How to create a List of Lists in Scala in functional programming style

Question

I want to create a function in Scala that given a List[Int] returns a List[List[Int]]. For example getCombs(List(1,2)) should returns List(List(1), List(2), List(1,2)).

I'm studying functional programming so I would like to accomplish my task using that paradigm.

I've created the following function and it works, but I think that there exist a better way to do the job in a functional programming style.

def getCombs(coins: List[Int]): List[List[Int]] = {

  var l = List(coins)
  var i = 0
  for (i <- 1 to coins.length - 1) {
    var it = coins.combinations(i)
    while (it.hasNext) {
      val el = it.next
      val newL = el :: l
      l = newL

    }

  }
  return l
}

marstran · Accepted Answer · 2016-05-29 16:18:27Z

2

I first create a range of all the lengths of combinations that I want to make, then I use flatMap to create all the combinations and make a list out of them:

def allCombinations(list: List[Int]): List[List[Int]] = {
    (1 to list.length).flatMap(list.combinations(_)).toList
}

answered May 29, 2016 at 16:18

marstran

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Comments

Keith Pinson · Accepted Answer · 2016-05-29 17:41:09Z

1

(1 to coins.length).flatMap { 
  coins.combinations(_)  
}.toList

edited May 29, 2016 at 17:41

Keith Pinson

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answered May 29, 2016 at 16:15

Dima

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3 Comments

marstran Over a year ago

This returns a List[Iterator[List[Int]]].

Keith Pinson Over a year ago

I changed the map to a flatMap...seems better to correct the code than give @Dima 's answer a down vote. :-)

Dima Over a year ago

Thanks, @KeithPinson. Some people just have to be like that ... :)

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