CODEWITHSUNDEEP
listhaskellrecursion
Rain
Rain

Reputation: 21

Rotate list by the input number

when I try to input the function below. I come up with how to rotate the func when the input is positive, however, when it is negative, I don't how to solve it. The code is below:

-- rotate : Takes a list, and a value, and rotates the list around
-- by the number of elements indicated
rotate :: [a] -> Int -> [a]
rotate ls m = case ls of 
    [] -> [] 
    x:xs 
       | m == 0 -> x:xs
       | otherwise -> rotate (xs ++ [x]) (m-1)

Upvotes: 1

Views: 163

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477598

A simple way to solve this is by seeing a rotation of m places to the left as a rotation of m mod n items to the right, with n the length of the list to rotate.

We thus can implement this as:

rotate :: [a] -> Int -> [a]
rotate xs m
  | m < 0 = rotate xs (m `mod` length xs)
rotate ls m = case ls of 
    [] -> [] 
    x:xs 
       | m == 0 -> x:xs
       | otherwise -> rotate (xs ++ [x]) (m-1)

It might also be better to look for a way to rotate more efficiently than rotating one position at a time.

Upvotes: 1

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