f# - 计算列表中元素的出现

Question

我有一个整数列表，并且多次出现的任何整数都会连续执行此操作。我想将其转换为元组列表，其中包含每个对象及其计数。

我想出了以下内容，但是temp的返回类型存在问题:“类型'int'与类型'a list'不匹配”。但是，这三种返回类型在我看来是一致的。我做错了什么？如果我所做的不是很好的F#，并且应该以完全不同的方式进行，请也告诉我。

let countoccurences list =
    match list with
    | x::xs -> let rec temp list collecting counted =
                    match list with
                    | x::xs when x=collecting -> temp xs collecting counted+1
                    | x::xs -> (collecting,counted)::temp xs x 1
                    | [] -> (collecting,counted)::[]
               temp xs x 1
    | [] -> []

Answer 1

在这一行:

| x::xs when x=collecting -> temp xs collecting counted+1

编译器将您的代码解释为

| x::xs when x=collecting -> (temp xs collecting counted)+1

但是你想要的是

| x::xs when x=collecting -> temp xs collecting (counted+1)

但是，即使进行了此更改，您算法的一个问题是 temp函数不是尾部递归的，这意味着在长列表中调用它可能会导致堆栈溢出(例如 countoccurences [1..10000]在我的计算机上失败)。如果这对您很重要，那么您应该重写 temp helper函数以使其具有尾递归性。最简单的方法是添加一个累加的列表参数，然后反转列表。

let countoccurences list =
    match list with
    | x::xs -> 
        let rec temp list collecting counted acc =
            match list with
            | x::xs when x = collecting -> temp xs collecting (counted+1) acc
            | x::xs -> temp xs x 1 ((collecting, counted)::acc)
            | [] -> (collecting, counted)::acc
        temp xs x 1 []
        |> List.rev
    | [] -> []