Method that add and remove an element from a List

Question

i'm writing a method that take a list: List[(String, Int)] , x:(String, Int) and a n: Int parameters and return a List[(String, Int)] The list parameter represents the input list, the x element represents the element to add to the list, the n represents the maximum dimension of the list.

The method has the following behavior:

1. First of all check if the list has n elements.
2. If list has less than n elements, the method add the x elements to the list.
3. Otherwise if the list contain an elements less than x, the method remove the minimum element from the list, and add the x element to the list.

I've implemented the method as the following:

def method(list: List[(String, Int)], x: (String, Int), n: Int) = {
            if(list.size < n)
                list :+ x
            else {
               var index = -1
               var min = 10000000//some big number
               for(el <- 0 to list.size) {
                 if(list(el)._2 < x._2 && list(el)._2 < min) {
                    min = list(el)._2
                    index = el
                  }
               } 
               if(index != -1) {
                   val newList = list.drop(index)
                   newList :+ x
               }
               else list


              }


        }

Exist a way to express this behavior in a more clean way??

Answer 1

First, a corrected version of what you posted (but it appears, not what you intended)

def method(list: List[(String, Int)], x: (String, Int), n: Int) = {
    if(list.size < n)
        list :+ x
    else {
       var index = -1
       for(i <- 0 until list.size) {
         val el = list(i)
          if(el._2 < x._2)
            index = i
       }
       if(index != -1) {
           val (before, after) = list.splitAt(index)
           val newList = before ++ after.tail
           newList :+ x
       }
       else list
      }
}   

And some tests

val l1 = List(("one", 1))            

val l2 = method(l1, ("three", 3), 2)
//> l2  : List[(String, Int)] = List((one,1), (three,3))
val l3 = method(l2, ("four", 4), 2)
//> l3  : List[(String, Int)] = List((one,1), (four,4))
val l4 = method(l2, ("zero", 0), 2)
//> l4  : List[(String, Int)] = List((one,1), (three,3))

Neater version (but still not meeting the spec as mentioned in a comment from the OP)

 def method(list: List[(String, Int)], x: (String, Int), n: Int) = {
    if (list.size < n)
      list :+ x
    else {
      val (before, after) = list.span(_._2 >= x._2)
      if (after.isEmpty)
        list
      else
        before ++ after.tail :+ x
    }
  } 

Another version that always removes the minimum, if that's less than x.

 def method(list: List[(String, Int)], x: (String, Int), n: Int) = {
    if (list.size < n)
      list :+ x
    else {
      val smallest = list.minBy(_._2)
      if (smallest._2 < x._2) {
        val (before, after) = list.span(_ != smallest)
        before ++ after.tail :+ x
      } else
        list
    }
  }

and its test results

val l1 = List(("one", 1))


val l2 = method(l1, ("three", 3), 2) 
//> l2  : List[(String, Int)] = List((one,1), (three,3))
val l3 = method(l2, ("four", 4), 2) 
//> l3  : List[(String, Int)] = List((three,3), (four,4))
val l4 = method(l2, ("zero", 0), 2)
//> l4  : List[(String, Int)] = List((one,1), (three,3))

but as I say in a comment, better to use a method that processes the whole sequence of Xs and returns the top N.

Answer 2

I would separate the ordering from the method itself:

def method[A: Ordering](list: List[A], x: A, n: Int) = ...

What do you want to do if the list contains more than one element less than x? If you're happy with replacing all of them, you could use map rather than your handwritten loop:

list map {
    case a if Ordering.lt(a, x) => x
    case a => a
}

If you only want to replace the smallest element of the list, maybe it's neater to use min?

val m = list.min
if(m < x) {
  val (init, m :: tail) = list.splitAt(list.indexOf(m))
  init ::: x :: tail
else list