[Haskell-beginners] ODP: Re: any feedback on this solution

Wysłane z telefonu Samsung -------- Oryginalna wiadomość -------- Od: Roelof Wobben Data: 07.02.2015 17:01 (GMT+01:00) Do: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Temat: Re: [Haskell-beginners] any feedback on this solution Oke, toDigitsRev was one of the steps of this exercise. but I can remove it. Roelof Joel Williamson schreef op 7-2-2015 om 16:56: Roelof, It doesn't look like you ever call toDigitsRev. To keep the solution clearer, you should remove any unneeded definitions. Joel On Sat, 7 Feb 2015 07:50 Kees Bleijenberg wrote: Roelof, Say, you find after a few months a bug or a better algorithm for toDigits, then you have to remember that sumDigits uses this algorithm (in a bit different form) too. In this case the functions are so small that it is not a problem. If you apply toDigits to all elements of a list of int's and you get almost the solution apply toDigits to alle elements of [4,12,3] and you get [[4],[1,2],[3]], which is a small step to [4,1,2,3] Kees -----Oorspronkelijk bericht----- Van: Beginners [mailto:beginners-bounces@haskell.org] Namens Roelof Wobben Verzonden: zaterdag 7 februari 2015 11:02 Aan: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Onderwerp: Re: [Haskell-beginners] any feedback on this solution 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@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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