SML大于两个列表

Question

Need help with an SML problem I can't seem to solve. Basically I have two list and I need to return the greater from each list.

Example call:

Greater([8,4,12,5,6],[2,6,14,4,5]);

Would return (8,6,14,6).

I just started working with lists in SML and not even sure where to start on this.

Answer 1

val greater = List.map Int.max o ListPair.zip

Or to expand that out:

fun greater(nil, nil)     = nil
  | greater(x::xs, y::ys) = (if x > y then x else y)::greater(xs, ys)
  | greater(_, _)         = raise Domain

Answer 2

Assuming you have two lists of equal lengths you could define this function:

fun greater nil nil = nil
  | greater (x::xs) (y::ys) = 
       if x > y then x::(greater xs ys) else y::(greater xs ys);

Or alternatively, by reducing the problem to a mapping from a list of int pairs to a list of ints:

fun zip nil nil = nil
  | zip (x::xs) (y::ys) = (x,y)::(zip xs ys);

fun max (x,y) = if x > y then x else y;

fun greater xs ys = map max (zip xs ys);

Note that lists in ML are denoted as [1,2,3] and not (1,2,3) , which is a tuple with three elements. This function is then called as:

- greater [1,2,3,4] [2,3,4,5];
val it = [2,3,4,5] : int list

- greater [~1,8,3] [8,~1,~2];
val it = [8,8,3] : int list

Answer 3

甚至更好：

val greater = ListPair.map Int.max