Haskell中是否有“继续”？

Posted 2021-04-29

我最近开始学习Haskell，目前正在尝试编写基本的Haskell函数。

我已经编写了一个名为intToRoman的函数，该函数应该将整数转换为罗马数字。它将整数中的数字除以整数（1400-> [1,4,0​​,0]），然后将每个数字转换为罗马数字，并考虑列表的长度，以确定何时是一千或一百。

但是，它不会停止并且也会检查零。例如，数字1400将返回：

 MCD** Exception: Map.!: given key is not an element in the map
 CallStack (from HasCallStack)

这里是代码本身：

mapInttoRoman:: M.Map Int String
mapInttoRoman = M.fromList
          [(1,"I"),(4,"IV"),(5,"V"),(9,"IX"),(10,"X"),(40,"XL")
          ,(50,"L"),(100,"C"),(400,"CD"),(900,"CM"),(500,"D"),(1000,"M")]

listOfInt :: Int -> [Int]
listOfInt 0 = [ ]
listOfInt c = listOfInt(c`div`10) ++ [c`mod`10]

duplicate :: String -> Int -> String
duplicate string n = concat $ replicate n string

intToRoman :: Int -> String
intToRoman 0 = ""
intToRoman c = createInt x (len-1)
 where x = listOfInt c
       len = length x
       createInt y xz = findRoman (head y) xz ++ createInt(drop 1 y)(xz-1)
        where findRoman b l
               | l < 1      = mapInttoRoman M.! b
               | b == 0     = " "
               | l == 3     = duplicate (mapInttoRoman M.! (10^l)) b
               | b == 1     = mapInttoRoman M.! (10^l)
               | otherwise  = mapInttoRoman M.! (b*10^l)

答案

我认为您使用的算法不正确。并且您使它变得比需要的更加专注。

也未完成列表，我添加了90和900个案例。

查看此代码..我认为它非常简单，您将轻松获得它。

将合格的Data.Map导入为M

mapInttoRoman:: M.Map Int String
mapInttoRoman = M.fromList
          [(1,"I"),(4,"IV"),(5,"V"),(9,"IX"),(10,"X"),(40,"XL")
          ,(50,"L"),(90,"XC"),(100,"C"),(400,"CD"),(500,"D"),(900,"CM"),(1000,"M")]

intToRoman :: Int -> String
intToRoman 0 = ""
intToRoman n | M.member n mapInttoRoman = mapInttoRoman M.! n
             | otherwise = val ++ intToRoman (n-key) 
             where
                keys = reverse (M.keys mapInttoRoman)
                key  = maxValidKey n keys
                val  = mapInttoRoman M.! key
                maxValidKey n (k:ks) 
                  | n >= k = k
                  | otherwise = maxValidKey n ks 

测试：