列表中的最后一个元素使用ocaml List.fold_left [英] last element in list using ocaml List.fold_left

问题描述

  let last（xs：'a list） ：'a = 
让rec aux xs prev = 
与
 |匹配xs []  - > prev 
 | x :: ys  - > 
中的aux ys x与
 |匹配xs []  - >失败没有元素
 | x :: xs  - > aux xs x 
  

如何使用 OCaml中的List.fold_left 函数？
提前致谢！

解决方案

fold_left 从头部到尾部，因此传递给 fold_left 的函数应该只将累加器替换为列表的当前元素。因此，简单地说，

  let last = function 
 | x :: xs  - > List.fold_left（fun_y  - > y）x xs 
 | []  - > failwithno element
  

您可以直接编写函数，不需要 aux

  let rec last = function 
 | x :: []  - > x 
 | _ :: xs  - >最后xs 
 | []  - > failwithno element
  




I can find the last element of a list by the following code.

let last (xs:'a list) : 'a =
    let rec aux xs prev =
        match xs with
        | [] -> prev
        | x::ys -> aux ys x in
    match xs with
    | [] -> failwith "no element"
    | x::xs -> aux xs x

How do I find the last element of the same list using the List.fold_left function in OCaml? Thanks in advance!

解决方案

fold_left accesses the list from the head to the tail, thus the function passed to fold_left should just replace the accumulator with the current element of the list. Thus simply,

let last = function
  | x::xs -> List.fold_left (fun _ y -> y) x xs
  | []    -> failwith "no element"

You can write your function directly, without the aux function.

let rec last = function
  | x::[] -> x
  | _::xs -> last xs
  | []    -> failwith "no element"

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