[英]Haskell- looping every second element of list
I want to be able to loop every second element of a given list. 我希望能够循环给定列表的每个第二个元素。 I can do this recursively as so:
我可以这样递归地执行此操作:
check validate (x:xs) = check (validate x) (tail xs)
But the problem is that I need a function that accepts a list as parameter, then returns a list consisting of only every second element in the list, starting with (and including) the first element of the list, and I do not think this is possible recursively. 但是问题是我需要一个函数,该函数接受一个列表作为参数,然后返回一个列表,该列表仅由列表中的每个第二个元素组成,并从列表的第一个元素开始(包括),而我不认为这是可能递归。
Can someone show me how to this using list comprehension? 有人可以告诉我如何使用列表理解吗? This would probably be the best approach.
这可能是最好的方法。
second (x:y:xs) = y : second xs;
second _ = []
List comprehension may not be useful. 列表理解可能没有用。
You can also try mutual recursion 您也可以尝试相互递归
first [] = []
first (x:xs) = x:second xs
second [] = []
second (x:xs) = first xs
such as 如
> first [1..10]
[1,3,5,7,9]
> second [1..10]
[2,4,6,8,10]
One of the Haskellish approaches would be something with map
, filter
, and zip
. Haskellish方法之一是使用
map
, filter
和zip
。
second xs = map fst $ filter (odd . snd) $ zip xs [1..]
If you really wanted to use list comprehension, you could use the parallel list comprehension extension. 如果您真的想使用列表理解,可以使用并行列表理解扩展。
{-# LANGUAGE ParallelListComp #-}
second xs = [ x | (x, n) <- [ (x, n) | x <- xs | n <- [1..] ], odd n ]
I think that the former is concise, though. 我认为前者是简洁的。
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