On 2014-02-11 15:54, James Toll wrote:
If I have a list of lists grouped by element value.
ghci> group [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,2,2,7] [[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[2,2],[7]]
I would like to take the subset of the outer list containing only the longest of the inner lists for any particular element.
So for this particular example, the desired output would be:
[[1,1,1,1],[2,2,2,2],[3,3],[5],[6],[7]] Any thoughts on how to do this would be appreciated.
After grouping the given list, so that you have [[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[2,2],[7]] Sort the list by comparing the first element of each list ghci> sortBy (comparing head) [[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[2,2],[7]] [[1,1,1,1],[2,2,2,2],[2,2,2],[2,2],[3,3],[5],[6],[7]] Then, group that again such that lists with equal elements get put into one list: ghci> groupBy ((==) `on` head) [[1,1,1,1],[2,2,2,2],[2,2,2],[2,2],[3,3],[5],[6],[7]] [[[1,1,1,1]],[[2,2,2,2],[2,2,2],[2,2]],[[3,3]],[[5]],[[6]],[[7]]] Finally, select the "maximum" of each inner list by comparing the length of the sub-sub-lists: ghci> map (maximumBy (comparing length)) [[[1,1,1,1]],[[2,2,2,2],[2,2,2],[2,2]],[[3,3]],[[5]],[[6]],[[7]]] You'll need "Data.List", "Data.Ord" and "Data.Function" for this. -- Frerich Raabe - raabe@froglogic.com www.froglogic.com - Multi-Platform GUI Testing