[Haskell-beginners] Padding List with Zeros

[Greg]

> I'm sorry to have posted incomplete and uncommented code, I should think
> twice next time.

I can hardly complain after having done the same two posts earlier...  =)

I botched the same condition, too-- go figure.



On Sep 15, 2010, at 1:54 AM, jean verdier wrote:

> You're right, i didn't compile with -fwarn-incomplete-patterns and it
> bite me.
> You may also write
> g as [] = map (\_ -> 0) as
> or some combination of take length and repeat.
> g as [] = take (length as) (repeat 0)
> 
> I'm sorry to have posted incomplete and uncommented code, I should think
> twice next time.
> 
> 
> On Wed, 2010-09-15 at 01:43 -0700, Greg wrote:
>> I think you also need to handle the case where the second parameter to g is empty (where list_A has non-matching elements after list_B is exhausted).
>> 
>> g (a:as) [] = 0 :  g as []
>> 
>> or something similar in between the two existing definitions?
>> 
>> 
>> On Sep 15, 2010, at 1:34 AM, jean verdier wrote:
>> 
>>> 
>>> 
>>> 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
>>>>>> _______________________________________________
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>>>>>> http://www.haskell.org/mailman/listinfo/beginners
>>>>> 
>>>> 
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