Re: [Haskell-beginners] How to select items from one list depending on another

On Wednesday 10 November 2010 16:13:39, David Virebayre wrote:

2010/11/10 C K Kashyap :

...

Hi, I have this problem at hand -

Given two lists

list1 = [0,1,2,0,1,4]

list2 = [1,2,3,4,5,6] I need to take items from the second list only when the corresponding item in list1 is non-zero.

Another wayOne way to do it, although not shorter :

list3 = catMaybes $ zipWith (\a b -> if a > 0 then Nothing else Just b) list1 list2

The function you zip with is \a b -> guard (a /= 0) >> return b, pointfree: (. return) . (>>) . guard . (/= 0) using the MonadPlus instance of Maybe. One could use any MonadPlus with a function [m a] -> [a] removing the mzeros, like catMaybes for Maybe, concat for []: import Control.Monad (guard) list3 = concat $ zipWith ((. return) . (>>) . guard . (/= 0)) list1 list2 but I wouldn't say that's better than list3 = map snd . filter ((/= 0) . fst) $ zip list1 list2 It's not shorter anyway.

David.