f＃列出负数的总和 [英] f# list sum with negative numbers

问题描述

我认为我为我的问题做了正确的解决方案，但我错了。



我必须在最后一个负数之后计算列表中每个元素的总和或者如果<列表中的
我对所有元素只有正的claculate总和。



代码我做了什么：不幸的是我必须这样做而不会使用只有高阶函数和列表模块。任何提示如何做到这一点甚至如何使用这个rec​​usrion？

I thought that i made right solutiuon for my problem but i was wrong.

I must calculate sum of every elements in list after last negative number or if
in the list i have only positive claculate sum for all elements.

code what i have done : Unfortunalety i must do it without recusion using only high order function and list module. Any hints how to do that or even how to make it with this recusrion ?

let rec sumList acc lst =
     match lst with
     | [] -> acc
     | hd::tl -> hd + sumList acc tl
     |hd::_ when hd < 0 ->  sumList acc lst

let sum lst = sumList 0 lst ;;

  sum [1;2;-5] ;;

推荐答案

最后一次非负数之后的所有正数的总和可以这样计算：

The sum of all positive numbers after the last non-negative can be calculated like this:

let lst = [-5; 4; -3; 2; 6; 10]
let afterLastNegative =
    lst
    |> List.rev
    |> Seq.takeWhile (fun x -> x > 0)
    |> Seq.sum
printfn "%A" afterLastNegative

List.rev反转列表然后Seq.takeWhile将获取数字，而函数

List.rev reverses the list then Seq.takeWhile will take numbers while the function

fun x -> x > 0

为true，如果x为正，则返回true。然后Seq.sum简单地将所有数字相加。一个稍微短的版本将是：

is true, and it returns true if x is positive. Then Seq.sum simply sums all the numbers taken. A slighly shorter version would be:

let lst = [-5; 4; -3; 2; 6; 10]
let afterLastNegative =
    lst
    |> List.rev
    |> Seq.takeWhile ((<) 0)
    |> Seq.sum
printfn "%A" afterLastNegative

完全一样。 （<）是函数运算符的名称，并且部分应用了一个运算符，而另一个运算符由Seq.takeWhile函数提供。

It does exactly the same. (<) is the name of the function operator lesser than and is partially applied with one operator, leaving the other to be supplied by the Seq.takeWhile function.

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