Lorenzo Isella
Dear All, I still have to find my way with immutable lists and list comprehension. Consider the following lists
A=[0,10,20,30,40,50] B=[0,10,50] (i.e. B is a subset of list A; list A is already ordered in increasing order and so is B). C=[2,1,-5] i.e. there is a corresponding element in C for every element in B.
Now, I would like to define a new list D having length equal to the length of A. The elements of D in the position of the elements of A in common with B are equal to the corresponding entries in C, whereas the other ones are zero i.e. D=[2,1,0,0,0,-5]. How can I achieve that? The first thought that comes to my mind is to define a list of zeros which I would modify according to my needs, but that is not allowed...
Here is a simple approach using Data.Map: import qualified Data.Map as M substSubset :: (Num b, Ord a) => [a] -> [b] -> [a] -> [b] substSubset ys zs = map (\x -> M.findWithDefault 0 x (M.fromList $ zip ys zs)) First build a map from [0, 10, 50] to [2, 1, -5], then map a function over [0, 10 .. 50], which tries to find each corresponding number in the map with a default of 0. But do you really need zero values? If your zeroes actually represent lack of a value, consider using Maybe instead: [Just 2, Just 1, Nothing, Nothing, Nothing, Just (-5)] instead of: [2, 1, 0, 0, 0, -5] Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/