int]) = new DiceHistImpl(rolls)  
  
  private def empty: DiceHist = DiceHist(IndexedSeq.fill(NRows)(0))

  def roll(times: Int): DiceHist =
    (1 to times).foldLeft(empty) { (hist, _) =>
      hist.roll()
  }

  private class DiceHistImpl(rolls: IndexedSeq[Int]) extends DiceHist {   
    def roll(): DiceHist = {
      val rollRes = Dice.roll() + Dice.roll()
      val i = rollRes - IndOffset
      DiceHist(rolls.updated(i, rolls(i) + 1))
    }

    override def toString =
      rolls.zipWithIndex.map { case (rollCardinality, i) =>
        val rollRes = i + IndOffset
        f"$rollRes%2d: " + ("X" * rollCardinality)
      }.mkString("\n")
  }
}
  
object Dice {
  def roll(): Int = nextInt(6) + 1
}

Well as you want me to put my comments then I will try to be very captiousness and strict :)

1. A dice might have not only 6 sides.. (http://www.awesomedice.com/image/cache/data/7-dice-set-premium-gemini-black-shell2-500x500.jpg)

2. Method def roll = .... If I didn't see the implementation then I would doubt what it returns... Consider the result type of ball.roll() or ship.turnAround() I would say that they just change the state of the object and return Unit rather than return some value...

3. Naming conventions... camelCase

  private val VecOffset = 2
  private val NStates = 11

4. Actually you do not need to have Vector as the algorithm for creating histograms does not require random access.

5. Too many string concatenation operations DiceHist(vec updated (i, vec(i) + 'X'))

6. Simulation & building histogram are tightly coupled. What if I just want to reuse your component to output histogram by my results that I got from somewhere else? Or just wanna get the results of your dice simulation and then put them into some Excel spreadsheet?...

7. Class/object/trait responsibility...

trait DiceHist {
  def roll: DiceHist
}

diceHistogram.roll().... I do not know what will happen if I roll some diagram :)