Ocaml 中的列表递归 [英] List Recursion in Ocaml

问题描述

这就是我想要实现的目标，使用递归返回到值低于给定值的列表:

This is what I want to achive, to return to a list with values that are below the given value with recursion:

# list_below 3 [7; 1; 0; 3];;
   - : int list = [1; 0]
# list_below 1 [-7; 1; 0; 3];;
   - : int list = [-7; 0]
# list_below 9.0 [4.2; 3.6; 5.0; 12.8];;
   - : float list = [4.2; 3.6; 5.0]

这是我到目前为止所写的内容，它似乎没有返回任何内容.

Here is what I wrote so far, and it does not appear to return anything.

let rec list_below thresh lst = 
 if List.hd lst > thresh then [] else
  List.hd lst :: list_below thresh (List.tl lst);;
;;

你能告诉我我的代码有什么问题吗?

Could you show me what is wrong with my code?

推荐答案

问题应该是 Jeffrey 为您指出的问题.

The problem should be what Jeffrey has pointed out for you.

您的问题表明您想要实现 list_below，但您的代码显示了 list_above.我会在这里使用 list_below.

Your questions says you want to implement list_below, but your code shows list_above. I'll stick to list_below here.

递归函数模式匹配.例如，下面的代码应该可以工作:

Recursive functions in Ocaml can be made quite intuitively if you use pattern matching. For example, the below code should work :

let rec list_below thresh lst =
  match lst with
  | [] -> []
  | hd :: tl -> if hd < thresh then hd :: (list_below thresh tl)
            else list_below thresh tl;;

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