How roughen (as opposed to flatten) a list in a functional style?

Question

For instance, I have a list (1 2 3 4 5 6 7 8 9 10 11), and want to roughen it by 3 elements (or another length) to get ((1 2 3) (4 5 6) (7 8 9) (10 11)). What pretty code could I use for this? Thanks.

Answer 1

def split[A](list : List[A], n : Int) : List[List[A]] = list match {
    case List() => List()
    case     _  => (list take n) :: split(list drop n, n)
}

Answer 2

List(1,2,3,4,5,6,7,8,9,10,11) grouped 3 toList

res0: List[List[Int]] = List(List(1, 2, 3), List(4, 5, 6), 
List(7, 8, 9), List(10, 11))

Answer 3

In Scala 2.8, List mixes in IterableLike which has the grouped method which returns an Iterator[List[T]], which in turn can be converted to List[List[T]].

List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11).grouped(3).toList

res3: List[List[Int]] = List(List(1, 2, 3), List(4, 5, 6), List(7, 8, 9), List(10, 11))

If you want a roughen method on List you can use an implicit conversion, something like:

scala> class RList[T](val l: List[T]) {def roughen(n: Int) = l.grouped(n).toList}
defined class RList

scala> implicit def list2rlist[T](l: List[T]) = new RList(l)
list2rlist: [T](l: List[T])RList[T]    

scala> List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) roughen 3
res5: List[List[Int]] = List(List(1, 2, 3), List(4, 5, 6), List(7, 8, 9), List(10, 11))

Answer 4

This is the best I could come up with:

def roughen(l:List[_],s:Int):List[_] ={

  if (l.isEmpty) return Nil
  val l2 = l.splitAt(s)
  l2._1 :: roughen(l2._2,s)

}

val l = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
roughen(l,3)
//returns: List(List(1, 2, 3), List(4, 5, 6), List(7, 8, 9), List(10))

Answer 5

Since you use the Clojure tag too...

There's a built-in function to do that in Clojure 1.2, also available in 1.1 in clojure.contrib.seq-utils.

(partition-all 3 [1 2 3 4 5 6 7 8 9 10 11])
; => ((1 2 3) (4 5 6) (7 8 9) (10 11))

See also partition and partition-by. Also note that partition and partition-all accept some optional arguments if you need something slightly different, see e.g. (doc partition) at the REPL.

Answer 6

Here's a Clojure 1.0-compatible implementation of roughen:

(defn roughen
  "Roughen sequence s by sub-grouping every n elements.
   e.gn (roughen '(a b c d) 2) -> ((a b) (c d))"
  [s n]
  (loop [result () s s]
    (cond (empty? s)
      result
      (< (count s) n)
      (concat result (list s))
      :default
      (recur (concat result (list (take n s))) (drop n s)))))

user=> (roughen '(a b c d e f g) 2)
((a b) (c d) (e f) (g))
user=> (roughen '(a b c d e f) 2)
((a b) (c d) (e f))
user=> (roughen '(a b c d e f) 4)
((a b c d) (e f))
user=> 

Answer 7

And another clojure version, written in more idiomatic clojure.

(defn roughen
  [n coll]
  (lazy-seq
    (when-let [s (seq coll)]
      (let [[l r] (split-at n s)]
        (cons l (roughen n r))))))

Note, that split-at traverses the input sequence twice. So you can replace the standard version with the following:

(defn split-at
  [n coll]
  (loop [n n, s coll, l []]
    (if-not (zero? n)
      (if-let [s (seq s)]
        (recur (dec n) (rest s) (conj l (first s)))
        [l nil])
      [l s])))

(Of course one would use partition and friends as already mentioned above.)