sum.scala

def iteration(op: (Double, Double) => Double, init: Int)(f: Int => Double)(a: Int, b:Int) :Double = {
  def iter(a: Int, result: Double) :Double = {
    if (a > b) result
    else iter(a+1, op(f(a),result))
  }
  iter(a,init)
}

//def sum(f: Int => Double)(a: Int, b: Int) :Double = {
//  def iter(a: Int, result: Double) :Double = {
//    if (a > b) result
//    else iter(a+1, f(a)+result)
//  }
//  iter(a,0)
//}
val sum = iteration( (x, y) => x + y, 0) _
println(sum(x => x)(0,10))

//def product(f: Int => Double)(a: Int, b: Int) :Double = {
//  def iter(a: Int, result: Double) :Double = {
//    if (a > b) result
//    else iter(a+1, f(a)*result)
//  }
//  iter(a,1)
//}
val product = iteration( (x ,y) => x * y, 1) _
println(product(x => x)(1,10))