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haskell - 哈斯克尔。两个列表的 INNER JOIN

问题描述

我一直在问谷歌并试图找到必要的工具来做到这一点，但似乎无法找到解决我的问题的方法。整个任务是使用字典来翻译列表并仅保留唯一值。例如以下输入：

dict = [("a", "aa"), ("b", "bb")]
list1 = ["a","b","c","a","a","g"]

应该产生以下输出：

result = ['aa', 'bb']

这是我到目前为止所困的地方：

main = do
    let dict = [("a", "aa"), ("b", "bb")]
    let list1 = ["a","b","c","g"]
    let keys = map fst dict
    let values = map snd dict
    let result = replace keys values list1
    print(result)

产生

["aa","bb","c","g"]

所以为了解决这个问题，我在想可能有一种方法可以使用filter或以map某种foldl方式进行内部连接，如下所示：

let result = innerJoin values result

目前我有一些看起来像这样的东西：

innerJoin :: (Eq a) =>[a] -> [a] -> [a]
innerJoin xs     []     = xs
innerJoin []     ys     = ys
innerJoin (x:xs) (y:ys) = if (elem x (y:ys))
                         then x: innerJoin xs ys
                         else innerJoin xs ys


main = do
    let dict = [("a", "aa"), ("b", "bb")]
    let list1 = ["a","b","c","g"]
    let keys = map fst dict
    let values = map snd dict
    let list2 = innerJoin keys list1
    let result = replace keys values list2
    print(result)

但它返回["aa","bb","c","g"]而不是预期的 ["aa","bb"]。最后我打算用 来完成它nub，但我正在努力找出 innerJoin 部分。

编辑：

感谢下面的答案，这里是问题的解决方案：

innerJoin xs     []     = []
innerJoin []     ys     = []
innerJoin (x:xs) (y:ys) = if (elem x (y:ys))
                         then x: innerJoin xs ys
                         else innerJoin xs ys
catMaybes ls = [x | Just x <- ls]
genList x [] = []
genList [] y = []
genList x (y:ys) = lookup y x: genList x ys
func dict list =  do
    let keys = map fst dict
    let list1 = innerJoin keys list
    catMaybes (genList dict list1)
test1 = func [("a", "aa"),("e", "bb")] ["a","b","c","g","a"]
test2 = func [(1,11),(2,11),(4,44)] [1,2,3,1,2]

标签： haskell

解决方案

你想要的功能是lookup. 例如：

lookup "a" [("a", "aa"), ("b", "bb")] = Just "aa"
lookup "b" [("a", "aa"), ("b", "bb")] = Just "bb"
lookup "x" [("a", "aa"), ("b", "bb")] = Nothing   -- "x" is not in the list

在这里有用的另一个功能是Data.Maybe.catMaybes：如果您有一个喜欢[Just 1, Just 2, Nothing, Just 3, Nothing]并应用catMaybes到它的列表，您会得到[1, 2, 3]. 所以你可以结合起来lookup得到catMaybes你catMaybes $ map (flip lookup dict) list1想要的解决方案。另一个改进是要注意Data.Maybe定义mapMaybe f xs为等效于catMaybe (map f xs)，因此您可以将其简化为mapMaybe (flip lookup dict) list1。

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