Scala best way of turning a Collection into a Map-by-key

Question

If I have a collection c of type T and there is a property p on T  of type P  say   what is the best way to do a map-by-extracting-key   val c  Collection T  val m  Map P  T    One way is the following   m   new HashMap P  T  c foreach   t   gt  m add  t getP  t      But now I need a mutable map  Is there a better way of doing this so that it s in 1 line and I end up with an immutable Map   Obviously I could turn the above into a simple library utility  as I would in Java  but I suspect that in Scala there is no need

User · Answer

This works for me   val personsMap   persons foldLeft scala collection mutable Map Int  PersonDTO            m  p    gt  m p id    p  m     The Map has to be mutable and the Map has to be return since adding to a mutable Map does not return a map

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Scala 2 13  instead of  quot breakOut quot  c map t   gt   t getP  t   to Mat  Scroll to  quot View quot   https   www scala-lang org blog 2017 02 28 collections-rework html

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How about using zip and toMap   myList zip myList map   length   toMap

User · Answer

What you re trying to achieve is a bit undefined  What if two or more items in c share the same p  Which item will be mapped to that p in the map   The more accurate way of looking at this is yielding a map between p and all c items that have it   val m  Map P  Collection T     This could be easily achieved with groupBy   val m  Map P  Collection T     c groupBy t   gt  t p    If you still want the original map  you can  for instance  map p to the first t that has it   val m  Map P  T    c groupBy t   gt  t p  map   case  p  ts    gt   p - gt  ts head

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In addition to  James Iry s solution  it is also possible to accomplish this using a fold   I suspect that this solution is slightly faster than the tuple method  fewer garbage objects are created    val list   List  this    maps    string    to    length   val map   list foldLeft Map String  Int        m  s    gt  m s    s length

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use map   on collection followed with toMap  val map   list map e   gt   e  e length   toMap

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You can construct a Map with a variable number of tuples   So use the map method on the collection to convert it into a collection of tuples and then use the      trick to convert the result into a variable argument   scala gt  val list   List  this    maps    string    to    length   map  s   gt   s  s length   list  List  java lang String  Int     List  this 4    maps 4    string 6    to 2    length 6    scala gt  val list   List  this    is    a    bunch    of    strings   list  List java lang String    List this  is  a  bunch  of  strings   scala gt  val string2Length   Map list map  s   gt   s  s length         string2Length  scala collection immutable Map java lang String Int    Map strings - gt  7  of - gt  2  bunch - gt  5  a - gt  1  is - gt  2  this - gt  4

User · Answer

This is probably not the most efficient way to turn a list to map  but it makes the calling code more readable  I used implicit conversions to add a mapBy method to List   implicit def list2ListWithMapBy T  list  List T    ListWithMapBy T        new ListWithMapBy list     class ListWithMapBy V  list  List V      def mapBy K  keyFunc  V   gt  K          list map a   gt  keyFunc a  - gt  a  toMap         Calling code example   val list   List  A    AA    AAA   list mapBy   length                     Map 1 - gt  A  2 - gt  AA  3 - gt  AAA    Note that because of the implicit conversion  the caller code needs to import scala s implicitConversions

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For what it s worth  here are two pointless ways of doing it   scala gt  case class Foo bar  Int  defined class Foo  scala gt  import scalaz    Scalaz   import scalaz   import Scalaz    scala gt  val c   Vector Foo 9   Foo 11   c  scala collection immutable Vector Foo    Vector Foo 9   Foo 11    scala gt  c map      Foo  bar   amp  amp  amp  identity  toMap res30  scala collection immutable Map Int Foo    Map 9 - gt  Foo 9   11 - gt  Foo 11    scala gt  c map      Foo  bar   gt  gt    Pair apply Int  Foo     curried  toMap res31  scala collection immutable Map Int Foo    Map 9 - gt  Foo 9   11 - gt  Foo 11

User · Answer

c map    getP  zip c   Works well and is very intuitiv

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This can be implemented immutably and with a single traversal by folding through the collection as follows   val map   c foldLeft Map P  T        m  t    gt  m    t getP - gt  t      The solution works because adding to an immutable Map returns a new immutable Map with the additional entry and this value serves as the accumulator through the fold operation   The tradeoff here is the simplicity of the code versus its efficiency  So  for large collections  this approach may be more suitable than using 2 traversal implementations such as applying map and toMap

User · Answer

You can use  c map  t   gt  t getP - gt  t  toMap   but be aware that this needs 2 traversals

User · Answer

Another solution  might not work for all types   import scala collection breakOut val m Map P  T    c map t   gt   t getP  t   breakOut    this avoids the creation of the intermediary list  more info here  Scala 2 8 breakOut

[scala] Scala best way of turning a Collection into a Map-by-key?

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