用文件夹在haskell中实现插入 [英] Implement insert in haskell with foldr
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问题描述
如何在haskell中使用文件夹实现插入. 我试过了:
How to implement insert using foldr in haskell. I tried:
insert'' :: Ord a => a -> [a] -> [a]
insert'' e xs = foldr (\x -> \y -> if x<y then x:y else y:x) [e] xs
没有骰子. 我必须在列表中插入元素e,以便它在大于或等于它的第一个元素之前出现.
No dice. I have to insert element e in list so that it goes before first element that is larger or equal to it.
示例:
insert'' 2.5 [1,2,3] => [1.0,2.0,2.5,3.0]
insert'' 2.5 [3,2,1] => [2.5,3.0,2.0,1.0]
insert'' 2 [1,2,1] => [1,2,2,1]
在最后一个示例中,前2个插入了一个.
In last example first 2 is inserted one.
谢谢@李.
我现在有这个:
insert'' :: Ord a => a -> [a] -> [a]
insert'' e xs = insert2 e (reverse xs)
insert2 e = reverse . snd . foldr (\i (done, l) -> if (done == False) && (vj e i) then (True, e:i:l) else (done, i:l)) (False, [])
where vj e i = e<=i
但是这不起作用:
insert'' 2 [1,3,2,3,3] => [1,3,2,2,3,3]
insert'' 2 [1,3,3,4] => [1,3,2,3,4]
insert'' 2 [4,3,2,1] => [4,2,3,2,1]
解决方案:
insert'' :: Ord a => a -> [a] -> [a]
insert'' x xs = foldr pom poc xs False
where
pom y f je
| je || x > y = y : f je
| otherwise = x : y : f True
poc True = []
poc _ = [x]
感谢@Pedro Rodrigues(它只是想将x> = y更改为x> y.)
Thanks @Pedro Rodrigues (It just nedded to change x>=y to x>y.)
(如何将其标记为已回答?)
(How to mark this as answered?)
推荐答案
这是我的看法:
insert :: Ord a => a -> [a] -> [a]
insert x xs = foldr aux initial xs False
where
aux y f done
| done || x > y = y : f done
| otherwise = x : y : f True
initial True = []
initial _ = [x]
但是,使用foldr
的恕我直言不是最适合此问题的方法,对我来说,以下解决方案更容易理解:
However IMHO using foldr
is not the best fit for this problem, and for me the following solution is easier to understand:
insert :: Int -> [Int] -> [Int]
insert x [] = [x]
insert x z@(y : ys)
| x <= y = x : z
| otherwise = y : insert x ys
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