创建直方图OCaml

Question

我的任务是创建一个直方图，输出它在列表中的元素的次数。

Input:[2;2;2;3;4;4;1] 
Output[(2, 3); (2, 2); (2, 1); (3, 1); (4, 2); (4, 1); (1, 1)] 
Expected output : [(2, 3); (3, 1); (4, 2); (1, 1)] 

My code: 

let rec count a ls = match ls with 
    |[]    -> 0 
    |x::xs when x=a -> 1 + count a xs 
    |_::xs   -> count a xs 

let rec count a = function 
    |[]    -> 0 
    |x::xs when x=a -> 1 + count a xs 
    |_::xs   -> count a xs 

let rec histo l = match l with 
|[] -> [] 
|x :: xs -> [(x, count x l)] @ histo xs ;; 

我做错了什么？

Answer 1

问题是xs包含的潜在元素等于x。这是你在输出中看到的内容：（2,3）表示列表中有3次2;那么xs等于[2; 2; 3; 4; 4; 1] ...等等。

另外（不影响结论）：你有2个定义的计数，但它们是相同的。

要实现直方图，使用Hashtbl：

let h = Hashtbl.create 1000;;  
List.iter (fun x -> let c = try Hashtbl.find h x with Not_found -> 0 in Hashtbl.replace h x (c+1)) your_list;; 
Hashtbl.fold (fun x y acc -> (x,y)::acc) h [];;