[Haskell-beginners] Padding List with Zeros
verdier.jean at gmail.com
Wed Sep 15 04:34:56 EDT 2010
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).
> 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
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