Pregunta Scala, ¿cómo puedo contar el número de ocurrencias en una lista


val list = List(1,2,4,2,4,7,3,2,4)

Quiero implementarlo así: list.count(2) (devuelve 3).


73
2017-07-12 09:20


origen


Respuestas:


Una versión más limpia de una de las otras respuestas es:

val s = Seq("apple", "oranges", "apple", "banana", "apple", "oranges", "oranges")

s.groupBy(identity).mapValues(_.size)

dando una Map con un conteo para cada artículo en la secuencia original:

Map(banana -> 1, oranges -> 3, apple -> 3)

La pregunta pregunta cómo encontrar el conteo de un artículo específico. Con este enfoque, la solución requeriría mapear el elemento deseado a su valor de recuento de la siguiente manera:

s.groupBy(identity).mapValues(_.size)("apple")

107
2018-02-13 08:11



las colecciones scala tienen count: list.count(_ == 2)


104
2017-07-12 09:24



Tuve el mismo problema que Sharath Prabhal, y obtuve otra solución (para mí más clara):

val s = Seq("apple", "oranges", "apple", "banana", "apple", "oranges", "oranges")
s.groupBy(l => l).map(t => (t._1, t._2.length))

Como resultado:

Map(banana -> 1, oranges -> 3, apple -> 3)

37
2017-12-02 11:58



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 => 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 => (x, l.count(_ == x)))
// outputs => List[(Int, Int)] = List((1,1), (2,3), (4,3), (2,3), (4,3), (7,1), (3,1), (2,3), (4,3))

12
2017-07-12 14:24



list.groupBy(i=>i).mapValues(_.size)

da

Map[Int, Int] = Map(1 -> 1, 2 -> 3, 7 -> 1, 3 -> 1, 4 -> 3)

Tenga en cuenta que puede reemplazar (i=>i) con construido en identity función:

list.groupBy(identity).mapValues(_.size)

11
2017-12-13 22:55



Me encontré con el mismo problema pero quería contar varios elementos de una vez ...

val s = Seq("apple", "oranges", "apple", "banana", "apple", "oranges", "oranges")
s.foldLeft(Map.empty[String, Int]) { (m, x) => m + ((x, m.getOrElse(x, 0) + 1)) }
res1: scala.collection.immutable.Map[String,Int] = Map(apple -> 3, oranges -> 3, banana -> 1)

https://gist.github.com/sharathprabhal/6890475


6
2017-10-10 13:25



Si quieres usarlo como list.count(2) tienes que implementarlo usando un Clase implícita.

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

6
2018-05-23 19:49



Respuesta corta:

import scalaz._, Scalaz._
xs.foldMap(x => Map(x -> 1))

Respuesta larga:

Utilizando Scalaz, dado, por ejemplo,

import scalaz._, Scalaz._

val xs = List('a, 'b, 'c, 'c, 'a, 'a, 'b, 'd)

luego todos estos (en el orden de simplificado a simplificado)

xs.map(x => Map(x -> 1)).foldMap(identity)
xs.map(x => Map(x -> 1)).foldMap()
xs.map(x => Map(x -> 1)).suml
xs.map(_ -> 1).foldMap(Map(_))
xs.foldMap(x => Map(x -> 1))

rendimiento

Map('b -> 2, 'a -> 3, 'c -> 2, 'd -> 1)

5
2017-09-17 20:21



Es interesante observar que el mapa con valor 0 predeterminado, diseñado intencionalmente para este caso, muestra el peor rendimiento (y no tan conciso como groupBy)

    type Word = String
    type Sentence = Seq[Word]
    type Occurrences = scala.collection.Map[Char, Int]

  def woGrouped(w: Word): Occurrences = {
        w.groupBy(c => c).map({case (c, list) => (c -> list.length)})
  }                                               //> woGrouped: (w: forcomp.threadBug.Word)forcomp.threadBug.Occurrences

  def woGetElse0Map(w: Word): Occurrences = {
        val map = Map[Char, Int]()
        w.foldLeft(map)((m, c) => m + (c -> (m.getOrElse(c, 0) + 1)) )
  }                                               //> 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) => m + (c -> (m(c) + 1)) )
  }                                               //> 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) => m + (c -> (m(c) + 1)) )
    }                                             //> 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) => m += (c -> (m(c) + 1)) )
  }                                               //> mmDef: (w: forcomp.threadBug.Word)forcomp.threadBug.Occurrences

    val functions = List("grp" -> woGrouped _, "mtbl" -> mmDef _, "else" -> woGetElse0Map _
    , "dfl0" -> woDeflt0Map _, "hash" -> dfltHashMap _
    )                                  //> functions  : List[(String, String => scala.collection.Map[Char,Int])] = Lis
                                                  //| t((grp,<function1>), (mtbl,<function1>), (else,<function1>), (dfl0,<functio
                                                  //| n1>), (hash,<function1>))


    val len = 100 * 1000                      //> 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 => 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) => title + " " + r} mkString " , ")
        }

        var groupResults = List.fill(functions.length)(1)

        val integrals = for (i <- (1 to 10)) yield {
            val results = functions map (f => (1 to 33).foldLeft(0) ((acc,_) => run(f._2)))
            println (log (results))
                groupResults = (results zip groupResults) map {case (r, gr) => r + gr}
                log(groupResults).toUpperCase
        }

        integrals foreach println

    }                                         //> 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]) {
    }

produce

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

Es curioso que lo más conciso groupBy ¡es más rápido que un mapa mutable!


4
2017-07-13 13:57



Aquí hay otra opción:

scala> 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> list.groupBy(x => x) map { case (k,v) => k-> v.length }
res74: scala.collection.immutable.Map[Int,Int] = Map(1 -> 1, 2 -> 3, 7 -> 1, 3 -> 1, 4 -> 3)

1
2018-04-21 18:57