SML: Remove the entry from the List

问题

How can I delete the element elem in list L? If the list does not contain elem, then the function should return the list unchanged.

For instance:

L = [1, 3, 4, 0, 5, 7]    
elem = 5

So far I have the following function:

fun removeElem elem myList[] = myList
  | removeElem (myList::tl) = if mem myList elem then
                                rm elem myList[]
                              else
                                removeElem elem tl

回答1:

You can turn the question around and ask how to keep only those items not equal to elem. This fits cleanly with filter:

fun removeElem elem myList = filter (fn x => x <> elem) myList

回答2:

This code will accomplish what you want to do: remove the element (actually it will remove all instances of the element if there are more than one) and return the rest of the list as-is:

fun remove_element (list, element) =
    case list of
    [] => []
      | list_head::list_tail => let val a = remove_element(list_tail, element)
                       in
                           if list_head = element
                           then a
                           else list_head::a
                       end

回答3:

fun delete (s,[])     = []
  | delete (s,x::xs') = 
    if s = x then xs' (* more efficient than call delete function again *)
    else x::delete(s, xs') 

回答4:

fun remove_element (elemlist, elem) = 
  case elemlist of
  [] => []
  | head::tail => if elem = head
                  then remove_element (tail, elem)
                  else head::remove_element (tail, elem)

Output SML/NJ:

val remove_element = fn : ''a list * ''a -> ''a list
val it = () : unit
(* matching *)
- remove_element ([1,2,3,4,5], 4);
val it = [1,2,3,5] : int list
(* non matching *)
- remove_element ([1,2,3,4,5], 7);
val it = [1,2,3,4,5] : int list
(* multiple instances *)
- remove_element ([1,3,4,4,5],4);
val it = [1,3,5] : int list

回答5:

With no libraries or extra functions

fun remv(L, c) = 
  if null(L) then nil
  else if c=hd(L) then remv(tl(L), c)
  else hd(L)::remv(tl(L), c);

回答6:

You can try this code.

fun remove_element (elemlist:int list, elem:int) = 
  case elemlist of
  [] => []
  | head::tail => if elem = head
                  then remove_element (tail, elem)
                  else head::remove_element (tail, elem)

回答7:

also you can use this function to remove duplicate entries from a list:

fun remove_duplicates(xs: ''a list) =
    let 
        fun helper(ds: ''a list, m: ''a) =
            if null ds 
            then []
            else if hd ds = m
            then helper(tl ds, m)
            else hd ds :: helper(tl ds, m)
    in 
        if null xs
        then []
        else hd xs :: remove_duplicates( helper(tl xs, hd xs) )
    end

来源：https://stackoverflow.com/questions/8558700/sml-remove-the-entry-from-the-list