Efficient iteration with index in Scala

Question

Since Scala does not have old Java style for loops with index       does not work val xs   Array  first    second    third   for  i 0  i lt xs length  i        println  String      i     is     xs i       How can we iterate efficiently  and without using var s   You could do this  val xs   Array  first    second    third   val indexed   xs zipWithIndex for  x  lt - indexed  println  String      x  2     is     x  1    but the list is traversed twice - not very efficient

User · Answer

Indeed  calling zipWithIndex on a collection will traverse it and also create a new collection for the pairs   To avoid this  you can just call zipWithIndex on the iterator for the collection   This will just return a new iterator that keeps track of the index while iterating  so without creating an extra collection or additional traversing   This is how scala collection Iterator zipWithIndex is currently implemented in 2 10 3     def zipWithIndex  Iterator  A  Int     new AbstractIterator  A  Int         var idx   0     def hasNext   self hasNext     def next           val ret    self next  idx        idx    1       ret             This should even be a bit more efficient than creating a view on the collection

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How about this   val a   Array  One    Two    Three   a foldLeft 0    i  x    gt   println i          x   i   1       Output   0  One 1  Two 2  Three

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Looping in scala is pretty simple  Create any array of your choice for ex   val myArray   new Array String  3  myArray 0   0   myArray 1   1   myArray 2   2     Types of loops   for data  lt - myArray println data   for  i  lt - 0 until myArray size  println i          myArray i

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Some more ways to iterate   scala gt   xs foreach  println   first second third   foreach  and similar  map  which would return something  the results of the function  which is  for println  Unit  so a List of Units   scala gt  val lens   for  x  lt - xs  yield  x length   lens  Array Int    Array 5  6  5    work with the elements  not the index  scala gt         xs           res21  java lang String   firstsecondthird   folding   for int i 0  j 0  i j lt 100  i  j 2  j  i 2              can be done with recursion    def ijIter  i  Int   0  j  Int   0  carry  Int   0    Int     if  i   j  gt   100  carry else      ijIter  i 2 j  j i 2  carry   3   2   i - 4   j   10     The carry-part is just some example  to do something with i and j  It needn t be an Int    for simpler stuff  closer to usual for-loops   scala gt   1 until 4  res43  scala collection immutable Range with scala collection immutable Range ByOne   Range 1  2  3   scala gt   0 to 8 by 2     res44  scala collection immutable Range   Range 0  2  4  6  8   scala gt   26 to 13 by -3  res45  scala collection immutable Range   Range 26  23  20  17  14    or without order   List  1  3  2  5  9  7  foreach  print

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There s nothing in the stdlib that will do it for you without creating tuple garbage  but it s not too hard to write your own   Unfortunately I ve never bothered to figure out how to do the proper CanBuildFrom implicit raindance to make such things generic in the type of collection they re applied to  but if it s possible  I m sure someone will enlighten us      def foreachWithIndex A  as  Traversable A   f   Int A    gt  Unit      var i   0   for  a  lt - as        f i  a      i    1        def mapWithIndex A B  in  List A   f   Int A    gt  B   List B        def mapWithIndex0 in  List A   gotSoFar  List B   i  Int   List B          in match         case Nil           gt  gotSoFar reverse       case one    more   gt  mapWithIndex0 more  f i  one     gotSoFar  i 1              mapWithIndex0 in  Nil  0        Tests       Test def testForeachWithIndex       var out   List Int      ScalaUtils foreachWithIndex List 1 2 3 4      i  num    gt      out     i   num       assertEquals List 0 2 6 12  out      Test def testMapWithIndex       val out   ScalaUtils mapWithIndex List 4 3 2 1      i  num    gt      i   num        assertEquals List 0 3 4 3  out

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One more way   scala gt  val xs   Array  first    second    third   xs  Array java lang String    Array first  second  third   scala gt  for  i  lt - xs indices           println i          xs i   0  first 1  second 2  third

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I have the following approaches  object HelloV2       def main args  Array String             Efficient iteration with index in Scala         Approach  1      var msg             for  i  lt - args indices                msg   args i                var msg1             Approach  2      for  i  lt - 0 until args length                 msg1     args i                   Approach  3      var msg3         args foreach         arg   gt          msg3     arg                 println  msg      msg          println  msg1      msg1          println  msg3      msg3

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The proposed solutions suffer from the fact that they either explicitly iterate over a collection or stuff the collection into a function  It is more natural to stick with the usual idioms of Scala and put the index inside the usual map- or foreach-methods  This can be done using memoizing  The resulting code might look like  myIterable map  doIndexed someFunction     Here is a way to achieve this purpose  Consider the following utility   object TraversableUtil       class IndexMemoizingFunction A  B  f   Int  A    gt  B  extends Function1 A  B            private var index   0         override def apply a  A   B                 val ret   f index  a              index    1             ret                      def doIndexed A  B  f   Int  A    gt  B   A   gt  B             new IndexMemoizingFunction f            This is already all you need  You can apply this for instance as follows   import TraversableUtil   List  a   b   c   map doIndexed  i  char    gt  char   i     which results in the list  List 97  99  101    This way  you can use the usual Traversable-functions at the expense of wrapping your effective function  Enjoy

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A simple and efficient way  inspired from the implementation of transform in SeqLike scala      var i   0     xs foreach   el   gt        println  String      i     is     xs i         i    1

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Much worse than traversing twice  it creates an intermediary array of pairs   You can use view  When you do collection view  you can think of subsequent calls as acting lazily  during the iteration  If you want to get back a proper fully realized collection  you call force at the end  Here that would be useless and costly  So change your code to  for  x i   lt - xs view zipWithIndex  println  String      i     is     x

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It has been mentioned that Scala does have syntax for for loops   for  i  lt - 0 until xs length        or simply  for  i  lt - xs indices        However  you also asked for efficiency  It turns out that the Scala for syntax is actually syntactic sugar for higher order methods such as map  foreach  etc  As such  in some cases these loops can be inefficient  e g  How to optimize for-comprehensions and loops in Scala    The good news is that the Scala team is working on improving this  Here s the issue in the bug tracker  https   issues scala-lang org browse SI-4633    For utmost efficiency  one can use a while loop or  if you insist on removing uses of var  tail recursion   import scala annotation tailrec   tailrec def printArray i  Int  xs  Array String       if  i  lt  xs length        println  String      i     is     xs i       printArray i 1  xs        printArray 0  Array  first    second    third      Note that the optional  tailrec annotation is useful for ensuring that the method is actually tail recursive  The Scala compiler translates tail-recursive calls into the byte code equivalent of while loops

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Actually  scala has old Java-style loops with index   scala gt  val xs   Array  first   second   third   xs  Array java lang String    Array first  second  third   scala gt  for  i  lt - 0 until xs length         println  String       i     is    xs i    String   0 is first String   1 is second String   2 is third   Where 0 until xs length or 0 until xs length  is a RichInt method which returns Range suitable for looping   Also  you can try loop with to   scala gt  for  i  lt - 0 to xs length-1         println  String       i     is    xs i   String   0 is first String   1 is second String   2 is third

[scala] Efficient iteration with index in Scala

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