# [Haskell-beginners] Padding List with Zeros

Lorenzo Isella lorenzo.isella at gmail.com
Wed Sep 15 03:28:49 EDT 2010

```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
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>

```

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