haskell - 添加列表中每第 5 个(或第 n 个)元素的方法是什么？

Question

我一直在尝试解决此 question 提供的示例在 StackOverflow 上。我误解了它。我想从列表中删除每 5 个元素并将它们一起放入另一个列表中。

这是迄今为止我最好的尝试。

fifth(_:_:_:_xs) = xs fifth xs
fifth [] = []

Answer 1

最好将您的关注点分开:创建一个函数，对列表进行“切片”并获取每五个元素，然后对该列表求和。

fifth :: [a] -> [a]
fifth (x:_:_:_:_:xs) = x : fifth xs
fifth (x:_) = [x]
fifth [] = []

然后对元素列表求和:

sumFifth :: Num a => [a] -> a
sumFifth = sum . <b>fifth</b>