从字符串中删除每个第 n 个元素

Question

如何删除字符串的每个第 n 个元素？

我猜你会以某种方式使用drop 函数。

像这样丢弃第一个 n，你怎么能改变它，只丢弃第 n 个，然后是之后的第 n 个，依此类推，而不是全部？

dropthem n xs = drop n xs

Answer 1

我喜欢以下解决方案：

del_every_nth :: Int -> [a] -> [a]    
del_every_nth n = concat . map init . group n 

您只需要定义一个函数group，它将一个列表分组为长度为n 的部分。但这很容易：

group :: Int -> [a] -> [[a]]
group n [] = []
group n xs = take n xs : group n (drop n xs)

Answer 2

remove_every_nth :: Int -> [a] -> [a]
remove_every_nth n = foldr step [] . zip [1..]
    where step (i,x) acc = if (i `mod` n) == 0 then acc else x:acc

函数的作用如下：

zip [1..] 用于索引列表中的所有项目，例如zip [1..] "foo" 变为 [(1,'f'), (2,'o'), (3,'o')]。

然后使用right fold 处理索引列表，该right fold 会累积索引不能被n 整除的每个元素。

这里有一个稍长的版本，基本上做同样的事情，但避免了来自zip [1..] 的额外内存分配，并且不需要计算模数。

remove_every_nth :: Int -> [a] -> [a]
remove_every_nth = recur 1
    where recur _ _ []     = []
          recur i n (x:xs) = if i == n
            then recur 1 n xs
            else x:recur (i+1) n xs

Answer 3

简单。取 (n-1) 个元素，然后跳过 1 个，冲洗并重复。

dropEvery _ [] = []
dropEvery n xs = take (n-1) xs ++ dropEvery n (drop n xs)

为了效率，还是采用showS风格

dropEvery n xs = dropEvery' n xs $ []
    where dropEvery' n [] = id
          dropEvery' n xs = (take (n-1) xs ++) . dropEvery n (drop n xs)

Answer 4

-- groups is a pretty useful function on its own!
groups :: Int -> [a] -> [[a]]
groups n = map (take n) . takeWhile (not . null) . iterate (drop n)

removeEveryNth :: Int -> [a] -> [a]
removeEveryNth n = concatMap (take (n-1)) . groups n

Answer 5

尝试结合take 和drop 来实现这一点。

take 3 "hello world" = "hel"
drop 4 "hello world" = "o world"