Preferred way to create a Scala list

Question

There are several ways to construct an immutable list in Scala  see contrived example code below   You can use a mutable ListBuffer  create a var list and modify it  use a tail recursive method  and probably others that I don t know about   Instinctively  I use the ListBuffer  but I don t have a good reason for doing so  Is there a preferred or idiomatic method for creating a list  or are there situations that are best for one method over another   import scala collection mutable ListBuffer     THESE are all the same as  0 to 3 toList  def listTestA          var list List Int    Nil      for i  lt - 0 to 3           list   list     List i      list     def listTestB          val list   new ListBuffer Int         for  i  lt - 0 to 3           list    i     list toList     def listTestC          def  add l List Int   i Int  List Int    i match           case 3   gt  l     List 3          case     gt   add l     List i   i  1             add Nil  0

User · Answer

Uhmm   these seem too complex to me  May I propose  def listTestD    0 to 3  toList   or  def listTestE   for  i  lt -  0 to 3  toList  yield i

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As a new scala developer i wrote small test to check list creation time with suggested methods above  It looks like   for   p  lt -   0 to x     yield p  toList  the fastest approach     import java util Date object Listbm      final val listSize   1048576   final val iterationCounts   5   def getCurrentTime  BigInt    new Date  getTime    def createList T    f   Int   gt  T    size   Int    T   f   size         returns function time execution   def experiment T    f   Int   gt  T     iterations  Int     size  Int     Int           val start time   getCurrentTime     for   p  lt - 0 to iterations    createList   f     size       return  getCurrentTime - start time  toInt         def printResult   f     gt  Int     Unit   println    execution time     f       def main  args   Array String            args 0  match          case  for    gt   printResult   experiment   x   gt   for   p  lt -   0 to x     yield p  toList      iterationCounts     listSize           case  range     gt   printResult   experiment   x   gt    0 to x   toList     iterationCounts     listSize           case        gt  printResult   experiment   x   gt    0 to x     List Int                 iterationCounts     listSize           case     gt  println    please use  for  range or    n

User · Answer

just an example that uses collection breakOut  scala gt  val a   List Int     for  x  lt - 1 to 10   yield x   3  collection breakOut  a  List Int    List 3  6  9  12  15  18  21  24  27  30   scala gt  val b   List Int     1 to 10  map     3  collection breakOut  b  List Int    List 3  6  9  12  15  18  21  24  27  30

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Note  This answer is written for an old version of Scala   The Scala collection classes are going to be redesigned as of Scala 2 8  so be prepared to change the way you create lists very soon   What is the forward compatible way of creating a List  I have no idea since I haven t read the 2 8 docs yet   A PDF document describing the proposed changes of the collection classes

User · Answer

ListBuffer is a mutable list which has constant-time append  and constant-time conversion into a List   List is immutable and has constant-time prepend and linear-time append   How you construct your list depends on the algorithm you ll use the list for and the order in which you get the elements to create it   For instance  if you get the elements in the opposite order of when they are going to be used  then you can just use a List and do prepends  Whether you ll do so with a tail-recursive function  foldLeft  or something else is not really relevant   If you get the elements in the same order you use them  then a ListBuffer is most likely a preferable choice  if performance is critical   But  if you are not on a critical path and the input is low enough  you can always reverse the list later  or just foldRight  or reverse the input  which is linear-time   What you DON T do is use a List and append to it  This will give you much worse performance than just prepending and reversing at the end

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And for simple cases    val list   List 1 2 3

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Using List tabulate  like this   List tabulate 3   x   gt  2 x   res  List 0  2  4   List tabulate 3       gt  Math random   res  List 0 935455779102479  0 6004888906328091  0 3425278797788426   List tabulate 3       gt   Math random 10  toInt   res  List 8  0  7

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You want to focus on immutability in Scala generally by eliminating any vars  Readability is still important for your fellow man so   Try   scala gt  val list   for i  lt - 1 to 10  yield i list  scala collection immutable IndexedSeq Int    Vector 1  2  3  4  5  6  7  8  9  10    You probably don t even need to convert to a list in most cases     The indexed seq will have everything you need   That is  you can now work on that IndexedSeq   scala gt  list foldLeft 0       res0  Int   55

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To create a list of string  use the following   val l   List  is    am    are    if

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I always prefer List and I use  fold reduce  before  for comprehension    However   for comprehension  is preferred if nested  folds  are required   Recursion is the last resort if I can not accomplish the task using  fold reduce for    so for your example  I will do     0 to 3     List Int               before I do    for  x  lt - 0 to 3  yield x  toList   Note  I use  foldRight      instead of  foldLeft      here because of the order of    s   For a version that does not throw StackOverflowException  use  foldLeft  instead

[scala] Preferred way to create a Scala list

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