[Haskell-beginners] Padding List with Zeros

[jean verdier]

list_A = [0,10,20,30,40,50]
list_B = [0,10,50] 
list_C = [2,1,-5]

f a b c = g a (zip b c)

g []     _              = []
g (a:as) xs@((b,c):xs')
  | a > b  = error "b is not a subset of a"
  | a == b = c : g as xs'
  | a < b  = 0 : g as xs
             
main = do
  print (f list_A list_B list_C)



On Wed, 2010-09-15 at 09:28 +0200, Lorenzo Isella wrote:
> Hi Antoine,
> Unfortunately these are really truly lists and not sets (for instance, 
> the ordering of elements matter and some of them may be repeated).
> 
> Lorenzo
> 
> On 09/15/2010 01:55 AM, Antoine Latter wrote:
> > Are these truly lists, or would you be better suited using Sets, Maps or
> > IntMaps?
> >
> > Then you can use some of the unionWith functions to decide what to
> > insert, or you can simply wrap the looking functions to return zero on
> > failure.
> >
> 
> 
> 
> > Antoine
> >
> > On Sep 14, 2010 6:35 PM, "Lorenzo Isella" <lorenzo.isella at gmail.com
> > <mailto:lorenzo.isella at gmail.com>> wrote:
> >  > 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...
> >  > Many thanks
> >  >
> >  > Lorenzo
> >  > _______________________________________________
> >  > Beginners mailing list
> >  > Beginners at haskell.org <mailto:Beginners at haskell.org>
> >  > http://www.haskell.org/mailman/listinfo/beginners
> >
> 
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners