遍历Haskell中的列表拆分功能 [英] Walk through a list split function in Haskell

问题描述

这是我之前的问题的后续操作./p>

我试图从此处了解Haskell中的列表拆分示例:

 文件夹(\ a〜(x，y)->(a:y，x))([]，[]) 

我可以阅读Haskell并知道 folder 是什么，但不理解此代码.您能否指导我看一下这段代码，并对其进行更详细的说明?

解决方案

让我们尝试在示例输入列表上运行此功能，例如 [1,2,3,4,5] :

1. 我们从 foldr(\ a〜(x，y)->((a:y，x))([]，[])[1,2,3,4,5] .这里的 a 是列表的第一个元素，而(x，y)的开头是([[]，[])，所以(a:y，x)返回([1]，[]).
2. 输入列表的下一个元素是 a = 2 ，而(x，y)=([1]，[])，所以(a:y，x)=([2]，[1]).请注意，列表的顺序已交换.每次迭代将再次交换列表.但是，输入列表的下一个元素将始终添加到第一个列表中，这就是拆分的方式.
3. 输入列表的下一个元素是 a = 3 和(x，y)=([2]，[1])，所以(a:y，x)=([3,1]，[2]).
4. 输入列表的下一个元素是 a = 4 和(x，y)=([3,1]，[2])，所以<代码>(a:y，x)=([4,2]，[3,1]).
5. 输入列表的下一个元素是 a = 4 和(x，y)=([4,2]，[3,1])，因此(a:y，x)=([5,3,1]，[4,2]).
6. 没有更多元素了，所以返回值为([5,3,1]，[4,2]).

如演练所示，split函数的工作方式是维护两个列表，在每次迭代时交换它们，并将输入的每个元素附加到另一个列表中.

This is a follow up to my previous question.

I am trying to understand the list splitting example in Haskell from here:

foldr (\a ~(x,y) -> (a:y,x)) ([],[])

I can read Haskell and know what foldr is but don't understand this code. Could you walk me through this code and explain it in more details ?

解决方案

Let’s try running this function on a sample input list, say [1,2,3,4,5]:

1. We start with foldr (\a ~(x,y) -> (a:y,x)) ([],[]) [1,2,3,4,5]. Here a is the first element of the list, and (x,y) start out as ([],[]), so (a:y,x) returns ([1],[]).
2. The next element of the input list is a = 2, and (x,y) = ([1],[]), so (a:y,x) = ([2],[1]). Note that the order of the lists has swapped. Each iteration will swap the lists again; however, the next element of the input list will always be added to the first list, which is how the splitting works.
3. The next element of the input list is a = 3, and (x,y) = ([2],[1]), so (a:y,x) = ([3,1],[2]).
4. The next element of the input list is a = 4, and (x,y) = ([3,1],[2]), so (a:y,x) = ([4,2],[3,1]).
5. The next element of the input list is a = 4, and (x,y) = ([4,2],[3,1]), so (a:y,x) = ([5,3,1],[4,2]).
6. There are no more elements left, so the return value is ([5,3,1],[4,2]).

As the walkthrough shows, the split function works by maintaining two lists, swapping them on each iteration, and appending each element of the input to a different list.

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