无法找到有序类型的证据参数的隐含值[T]

Question

I am actually blocked on this for about 4 hours now. I want to get a List of Pairs[String, Int] ordered by their int value. The function partiotion works fine, so should the bestN, but when loading this into my interpreter I get:

<console>:15: error: could not find implicit value for evidence parameter of type Ordered[T]

on my predicate. Does someone see what the problem is? I am really desperate at the moment...

This is the code:

def partition[T : Ordered](pred: (T)=>Boolean, list:List[T]): Pair[List[T],List[T]] = {
    list.foldLeft(Pair(List[T](),List[T]()))((pair,x) => if(pred(x))(pair._1, x::pair._2) else (x::pair._1, pair._2))
}

def bestN[T <% Ordered[T]](list:List[T], n:Int): List[T] = {
    list match {
        case pivot::other => {
            println("pivot: " + pivot)
            val (smaller,bigger) = partition(pivot <, list)
            val s = smaller.size
            println(smaller)
            if (s == n) smaller 
            else if (s+1 == n) pivot::smaller
            else if (s < n) bestN(bigger, n-s-1) 
            else bestN(smaller, n)
        }
        case Nil => Nil
    }
}

class OrderedPair[T, V <% Ordered[V]] (t:T, v:V) extends Pair[T,V](t,v) with Ordered[OrderedPair[T,V]] {
    def this(p:Pair[T,V]) = this(p._1, p._2)
    override def compare(that:OrderedPair[T,V]) : Int = this._2.compare(that._2)
}

Edit: The first function divides a List into two by applying the predicate to every member, the bestN function should return a List of the lowest n members of the list list. And the class is there to make Pairs comparable, in this case what I want do do is:

val z = List(Pair("alfred",1),Pair("peter",4),Pair("Xaver",1),Pair("Ulf",2),Pair("Alfons",6),Pair("Gulliver",3))

with this given List I want to get for example with:

bestN(z, 3)

the result:

(("alfred",1), ("Xaver",1), ("Ulf",2))

Answer 1

It looks like you don't need an Ordered T on your partition function, since it just invokes the predicate function.

The following doesn't work (presumably) but merely compiles. Other matters for code review would be the extra braces and stuff like that.

package evident

object Test extends App {

  def partition[T](pred: (T)=>Boolean, list:List[T]): Pair[List[T],List[T]] = {
    list.foldLeft(Pair(List[T](),List[T]()))((pair,x) => if(pred(x))(pair._1, x::pair._2) else (x::pair._1, pair._2))
  }

  def bestN[U,V<%Ordered[V]](list:List[(U,V)], n:Int): List[(U,V)] = {
    list match {
      case pivot::other => {
        println(s"pivot: $pivot and rest ${other mkString ","}")
        def cmp(a: (U,V), b: (U,V)) = (a: OrderedPair[U,V]) < (b: OrderedPair[U,V])
        val (smaller,bigger) = partition(((x:(U,V)) => cmp(x, pivot)), list)
        //val (smaller,bigger) = list partition ((x:(U,V)) => cmp(x, pivot))
        println(s"smaller: ${smaller mkString ","} and bigger ${bigger mkString ","}")
        val s = smaller.size
        if (s == n) smaller
        else if (s+1 == n) pivot::smaller
        else if (s < n) bestN(bigger, n-s-1)
        else bestN(smaller, n)
      }
      case Nil => Nil
    }
  }

  implicit class OrderedPair[T, V <% Ordered[V]](tv: (T,V)) extends Pair(tv._1, tv._2) with Ordered[OrderedPair[T,V]] {
    override def compare(that:OrderedPair[T,V]) : Int = this._2.compare(that._2)
  }

  val z = List(Pair("alfred",1),Pair("peter",4),Pair("Xaver",1),Pair("Ulf",2),Pair("Alfons",6),Pair("Gulliver",3))
  println(bestN(z, 3))
}

I found the partition function hard to read; you need a function to partition all the parens. Here are a couple of formulations, which also use the convention that results accepted by the filter go left, rejects go right.

