Scala how can I count the number of occurrences in a list

Question

val list   List 1 2 4 2 4 7 3 2 4    I want to implement it like this  list count 2   returns 3

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Here is a pretty easy way to do it   val data   List  it    was    the    best    of    times    it    was                      the    worst    of    times   data foldLeft Map String Int    withDefaultValue 0      case  acc  letter    gt      acc    letter - gt   1   acc letter           gt  Map worst - gt  1  best - gt  1  it - gt  2  was - gt  2  times - gt  2  of - gt  2  the - gt  2

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Starting Scala 2 13  the groupMapReduce method does that in one pass through the list      val seq   Seq  apple    oranges    apple    banana    apple    oranges    oranges   seq groupMapReduce identity      gt  1            immutable Map String Int    Map banana - gt  1  oranges - gt  3  apple - gt  3  seq groupMapReduce identity      gt  1          apple      Int   3   This    groups list elements  group part of groupMapReduce  maps each grouped value occurrence to 1  map part of groupMapReduce  reduces values within a group of values         by summing them  reduce part of groupMapReduce     This is a one-pass version of what can be translated by   seq groupBy identity  mapValues   map     gt  1  reduce

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using cats  import cats implicits     Alphabet  toLowerCase   map c   gt  Map c - gt  1   toList combineAll  Alphabet  toLowerCase   map c   gt  Map c - gt  1   toList foldMap identity

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scala collections do have count  list count      2

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If you want to use it like list count 2  you have to implement it using an Implicit Class   implicit class Count T  list  List T       def count n  T   Int   list count      n     List 1 2 4 2 4 7 3 2 4  count 2      returns 3 List 1 2 4 2 4 7 3 2 4  count 5      returns 0

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It is interesting to note that the map with default 0 value  intentionally designed for this case demonstrates the worst performance  and not as concise as groupBy       type Word   String     type Sentence   Seq Word      type Occurrences   scala collection Map Char  Int     def woGrouped w  Word   Occurrences             w groupBy c   gt  c  map  case  c  list    gt   c - gt  list length                                                         gt  woGrouped   w  forcomp threadBug Word forcomp threadBug Occurrences    def woGetElse0Map w  Word   Occurrences             val map   Map Char  Int            w foldLeft map   m  c    gt  m    c - gt   m getOrElse c  0    1                                                          gt  woGetElse0Map   w  forcomp threadBug Word forcomp threadBug Occurrences    def woDeflt0Map w  Word   Occurrences             val map   Map Char  Int    withDefaultValue 0          w foldLeft map   m  c    gt  m    c - gt   m c    1                                                          gt  woDeflt0Map   w  forcomp threadBug Word forcomp threadBug Occurrences    def dfltHashMap w  Word   Occurrences             val map   scala collection immutable HashMap Char  Int    withDefaultValue 0          w foldLeft map   m  c    gt  m    c - gt   m c    1                                                          gt  dfltHashMap   w  forcomp threadBug Word forcomp threadBug Occurrences      def mmDef w  Word   Occurrences             val map   scala collection mutable Map Char  Int    withDefaultValue 0          w foldLeft map   m  c    gt  m     c - gt   m c    1                                                          gt  mmDef   w  forcomp threadBug Word forcomp threadBug Occurrences      val functions   List  grp  - gt  woGrouped     mtbl  - gt  mmDef     else  - gt  woGetElse0Map          dfl0  - gt  woDeflt0Map     hash  - gt  dfltHashMap                                             gt  functions    List  String  String   gt  scala collection Map Char Int      Lis                                                       t  grp  lt function1 gt     mtbl  lt function1 gt     else  lt function1 gt     dfl0  lt functio                                                       n1 gt     hash  lt function1 gt          val len   100   1000                         gt  len    Int   100000     def test len  Int            val data  String   scala util Random alphanumeric take len  toList mkString         val firstResult   functions head  2 data           def run f  Word   gt  Occurrences   Int                 val time1   System currentTimeMillis               val result  f data              val time2    System currentTimeMillis   - time1              assert result toSet    firstResult toSet              time2 toInt                    def log results  Seq Int                          functions zip results  map  case   title      r    gt  title         r  mkString                           var groupResults   List fill functions length  1           val integrals   for  i  lt -  1 to 10   yield               val results   functions map  f   gt   1 to 33  foldLeft 0    acc      gt  run f  2                println  log  results                   groupResults    results zip groupResults  map  case  r  gr    gt  r   gr                  log groupResults  toUpperCase                    integrals foreach println                                                   gt  test   len  Int Unit       test len      test len   2     GRP 14   mtbl 11   else 31   dfl0 36   hash 34    GRP 91   MTBL 111      println  Done       def main args  Array String             produces  grp 5   mtbl 5   else 13   dfl0 17   hash 17 grp 3   mtbl 6   else 14   dfl0 16   hash 16 grp 3   mtbl 6   else 13   dfl0 17   hash 15 grp 4   mtbl 5   else 13   dfl0 15   hash 16 grp 23   mtbl 6   else 14   dfl0 15   hash 16 grp 5   mtbl 5   else 13   dfl0 16   hash 17 grp 4   mtbl 6   else 13   dfl0 16   hash 16 grp 4   mtbl 6   else 13   dfl0 17   hash 15 grp 3   mtbl 5   else 14   dfl0 16   hash 16 grp 3   mtbl 6   else 14   dfl0 16   hash 16 GRP 5   MTBL 5   ELSE 13   DFL0 17   HASH 17 GRP 8   MTBL 11   ELSE 27   DFL0 33   HASH 33 GRP 11   MTBL 17   ELSE 40   DFL0 50   HASH 48 GRP 15   MTBL 22   ELSE 53   DFL0 65   HASH 64 GRP 38   MTBL 28   ELSE 67   DFL0 80   HASH 80 GRP 43   MTBL 33   ELSE 80   DFL0 96   HASH 97 GRP 47   MTBL 39   ELSE 93   DFL0 112   HASH 113 GRP 51   MTBL 45   ELSE 106   DFL0 129   HASH 128 GRP 54   MTBL 50   ELSE 120   DFL0 145   HASH 144 GRP 57   MTBL 56   ELSE 134   DFL0 161   HASH 160 grp 7   mtbl 11   else 28   dfl0 31   hash 31 grp 7   mtbl 10   else 28   dfl0 32   hash 31 grp 7   mtbl 11   else 28   dfl0 31   hash 32 grp 7   mtbl 11   else 28   dfl0 31   hash 33 grp 7   mtbl 11   else 28   dfl0 32   hash 31 grp 8   mtbl 11   else 28   dfl0 31   hash 33 grp 8   mtbl 11   else 29   dfl0 38   hash 35 grp 7   mtbl 11   else 28   dfl0 32   hash 33 grp 8   mtbl 11   else 32   dfl0 35   hash 41 grp 7   mtbl 13   else 28   dfl0 33   hash 35 GRP 7   MTBL 11   ELSE 28   DFL0 31   HASH 31 GRP 14   MTBL 21   ELSE 56   DFL0 63   HASH 62 GRP 21   MTBL 32   ELSE 84   DFL0 94   HASH 94 GRP 28   MTBL 43   ELSE 112   DFL0 125   HASH 127 GRP 35   MTBL 54   ELSE 140   DFL0 157   HASH 158 GRP 43   MTBL 65   ELSE 168   DFL0 188   HASH 191 GRP 51   MTBL 76   ELSE 197   DFL0 226   HASH 226 GRP 58   MTBL 87   ELSE 225   DFL0 258   HASH 259 GRP 66   MTBL 98   ELSE 257   DFL0 293   HASH 300 GRP 73   MTBL 111   ELSE 285   DFL0 326   HASH 335 Done   It is curious that most concise groupBy is faster than even mutable map

