CODEWITHSUNDEEP
haskell
user9525655
user9525655

Reputation:

How to reverse a created list in Haskell?

I need guidance to how to make a perfect reverse in my code of Haskell. My code:

input :: Int -> [Int]
input x = check_1 x
check_1 x = check_2 (case x of
  0 -> []
  x -> check_1 (x`div`10) ++ [x`mod`10])
check_2 x = reverse x

My output: 

Prelude -> input 12345
Prelude -> [5,3,1,2,4]

How I want:

Prelude -> input 12345
Prelude -> [5,4,3,2,1]

Please assist me in calling the check_2 function is a better way to reverse the list. Thanks

Upvotes: 0

Views: 567

Answers (1)

sepp2k
sepp2k

Reputation: 370112

The problem is that you're reversing the list after each added item, so it ends up a jumble. You want to reverse once after you've added all the items. You can achieve that by getting rid of check_2 and changing input to:

input x = reverse $ check x

However a better solution in this case would be to not reverse at all, but creating the list in the right order in the first place: If you change

check_1 (x`div`10) ++ [x`mod`10])

to

x `mod` 10 : check_1 (x `div` 10)

it will not only be more efficient because you got rid of the ++, but the list will also be in reversed order from the start.

Upvotes: 4

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