converting recursive function from python to scala

Question

I'm trying to convert this recursive function from python code to scala. In python:

    def func(x,y,z):
        if x >= y:
            return 0.0
        if y>= 12:
            return 1.0/6**z
        
        probability = 0.0
        
        for val in [1,2,3,4]:
            probability += func(x,y+val,z+1)        
        for val in [5,6]:
            probability += func(x+val,y,z+1)

        return probability

    print(func(1,7,0))

In scala, my the code became

object Prob extends App {

  println(func(1, 7, 0))

  def func(y: Int, x: Int, z: Int): Double = { 
    if (x>= y) {
      return 0.0;
    }   
    if (y>= 12) {
      return scala.math.pow(1.0/6,z);
    }
      
    var probability : Double = 0.0;
      
    for (i <- 1 to 4) {
        probability += func(x,y+i,z+1);
    }
    for (i <- 5 to 6) {
        probability += func(x+i,y,z+1);
    }
    return probability;
  }
}

Unfortunately, while the python code returns the correct value of 0.6, the scala code returns 0.0.

Where's the error in the scala code?

Clashsoft · Accepted Answer · 2021-01-23 11:55:50Z

4

You swapped the order of parameters:

def func(hunter,goose,num):

def func(goose: Int, hunter: Int, num: Int): Double = {

Since you call func(1, 7, 0), in scala you immediately hit the hunter >= goose case, which returns 0.

answered Jan 23, 2021 at 11:55

Clashsoft

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Comments

Luis Miguel Mejía Suárez · Accepted Answer · 2021-01-23 17:30:52Z

3

This is an equivalent and more idiomatic implementation, which takes advantages of the functional capabilities instead of just being an imperative copy & paste of the original code.

def func(x: Int, y: Int, z: Int): Double =
  if (x >= y)
    0.0d
  else if (y >= 12)
    math.pow(1.0d / 6, z)
  else
    (1 to 6).foldLeft(0.0d) {
      case (probability, i) =>
        val r =
          if (i <= 4) func(x, y + i, z + 1)
          else func(x + i, y, z + 1)
      
        probability + r
    }

Changes I made:

I removed return since it is not needed and not recommended, by taking advantage of the fact that if is an expression and not a statement.
I removed unnecessary mutability by taking advantage of higher-order functions provided by the standard library.
Style changes like removing ; (which is common on most, if not all, Scala programs) and removing some braces (which is more of my personal taste).

You can see the code running here.

edited Jan 23, 2021 at 17:30

answered Jan 23, 2021 at 13:46

Luis Miguel Mejía Suárez

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4 Comments

user Over a year ago

May I ask why you used case? Is it a stylistic thing? (because I thought that desugared to creating and destructuring a tuple)

Luis Miguel Mejía Suárez Over a year ago

@user yeah it shouldn't be needed because foldLeft expects a function of two arguments, I am just used to it.

user Over a year ago

Oh ok. A map and a sum might also work, perhaps with a view, although it doesn't match the OP's code that much.

Luis Miguel Mejía Suárez Over a year ago

@user yeah I also thought about that, but introducing views or iterators seemed that too much IMHO.

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