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Haskell递归函数

发布于 2024-08-31 03:53:28 字数 229 浏览 6 评论 0原文

请帮助我编写一个函数，该函数接受两个参数：整数列表和索引（int），并返回表中指定索引位置上具有负值的整数列表。

该函数将具有此签名MyReverse :: [Int]->Int->[Int]。

例如：myReverse [1,2,3,4,5] 3 = [1,2,-3,4,5]。

如果索引大于列表的长度或小于0，则返回相同的列表。

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甜味拾荒者 2024-09-07 03:53:28

myReverse :: [Int] -> Int -> [Int]
myReverse [] n = []
myReverse (x:xs) n
 | n < 0     = x:xs
 | n == 0    = (-x):xs
 | otherwise = x:(myReverse xs (n-1))

这是从 0 开始索引数组；您的示例从 1 建立索引，但对于 n == 0 的情况未定义。将其从 1 索引的修复应该相当明显:)

此外，您的大小写不一致； MyReverse 与 myReverse 不同，只有后者作为函数才有效。

结果，在 GHCi 中：

*Main> myReverse [10,20,30,40,50] 0
[-10,20,30,40,50]
*Main> myReverse [10,20,30,40,50] 2
[10,20,-30,40,50]
*Main> myReverse [10,20,30,40,50] 3
[10,20,30,-40,50]
*Main> myReverse [10,20,30,40,50] 5
[10,20,30,40,50]
*Main> myReverse [10,20,30,40,50] (-1)
[10,20,30,40,50]

执行相同操作的更通用版本，使用 myReverse 的毫无意义的定义：

myGeneric :: (a -> a) -> [a] -> Int -> [a]
myGeneric f [] n = []
myGeneric f (x:xs) n
 | n < 0     = x:xs
 | n == 0    = (f x):xs
 | otherwise = x:(myGeneric f xs (n-1))

myReverse :: [Int] -> Int -> [Int]
myReverse = myGeneric negate

myReverse :: [Int] -> Int -> [Int]
myReverse [] n = []
myReverse (x:xs) n
 | n < 0     = x:xs
 | n == 0    = (-x):xs
 | otherwise = x:(myReverse xs (n-1))

That's indexing the array from 0; your example indexes from 1, but is undefined for the case n == 0. The fix to take it to index from 1 should be fairly obvious :)

Also, your capitalisation is inconsistent; MyReverse is different to myReverse, and only the latter is valid as a function.

Results, in GHCi:

*Main> myReverse [10,20,30,40,50] 0
[-10,20,30,40,50]
*Main> myReverse [10,20,30,40,50] 2
[10,20,-30,40,50]
*Main> myReverse [10,20,30,40,50] 3
[10,20,30,-40,50]
*Main> myReverse [10,20,30,40,50] 5
[10,20,30,40,50]
*Main> myReverse [10,20,30,40,50] (-1)
[10,20,30,40,50]

More generic version that does the same thing, using a pointless definition for myReverse:

myGeneric :: (a -> a) -> [a] -> Int -> [a]
myGeneric f [] n = []
myGeneric f (x:xs) n
 | n < 0     = x:xs
 | n == 0    = (f x):xs
 | otherwise = x:(myGeneric f xs (n-1))

myReverse :: [Int] -> Int -> [Int]
myReverse = myGeneric negate

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梅倚清风 2024-09-07 03:53:28

myReverse :: [Int] ->整数-> [国际]
myReverse [] _ = []
myReverse 列表 n
|长度列表< n = 列表
myReverse (x:xs) n
|n == 0 = -x : myReverse xs (n-1)
|否则 = x : myReverse xs (n-1)

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烟织青萝梦 2024-09-07 03:53:28

myReverse :: [Int] -> Int -> [Int]
myReverse [] _ = []
myReverse list n
   |length list < n = list
myReverse (x:xs) n
   |n == 0 = -x : myReverse xs (n-1)
   |otherwise = x : myReverse xs (n-1)

myReverse :: [Int] -> Int -> [Int]
myReverse [] _ = []
myReverse list n
   |length list < n = list
myReverse (x:xs) n
   |n == 0 = -x : myReverse xs (n-1)
   |otherwise = x : myReverse xs (n-1)

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