[英]How to deal with recursing through a list in Haskell (transpose operation)
While I understand that there may be transpose or ZipList functions in Haskell, I am trying to build my own transpose function that will take n lists of equal length m and transpose them into m lists of length n .虽然我知道 Haskell 中可能有转置或 ZipList 函数,但我正在尝试构建我自己的转置函数,该函数将采用n个等长m 的列表并将它们转置为m个长度为n 的列表。
So far I have the function nearly working with the following code:到目前为止,我的功能几乎可以使用以下代码:
list = [[1,2,3],[4,5,6],[7,8,9]]
head' (x:xs) = x
head'' [] = []
head'' (xs:lxs) = head' xs:head'' lxs
tail' [] = []
tail' (x:xs) = xs
tail'' [] = []
tail'' (xs:lxs) = tail' xs:tail'' lxs
merge (xs:lxs) = (head' xs:head'' lxs):(merge (tail' xs:tail'' lxs))
and I get the following output when I run > merge list
in ghci I get:当我在 ghci 中运行 > merge list
时,我得到以下输出:
[[1,4,7],[2,5,8],[3,6,9],[*** Exception: list2.hs:16:1-16: Non-exhaustive patterns in function head'
which I am pretty sure means that the base case of the empty list on my head'
function is missing.我很确定这意味着我head'
的空列表的基本情况丢失了。 The list is transposed, just not closed.该列表已转置,只是未关闭。 How do I deal with that problem in this case?在这种情况下我该如何处理这个问题? I have an inkling that it might have to do with Maybe
, but I'm having trouble implementing it that way.我有一种暗示,它可能与Maybe
,但我无法以这种方式实现它。
You need to add exit conditions:您需要添加退出条件:
merge [] = []
merge ([]:xss) = merge xss
map
is all you need, in addition to the existing head
and tail
functions.除了现有的head
和tail
功能之外, map
就是您所需要的。 For simplicity, this assumes that the input is always a non-empty list (ie, xs
might be [[],[],[]]
, but never []
alone, so there's no problem with using head
or tail
.)为简单起见,这里假设输入始终是一个非空列表(即, xs
可能是[[],[],[]]
,但绝不是单独的[]
,因此使用head
或tail
没有问题。)
> map head list
[1,4,7]
> map tail list
[[2,3],[5,6],[8,9]]
> let foo xs = if null (head xs) then [] else map head xs : foo (map tail xs)
> foo list
[[1,4,7],[2,5,8],[3,6,9]]
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