I have to write a function that flattens a list of lists.
For example flatten [] = []
or flatten [1,2,3,4] = [1,2,3,4]
or flatten [[1,2],[3],4,5]] = [1,2,3,4,5]
I'm having trouble with the being able to match the type depending on what is given to the flatten function.
Here's what I have:
data A a = B a | C [a] deriving (Show, Eq, Ord)
flatten::(Show a, Eq a, Ord a)=>A a -> A a
flatten (C []) = (C [])
flatten (C (x:xs) ) = (C flatten x) ++ (C flatten xs)
flatten (B a) = (C [a])
From what I can tell the issue is that the ++
operator is expecting a list for both of its arguments and I'm trying to give it something of type A
. I've added the A
type so the function can either get a single element or a list of elements.
Does anyone know a different way to do this differently, or explain what I can do to fix the type error?
Flattening lists means converting a multidimensional or nested list into a one-dimensional list. For example, the process of converting this [[1,2], [3,4]] list to [1,2,3,4] is called flattening.
To flatten a list of lists in Python, use the numpy library, concatenate(), and flat() function. Numpy offers common operations, including concatenating regular 2D arrays row-wise or column-wise. We also use the flat attribute to get a 1D iterator over the array to achieve our goal.
It's a bit unclear what you are asking for, but flattening a list of list is a standard function called concat
in the prelude with type signature [[a]] -> [a]
.
If you make a data type of nested lists as you have started above, maybe you want to adjust your data type to something like this:
data Lists a = List [a] | ListOfLists [Lists a]
Then you can flatten these to a list;
flatten :: Lists a -> [a]
flatten (List xs) = xs
flatten (ListOfLists xss) = concatMap flatten xss
As a test,
> flatten (ListOfLists [List [1,2],List [3],ListOfLists [List [4],List[5]]])
[1,2,3,4,5]
Flatten via list comprehension.
flatten arr = [y | x<- arr, y <- x]
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