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我想要一个带有签名 :: [(Int,Char)] -> [String] 的函数，它接受每个 Char 并根据相应对中的整数值将它们组合成一个字符串列表......

IE。

[(0,'A'),(1,'B'),(2,'C'),(3,'D'),(0,'E'),(1,'F'),(2,'G'),(3,'H'),(0,'I'),(1,'J'),(2,'K'),(3,'L'),(0,'M'),(1,'N'),(2,'O'),(3,'P')]

应该生成列表：["MIEA","NJFB","OKGC","PLHD"]

我试过了：

toList :: [(Int,Char)] -> Int -> [String]
toList [] a = []
toList (x:xs) a = [snd(x) | n <-[0..(a-1)], fst(x) == n]:toList xs a

但这只是给了我：

["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P"]

任何建议将不胜感激。

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2 回答 2

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您需要按索引对输入进行排序，然后将所有具有相同索引的对分组。类似下一个：

map (reverse . map snd) $ groupBy ((==) `on` fst) $ sortBy (compare `on` fst) $ input

无$运算符：

import Data.List
import Data.Function

toList :: [(Int, Char)] -> [String]
toList input = map (reverse . map snd) grouped
  where
  sorted = sortBy (compare `on` fst) input    -- sorted by the index
  grouped = groupBy ((==) `on` fst) sorted    -- grouped by the index
于 2013-02-03T21:10:35.743 回答
1

现在我的作业截止日期已经过去，我可以包括我的整个问题......该功能被要求将模拟的“图片”顺时针旋转 90 度。我想出的解决方案（使用简单的 Haskell 代码，因为这是我们的第一个任务）如下。虽然它适用于简单的“方形”字符串列表，但它似乎不适用于更多矩形列表......不知道为什么。

{- The function rotate90 rotates a "picture" (List of Strings) through
   90 degrees clockwise and prints the list to the standard output device
   using the printPicture function defined in the Thompson Picture.hs file.
-}

-- the following String list is a sample used to test implementation:
pic :: [String]
pic = [ "ABCD", "EFGH", "IJKL", "MNOP", "QRST" ]

rotate90 :: [String] -> IO ()
rotate90 aPic   = printPicture(convertToPic (listPairs aPic (maxOfList(widthOfLines aPic))     ((maxOfList(widthOfLines aPic))*(length(aPic)))) ([0..((maxOfList(widthOfLines aPic))-1)]))

-- takes a picture, the width and the number of Chars in a rectangle
-- bounding the picture (area) and converts it to a list of pairs with
-- an address and Char as per the cypher algorithm:
-- address = floor of (area -1)/ width
listPairs :: [String] -> Int -> Int -> [(Int,Char)]
listPairs [] w area = []
listPairs aPic w area   = [ (((n-1)`mod`w),(aPic!!(floor(fromIntegral(n-1)/(fromIntegral(w))))!!((n-1)`mod`w))) | n <- [1..area] ]

-- takes a list of tuples (pairs) and an Int which is the width of the input picture
-- it retunrs a list of Strings that correspond to the groupings of the pairs.
convertToPic :: [(Int,Char)] -> [Int] -> [String]
convertToPic [] (y:ys)  = []
convertToPic somePair []    = []
convertToPic somePair (y:ys)= reverse([ snd(x) | x <- somePair, ((fst(x)) == y)]):(convertToPic somePair ys)

-- takes a list of Int and returns the maximum value from the list as an Int
maxOfList :: [Int] -> Int
maxOfList []        = 0
maxOfList (x:xs)        = max x (maxOfList xs)

-- takes a list of type String and returns a list of Int of the length of each string
widthOfLines :: [String] -> [Int]
widthOfLines []     = []
widthOfLines (x:xs)     = (length x):(widthOfLines xs)

-- definition from Thompson (textbook) picture.hs file
printPicture :: [String] -> IO ()
printPicture = putStr . concat . map (++"\n")
于 2013-02-20T07:33:03.487 回答