Question

Edit: I must not have worded it clearly enough, but I’m looking for a function like the one below, but not exactly it.

Given a list, I wanted to be able to find the index of the largest element in the list
(So, list !! (indexOfMaximum list) == maximum list)
I wrote some code that seems pretty efficient, although I feel I’m reinventing the wheel somewhere.

indexOfMaximum :: (Ord n, Num n) => [n] -> Int
indexOfMaximum list =
   let indexOfMaximum' :: (Ord n, Num n) => [n] -> Int -> n -> Int -> Int
       indexOfMaximum' list' currIndex highestVal highestIndex
          | null list'                = highestIndex
          | (head list') > highestVal = 
               indexOfMaximum' (tail list') (1 + currIndex) (head list') currIndex
          | otherwise                 = 
               indexOfMaximum' (tail list') (1 + currIndex) highestVal highestIndex
   in indexOfMaximum' list 0 0 0

Now I want to return a list of the indices of the largest n numbers in the list.

My only solution is to store the top n elements in a list and replace (head list') > highestVal with a comparison across the n-largest-so-far list.

It feels like there has to be a more efficient way than to do this, and I also feel I’m making insufficient use of Prelude and Data.List. Any suggestions?

Answer 1

This solution associates each element with its index, sorts the list, so the smallest element is first, reverses it so the largest element is first, takes the first n elements, and then extracts the index.

maxn n xs = map snd . take n . reverse . sort $ zip xs [0..]