firstElement::Middle::LastElement REGEX

Question

If I want to check the input so that I can get the last element right away, is there a form of Regex to do:

fun someFunction (firstElement::MiddleOfList::LastElement)

So i can get last element

Answer 1

There appears to be a last function for lists in the List structure; is that what you need?

Answer 2

You cannot pattern match to find the last or middle element in a SML list.

A SML list is a linked list, which means if you want to find element x, you must first visit all the elements before it.

You could however use the library functions to find the middle element in a manner similar to this:

fun someFunction list = 
   let
      val (first, middle, last) = (hd list, List.nth (list, (length list) div 2), List.last list)
   in
      (* your code here *)
   end

This will however take time linear to the length of the list, but there is no asymtotically faster way using lists, if you need constant time access, you should consider using arrays.