使用Iterator将元组映射到元组

Question

Why is the following code does not work, and how can I overcome it using Iterator ?

def f(str : String) : (String, String) = {
  str.splitAt(1)
}
var with_id : Iterator[(String, Int)] = List(("test", 1), ("list", 2), ("nothing", 3), ("else", 4)).iterator

println(with_id.mkString(" "))

val result = with_id map { (s : String, i : Int) => (f(s), i) }

println(result.mkString(" "))

Expected output is:

(("t", "est"), 1) (("l", "ist"), 2) ...

Error:

Error:(28, 54) type mismatch;
found   : (String, Int) => ((String, String), Int)
required: ((String, Int)) => ?
val result = with_id map { (s : String, i : Int) => (f(s), i) }
                                                 ^

Answer 1

The problem is that (s : String, i : Int) => (f(s), i) is a Function2 (ie a function that takes 2 arguments):

scala> (s : String, i : Int) => (f(s), i)
res3: (String, Int) => ((String, String), Int) = <function2>

whereas .map expects a Function1 (taking a tuple as its argument).

You could define a Function1 with

scala> val g = (t: (String, Int)) => (f(t._1), t._2)
g: ((String, Int)) => ((String, String), Int) = <function1>

scala> val result = with_id map g
result: Iterator[((String, String), Int)] = non-empty iterator

But it seems much better (to me at least) to use the more idiomatic pattern matching anonymous function (note the added case ) :

scala> val result = with_id map { case (s : String, i : Int) => (f(s), i) }
result: Iterator[((String, String), Int)] = non-empty iterator

Answer 2

with_id.map expects a ((String, Int) => ?) function as input. That is, a function that takes a Tuple as input, not two parameters.

You can use it like this:

with_id map{ case (s,i) => (f(s), i)}  //match the input tuple to s and i