loops: having my cake and eating it too?

[gusl]

One programming annoyance when writing loops is wanting to access each element of the list without passing the index, i.e. with code like: for (el in list) while simultaneously wanting to know the index without writing i=0 and i++.

I'm not aware of any solutions to this in current usage.

Comments

[wjl.livejournal.com]

In a sufficiently expressive language (e.g. SML), it is not difficult to get the behavior you want with a higher-order function. For example:

  local
    fun mapi_aux i f [] = []
      | mapi_aux i f (x::xs) = f (i, x) :: mapi_aux (i+1) f xs
  in
    (* mapi : (int * 'a -> 'b) -> 'a list -> 'b list *)
    fun mapi f list = mapi_aux 0 f list
  end

Now instead of for (el in list) { ... code(i, el) ... } you just say mapi (fn (i, el) => ... code(i, el) ...) list.

The Standard ML Basis has things like Array.iteri and Vector.mapi predefined:

  - Array.appi;
  val it = fn : (int * 'a -> unit) -> 'a array -> unit
  - Vector.mapi;
  val it = fn : (int * 'a -> 'b) -> 'a vector -> 'b vector

[chrisamaphone]

you beat me to it! i had the source for mapi in my copy buffer :)