斯卡拉列表搜索遞歸

Question

我有一個示例文件（選項卡分隔），我必須使用線性遞歸來搜索給定狀態的樹列表。

輸入文件：

Quercus acerifolia mapleleaf oak MN 
    Quercus _acutidens  CA 
    Quercus acutissima sawtooth oak AL,GA,LA,MD,MS,NC,PA,VA 
    Quercus agrifolia California live oak CA 
    Quercus alba white oak AL,AR,CT,DC,DE,FL,GA,IA,IL,IN,KS,KY,LA 
    Quercus ajdfensis Ajo Mountain scrub oak ,MN 

    First Column - Tree Name(Genus Species) 
    Second Column - Common Tree name 
    Third Column - State Name 

代碼使用遞歸：

//declaring package 
    package HW10 

    //declaring object 
    object TreesStub { 

    //importing Source package for files import 
     import scala.io.Source 

     //assigning the file path to filename variable 
     val fileName = "trees.tsv" //tab separated 

     //defining Main function 
     def main(args: Array[String]): Unit = { 

     //reading source file from a file which is tabe separated 
     val treeList: List[String] = Source.fromFile(fileName).getLines.toList 

     //Creating mutable list to append element which found a match 
     var stateList = collection.mutable.ListBuffer[String]:() 

     //Checking the list if empty then print empty list else call the function 
     if(treeList.isEmpty) println("Empty List") 
     else searchTreesRecursively(state,treeList) //calling recurive func 

     //Calling recursive function search trees using "state" name and each line of //file(as second parameter)  
     def searchTreesRecursively(state: String, trees: List[String]): Unit = { 
    //matching the trees list 
     trees match { 
      case Nil => println(stateList) //If empty print the entire list 

      //taking each line and splitting the lines using and matching with the state //given  

      case x => x.map(x => (x.split("\t", -1))).filter(_.length > 2).map(x1=> if(x1(2).contains(state)) stateList+= x(0) 

      //calling function recursively for the rest of the elements of the list 
      else searchTreesRecursively(state,x1->next)}//next //element of list 

     } 
     } 

    } 

我在這裏想呼籲在case語句列表中的 「X1」 的每一個元素的遞歸函數。我不知道如何通過遞歸調用列表的第二個元素，我使用了「searchTreesRecursively（state，x1-> next）}」但是我得到的錯誤是「Type Mismatch」。

I know we can use x::xs for iterating the list, but i am not sure how i can use it to fit in this logic. Please let me know. 

Answer 1

的問題是，在這種模式匹配你的格局，

所以... Scala中的一個列表可以被認爲是類似，

list = elem1 :: elem2 :: elem3 :: Nil 

，或者

list = elem1 :: (elem2 :: (elem3 :: (Nil))) 

或者，

list = elem1 :: tail1 

其中，

tail1 = (elem2 :: (elem3 :: (Nil))) 

所以，如果讓我們說你想遞歸地做abcdef在這個列表的元素，

def abcdef(list: List[String]): Unit = { 
    case Nil => println("nil") 
    case head :: tail => { 
    println(head) 
    abcdef(tail) 
    } 
}