list - 输出一个元素我想要的次数

Question

我有这个代码:

lado ::  [([Char],Int)] -> [[Char]]
lado xs = [a | (a,b) <- xs]

我需要输出这个:

> lado [("A",3),("B",2),("C",1)]
["A","B","C","A","B","A"]

我必须输出 "A"3 次，然后输出 "B"2 次，然后输出 "C"1 次，但我只能用这段代码得到这个 ["A","B","C"]。

Answer 1

您可以使用递归来完成此操作:

lado :: [(a, Int)] -> [a]
-- base case
lado [] = []
-- take each of the non-zero elements then recurse
lado xs = map fst nonzero ++ lado subtracted
    where
        -- find elements with non-zero count
        nonzero = filter (\x -> snd x > 0) xs
        -- subtract one from the count for each of those elements
        subtracted = map (\(x, n) -> (x, n - 1)) nonzero