//
// demo of some simple functions on lists implemented with recursion
//

import scala.io.StdIn._

// get list of doubles from stdin, ending with "quit"
def readList() : List[Double] = {
  println("enter next value, 'quit' to end")
  val in = readLine
  if (in == "quit") {
    List[Double]()
  } 
  else {
    in.toDouble :: readList()
  }
}

// print elements of list
def printList(a:List[Double]) {
  if (!a.isEmpty) {
    println(a.head)
    printList(a.tail)
  }
}

// sum elements of list
// (returns zero for empty list)
def sumList(a:List[Double]) : Double = {
  if (!a.isEmpty) {
    a.head + sumList(a.tail)
  }
  else {
    0.0
  }
}

// perform reduction on list 
// (returns "identity" for empty list)
def combineList(a:List[Double], 
  combine:(Double,Double)=>Double, identity:Double
) : Double = 
{
  if (!a.isEmpty) {
    combine(a.head, combineList(a.tail, combine, identity))
  }
  else {
    identity
  }
}

// reduction functions analogous to those in array-demo

def add(x1:Double,x2:Double) : Double = { x1+x2 }

def newSumList(a:List[Double]) : Double = {
  //  combineList(a, add, 0.0)
  //  combineList(a, (x1,x2) => x1+x2, 0.0)
  combineList(a, _ + _, 0.0)
}

def newProductList(a:List[Double]) : Double = {
  combineList(a, (x1,x2) => x1*x2, 1.0)
}

def newMaxList(a:List[Double]) : Double = {
  combineList(a, (x1,x2) => x1 max x2, Double.MinValue)
}

def newMinList(a:List[Double]) : Double = {
  combineList(a, (x1,x2) => x1 min x2, Double.MaxValue)
}