Question

I have a high order function which would take:

results ["Red", "Blue", "Green", "Blue", "Blue", "Red"]

And return:

[(1,"Green"),(2,"Red"),(3,"Blue")]

I need to use the results function and create a new function called winner:

winner :: [Party ] -> Party
winner xs = 

which would output the most occurred colour and remove first element within tuples, if two colours have the same occurrences it output two the colours, for example:

winner ["Red", "Blue", "Green", "Blue", "Blue", "Red"]

output:

"blue"

so far I’ve tried using snd and tail but I keep getting errors.
Thank you in advance.

Answer 1

If you have the results function, a simple application of

maximumBy :: (a -> a -> Ordering) -> [a] -> a

would suffice

import Data.List -- for maximumBy
import Data.Ord  -- for comparing

winner = snd . maximumBy (comparing fst) . results

Seeing

if two colours have the same occurrences it output two the colours

you need a different type – and of course a different implementation, still easy with library functions, though:

import Data.List
import Data.Ord
import Data.Function

winners :: [Party] -> [Party]
winners = map snd . head . groupBy ((==) `on` fst) . sortBy (flip $ comparing fst) . results

Since results already produces a sorted list – I didn’t want to rely on that just from the example output – it is not necessary to sort again, and we can use

winners = map snd . last . groupBy ((==) `on` fst) . results

to produce the list of all most occurring elements. If only one most frequent element is desired, it could be obtained with

winner = snd . last . results

from the sorted frequency list.