[Haskell-beginners] any feedback on this solution

[Norbert Melzer]

Nope they don't! Lists and arrays are conceptionally very different!

An array is a fixed block of memory where the single elements are lined up
in order, you have constant time access here, but changing the size
requires you to completely reallocate the memory and copy old content over.

A list on the other hand side is dynamic sized, this is bought by a larger
memory footprint per element (pointer to the next one is added) and higher
time for random access.
Am 07.02.2015 11:59 schrieb "Roelof Wobben" <r.wobben at home.nl>:

>  You are right. Im was confusing it because several languages uses these
> words differencely.
>
> Roelof
>
>
>
> Norbert Melzer schreef op 7-2-2015 om 11:41:
>
> I'm sure you don't mean array here, but a list! The difference is crucial!
> Am 07.02.2015 11:01 schrieb "Roelof Wobben" <r.wobben at home.nl>:
>
>> Why,
>>
>> They have opposite terms.
>>
>> SumDigits takes a array and has a integer as output.
>> ToDigits takes a integer and puts  out a array.
>>
>> Roelof
>>
>>
>> Kees Bleijenberg schreef op 7-2-2015 om 10:56:
>>
>>> Roelof,
>>>
>>> Maybe it's better to reuse toDigits in sumDigits.
>>>
>>> Kees
>>>
>>> -----Oorspronkelijk bericht-----
>>> Van: Beginners [mailto:beginners-bounces at haskell.org] Namens Roelof
>>> Wobben
>>> Verzonden: zaterdag 7 februari 2015 10:37
>>> Aan: The Haskell-Beginners Mailing List - Discussion of primarily
>>> beginner-level topics related to Haskell
>>> Onderwerp: [Haskell-beginners] any feedback on this solution
>>>
>>> Hello,
>>>
>>> I finally solved exercise 1 where I had to write a programm which checks
>>> if a creditcard number is valid.
>>>
>>> I solved it this way :
>>>
>>> toDigits :: Integer -> [Integer]
>>> toDigits n
>>>      | n < 0 = []
>>>      | n < 10 = [n]
>>>      | otherwise =  toDigits (n `div` 10) ++ [n `mod` 10]
>>>
>>> -- | convert a number to a reversed array where a negative number will
>>> be a empty array
>>> toDigitsRev :: Integer -> [Integer]
>>> toDigitsRev 0 = [0]
>>> toDigitsRev n
>>>      | n < 0 = []
>>>      | n < 10 = [n]
>>>      | otherwise = n `mod` 10 : toDigitsRev (n `div` 10)
>>>
>>>    -- | Doubles every second number from the right.
>>> doubleEveryOther :: [Integer] -> [Integer]
>>> doubleEveryOther []         = []
>>> doubleEveryOther (x:[])     = [x]
>>> doubleEveryOther (x:(y:zs))
>>>      | length (x:(y:zs)) `mod` 2 /= 0 = [x] ++ (y * 2) :
>>> doubleEveryOther zs
>>>      | otherwise = [x *2]  ++ y : doubleEveryOther zs
>>>
>>>
>>> -- | sum all the digits of a array
>>> sumDigits :: [Integer] -> Integer
>>> sumDigits []  = 0
>>> sumDigits (x:zs)
>>>      | x < 10     = x + sumDigits zs
>>>      | otherwise  = x `mod` 10 + x `div` 10 + sumDigits zs
>>>
>>>
>>> -- | validate a number by looking if a number can be divided by 10
>>> validate :: Integer -> Bool
>>> validate n = sumDigits(doubleEveryOther(toDigits(n))) `mod` 10 == 0
>>>
>>>
>>> -- | The main entry point.
>>> main :: IO ()
>>> main = do
>>>       print $ validate  4012888888881881
>>>
>>> Any remarks about this solution.
>>>
>>> I know there are higher function solutions but that part is not
>>> descrived in chapter 1 .
>>>
>>> Roelof
>>>
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