def partition[T](p: T => Boolean, list: List[T]) = 
  ((List.empty[T], List.empty[T]) /: list) { (s, t) =>
    if (p(t)) (t :: s._1, s._2) else (s._1, t :: s._2)
  }
def partition2[T](p: T => Boolean, list: List[T]) =
  ((List.empty[T], List.empty[T]) /: list) {
    case ((is, not), t) if p(t) => (t :: is, not)
    case ((is, not), t)         => (is, t :: not)
  }
// like List.partition
def partition3[T](p: T => Boolean, list: List[T]) = {
  import collection.mutable.ListBuffer
  val is, not = new ListBuffer[T]
  for (t <- list) (if (p(t)) is else not) += t
  (is.toList, not.toList)
}

This might be closer to what the original code intended:

def bestN[U, V <% Ordered[V]](list: List[(U,V)], n: Int): List[(U,V)] = {
  require(n >= 0)
  require(n <= list.length)
  if (n == 0) Nil
  else if (n == list.length) list
  else list match {
    case pivot :: other =>
      println(s"pivot: $pivot and rest ${other mkString ","}")
      def cmp(x: (U,V)) = x._2 < pivot._2
      val (smaller, bigger) = partition(cmp, other)     // other partition cmp
      println(s"smaller: ${smaller mkString ","} and bigger ${bigger mkString ","}")
      val s = smaller.size
      if (s == n) smaller
      else if (s == 0) pivot :: bestN(bigger, n - 1)
      else if (s < n) smaller ::: bestN(pivot :: bigger, n - s)
      else bestN(smaller, n)
    case Nil => Nil
  }
}

Arrow notation is more usual:

  val z = List(
    "alfred" -> 1,
    "peter" -> 4,
    "Xaver" -> 1,
    "Ulf" -> 2,
    "Alfons" -> 6,
    "Gulliver" -> 3
  )

Answer 2

I suspect I am missing something, but I'll post a bit of code anyway.

For bestN , you know you can just do this?

val listOfPairs = List(Pair("alfred",1),Pair("peter",4),Pair("Xaver",1),Pair("Ulf",2),Pair("Alfons",6),Pair("Gulliver",3))
val bottomThree = listOfPairs.sortBy(_._2).take(3)

Which gives you:

List((alfred,1), (Xaver,1), (Ulf,2))

And for the partition function, you can just do this (say you wanted all pairs lower then 4):

val partitioned = listOfPairs.partition(_._2 < 4)

Which gives (all lower then 4 on the left, all greater on the right):

(List((alfred,1), (Xaver,1), (Ulf,2), (Gulliver,3)),List((peter,4), (Alfons,6)))

Answer 3

Just to share with you: this works! Thanks alot to all people who helped me, you're all great!

object Test extends App {

  def partition[T](pred: (T)=>Boolean, list:List[T]): Pair[List[T],List[T]] = {
    list.foldLeft(Pair(List[T](),List[T]()))((pair,x) => if(pred(x))(pair._1, x::pair._2) else (x::pair._1, pair._2))
  }

  def bestN[U,V<%Ordered[V]](list:List[(U,V)], n:Int): List[(U,V)] = {
    list match {
      case pivot::other => {
        def cmp(a: (U,V), b: (U,V)) = (a: OrderedPair[U,V]) <= (b: OrderedPair[U,V])
        val (smaller,bigger) = partition(((x:(U,V)) => cmp(pivot, x)), list)
        val s = smaller.size
        //println(n + " :" + s)
        //println("size:" + smaller.size + "Pivot: " + pivot + " Smaller part: " + smaller + " bigger: " + bigger)
        if (s == n) smaller
        else if (s+1 == n) pivot::smaller
        else if (s < n) bestN(bigger, n-s)
        else bestN(smaller, n)
      }
      case Nil => Nil
    }
  }

  class OrderedPair[T, V <% Ordered[V]](tv: (T,V)) extends Pair(tv._1, tv._2) with Ordered[OrderedPair[T,V]] {
    override def compare(that:OrderedPair[T,V]) : Int = this._2.compare(that._2)
  }
  implicit final def OrderedPair[T, V <% Ordered[V]](p : Pair[T, V]) : OrderedPair[T,V] = new OrderedPair(p)

  val z = List(Pair("alfred",1),Pair("peter",1),Pair("Xaver",1),Pair("Ulf",2),Pair("Alfons",6),Pair("Gulliver",3))
  println(bestN(z, 3))
  println(bestN(z, 4))
  println(bestN(z, 1))
}