User · Answer

Try this  should work     val list   List 1 2 4 2 4 7 3 2 4  list count    2     It will return 3

User · Answer

Here is another option   scala gt  val list   List 1 2 4 2 4 7 3 2 4  list  List Int    List 1  2  4  2  4  7  3  2  4   scala gt  list groupBy x   gt  x  map   case  k v    gt  k- gt  v length   res74  scala collection immutable Map Int Int    Map 1 - gt  1  2 - gt  3  7 - gt  1  3 - gt  1  4 - gt  3

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I ran into the same problem but wanted to count multiple items in one go    val s   Seq  apple    oranges    apple    banana    apple    oranges    oranges   s foldLeft Map empty String  Int      m  x    gt  m     x  m getOrElse x  0    1     res1  scala collection immutable Map String Int    Map apple - gt  3  oranges - gt  3  banana - gt  1    https   gist github com sharathprabhal 6890475

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Short answer   import scalaz    Scalaz   xs foldMap x   gt  Map x - gt  1     Long answer   Using Scalaz  given   import scalaz    Scalaz    val xs   List  a   b   c   c   a   a   b   d    then all of these  in the order of less simplified to more simplified   xs map x   gt  Map x - gt  1   foldMap identity  xs map x   gt  Map x - gt  1   foldMap   xs map x   gt  Map x - gt  1   suml xs map   - gt  1  foldMap Map     xs foldMap x   gt  Map x - gt  1     yield  Map  b - gt  2   a - gt  3   c - gt  2   d - gt  1

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I had the same problem as Sharath Prabhal  and I got another  to me clearer  solution    val s   Seq  apple    oranges    apple    banana    apple    oranges    oranges   s groupBy l   gt  l  map t   gt   t  1  t  2 length     With as result    Map banana - gt  1  oranges - gt  3  apple - gt  3

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val list   List 1  2  4  2  4  7  3  2  4     Using the provided count method this would yield the occurrences of each value in the list  l map x   gt  l count      x    List Int    List 1  3  3  3  3  1  1  3  3     This will yield a list of pairs where the first number is the number from the original list and the second number represents how often the first number occurs in the list  l map x   gt   x  l count      x       outputs   gt  List  Int  Int     List  1 1    2 3    4 3    2 3    4 3    7 1    3 1    2 3    4 3

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list groupBy i  gt i  mapValues   size    gives  Map Int  Int    Map 1 - gt  1  2 - gt  3  7 - gt  1  3 - gt  1  4 - gt  3    Note that you can replace  i  gt i  with built in identity function   list groupBy identity  mapValues   size

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scala gt  val list   List 1 2 4 2 4 7 3 2 4  list  List Int    List 1  2  4  2  4  7  3  2  4   scala gt  println list filter      2  size  3

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A somewhat cleaner version of one of the other answers is   val s   Seq  apple    oranges    apple    banana    apple    oranges    oranges    s groupBy identity  mapValues   size    giving a Map with a count for each item in the original sequence   Map banana - gt  1  oranges - gt  3  apple - gt  3    The question asks how to find the count of a specific item  With this approach  the solution would require mapping the desired element to its count value as follows   s groupBy identity  mapValues   size   apple

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I did not get the size of the list using length but rather size as one the answer above suggested it because of the issue reported here   val list   List  apple    oranges    apple    banana    apple    oranges    oranges   list groupBy x  gt x  map t   gt   t  1  t  2 size

[scala] Scala how can I count the number of occurrences in a list

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