[英]Haskell: Find the minimum in custom data type
So I have a custom data type, let's call it Struct
, defined like this: 因此,我有一个自定义数据类型,我们将其称为Struct
,定义如下:
data Struct = Struct [SubStruct] deriving (Read, Show)
data SubStruct = SubStruct (Int, Int) deriving (Read, Show)
What I need to do is to go through all the elements in Struct
and find the minimum based on fst
and then based on the snd
. 我需要做的是遍历Struct
所有元素,然后根据fst
然后根据snd
找到最小值。 How do I do that? 我怎么做? More specifically, I want to get another SubStruct
such as: 更具体地说,我想获得另一个SubStruct
例如:
SubStruct (-2,-5)
, based on the example in the code. SubStruct (-2,-5)
,基于代码中的示例。
Currently, i started by doing this: 目前,我首先这样做:
import Data.List
import Data.Function (on)
import Data.List (sortBy)
data Struct = Struct [SubStruct] deriving (Read, Show)
data SubStruct = SubStruct (Int, Int) deriving (Read, Show )
struct s sx = Struct(s:sx)
subStruct :: (Int, Int) -> SubStruct
subStruct (x, y) = SubStruct (x, y)
substructs = Struct $ [subStruct (0,1), subStruct (-2, 3), subStruct (4,-5)]
results xs = sortBy (compare `on` fst) (substructs xs)
But I get this error: 但是我得到这个错误:
Couldn't match expected type `t -> [(a, b)]'
with actual type `Struct'
Relevant bindings include
xs :: t (bound at bbox.hs:15:9)
results :: t -> [(a, b)] (bound at file.hs:15:1)
The function `substructs' is applied to one argument,
but its type `Struct' has none
In the second argument of `sortBy', namely `(substructs xs)'
In the expression: sortBy (compare `on` fst) (substructs xs)
Why not use the unzip
function. 为什么不使用unzip
功能。 If we define an auxiliary function: 如果我们定义一个辅助功能:
unSubStruct :: SubStruct -> (Int, Int)
unSubStruct (SubStruct p) = p
Then the function that returns the element that you want can be written as: 然后,可以将返回所需元素的函数编写为:
getMin :: Struct -> SubStruct
getMin (Struct l) = SubStruct (minimum xs, minimum ys)
where
(xs, ys) = unzip $ map unSubStruct l
Note that this will traverse the list twice. 请注意,这将遍历列表两次。 You can avoid that if you define your own version minimum
that works on pairs: 如果您定义自己的minimum
版本对可以工作,则可以避免这种情况:
getMin :: Struct -> SubStruct
getMin (Struct l) =
SubStruct $ foldr1 minPair $ map unSubStruct l
where
minPair (x0, y0) (x, y) = (min x0 x, min y0 y)
You have a list of SubStruct
, which is basically the same as a list of tuples. 您有一个SubStruct
列表,该列表与元组列表基本相同。
So, one solution using only common functions would be: 因此,仅使用通用功能的一种解决方案是:
result = SubStruct (min1, min2) where
min1 = minimum (map fst . list)
min2 = minimum (map snd . list)
list = case substructs of
Struct this -> map (\(SubStruct t) -> t) this
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.