Value of the last element of a list

Question

how to get the value of the last element of a List? I\'ve noted that List.hd (or .Head) return an item, while List.tl (or .Tail) returns a List.

Is rev the List and

Answer 1

The regular way to work with lists in F# is to use recursion. The first item in a list is the head (obviously) and the rest of the list is the tail (as oppose to the last item). So when a function recieves a list it processes the head and then recursively processes the rest of the list (the tail).

let reversedList = List.rev originalList
let tailItem = List.hd reversedList

Answer 2

As a novice F# developer, I don't see what the harm is in doing the following

let mylist = [1;2;3;4;5]

let lastValue = mylist.[mylist.Length - 1]

Imperative in nature? Yes but no need for recursion.

Answer 3

In general, if you need to do this, you're doing something wrong. Since F# lists are single-linked, accessing the last element is costly - O(N), where N is size of list. Try to rewrite your algorithm so that you always access the first element, not the last (which is O(1)). If you cannot do so, chances are good that your choice of list for a data structure wasn't correct in the first place.

Answer 4

You can call List.Head to get the first element of a list, such that the below expression evaluates to true:

let lst = [1;2;3;4;5]
List.head lst = 1

However, calling List.Tail will return every element in the list after the first element, such that the below expression is true:

let lst = [1;2;3;4;5]
List.tail lst = [2;3;4;5]

Like some other people have mentioned, there isn't an efficient way in F# to get the tail end of a list, basic lists just aren't built with that functionality in mind. If you really want to get the last element you're going to have to reverse your list first, and then take the new head (which was the previous tail).

let lst = [1;2;3;4;5]
(List.head (List.rev lst) ) = 5