February 2015 - Beginners

Code review request
by Douglas Christman 10 Feb '15

10 Feb '15

Hi everyone, After spending some time reading "Learn you a Haskell," I decided to try writing my first Haskell program (https://github.com/dobyrch/musicbox) It reads ASCII "sheet music" and uses the terminal bell to play the song as it displays the frequency of each note. I'm looking for any tips that would make my code look more idiomatic. Please point out any places where I could simplify the logic or use standard library functions in place of custom code. Style suggestions are also welcome--let me know if you see anything ugly that could be written in a more readable way. Thanks! Doug

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any feedback on this solution
by Roelof Wobben 08 Feb '15

08 Feb '15

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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ODP: Re: any feedback on this solution
by Marek Wawrzyczek 08 Feb '15

08 Feb '15

Wysłane z telefonu Samsung -------- Oryginalna wiadomość -------- Od: Roelof Wobben <r.wobben(a)home.nl> Data: 07.02.2015 17:01 (GMT+01:00) Do: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> 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 <K.Bleijenberg(a)lijbrandt.nl> 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 > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2015.0.5646 / Virus Database: 4281/9071 - Release Date: > 02/07/15 > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2015.0.5646 / Virus Database: 4281/9071 - Release Date: 02/07/15 _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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sum problem
by Roelof Wobben 07 Feb '15

07 Feb '15

Hello, The exercises of CIS 194 are really hard. Now I have to sum all the numbers in a array with a catch If a number is greater then 10 it has to be dived in single numbers. so [ 1 , 12, 3] will be [ 1, 1,2,3] and then sum up. So far I have this : -- | sum all the digits of a array sumDigits :: [Integer] -> Integer sumDigits n | n == 0 = acc | n < 10 = acc + n | otherwise = sumDigits acc + n `mod` 10 + n `rem` 10 But now I see a message that acc is not known which is not wierd. But when I do sumDigits n acc I see a message that sumDigits has only 1 parameter where I give it to, When I only had to sum up i is easy , I could use map [+] xs Anyone who can give a me tip without giving the answer otherwise I do not learn anything. Roelof

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recursion problem.
by Roelof Wobben 07 Feb '15

07 Feb '15

Hello, I have figured out how I can make from 123 [1,2,3] I did it this way : -- | convert a number to a array in pieces where a negative number will be a empty array. toDigits :: Integer -> [Integer] toDigits n | n <= 0 = [] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] but now when I do toDigits 0 , I see [] as output where I was expected [0] But when I do toDigits 100 I see the right output [ 1,0,0] which surprises me because I tought that with the first 0 there will be a [] Or is the n the second time [0,0] and the thirth time [0] So it will be like this : toDigits 100 to Digits [1] ++ [ 0,0] toDigits [1] ++ [0] ++ [0] which leads to [1,0,0] Roelof

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Re: [Haskell-beginners] length problem
by Roelof Wobben 06 Feb '15

06 Feb '15

Oke, I have solved it already. The problem was that I did list.length but Haskell uses length list Still too much Ruby in my system :( Roelof Francesco Ariis schreef op 6-2-2015 om 17:50: > (off-list) please consider not using html in your mails, it's quite difficult > to read them for us who plaintext-friendly client > > On Fri, Feb 06, 2015 at 05:47:24PM +0100, Roelof Wobben wrote: >> <html> >> <head> >> >> <meta http-equiv="content-type" content="text/html; charset=windows-1252"> >> </head> >> <body bgcolor="#FFFFFF" text="#000000"> >> Hello, >> >> I have to double every second element from the right. >> >> So for a even length array that means : 1 , 3 , 5 and so on >> and for a non even lenght that means the 2,4 and so on. >> >> So I thought I could solve that on this way : >> >> -- | Doubles every second number from the right. >> doubleEveryOther :: [Integer] -> [Integer] >> doubleEveryOther [] = [] >> doubleEveryOther (x:[]) = [x] >> doubleEveryOther (x:(y:zs)) >> | ((x:(y:zs)).length) `mod` 2 /= 0 = [x] ++ (y * 2) : >> doubleEveryOther zs >> | otherwise = [x *2] ++ y : doubleEveryOther zs >> >> >> >> but this does not work because I see this error message : >> >> <div class="ide-error-span">src/Main.hs@14:8-14:16 </div> >> <div class="ide-error-msg">Couldn't match expected type ‘Int >> -> c0’ with actual type >> <div class="CodeMirror cm-s-default" style="font-size: 14px;">[> class="cm-variable-2">Integer]</div> >> > class="ide-error-collapse-btn"> …> inline;"> >> In the first argument of ‘(.)’, namely ‘(x : (y : zs))’ >> In the first argument of ‘mod’, namely ‘((x : (y : zs)) . >> length)’ >> In the first argument of ‘(/=)’, namely ‘((x : (y : zs)) . >> length) `mod` 2’ >> >> >> Can anyone give me a better way to check if I have a even or odd >> length array ? >> >> Roelof >> >> </div> >> >> </body> >> </html> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >

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length problem
by Roelof Wobben 06 Feb '15

06 Feb '15

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Couldn't match expected type ‘IO ()’ with actual type [Integer]
by Roelof Wobben 06 Feb '15

06 Feb '15

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Subject: Re: File I/O: reading from, writing to,
by Doug McIlroy 05 Feb '15

05 Feb '15

same file in one function One more safety feature in the "proper way": in most systems it will guarantee that a concurrent reader sees a coherent file image. ------------------------------------------------------------ > I strongly urge you to use the proper way with temp file and renaming, even making it a function if you need to repeat this several times The "proper way" is fraught with pitfalls: potentially changes in file ownership and permissions, loss of linkage when the file is known by multiple names, and even total failure if you don't have write permission on the directory.^C The proper way does have one safety advantage. The original file is highly likely to survive should the writing of the new contents stop prematurely for some external reason. Doug

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Re: [Haskell-beginners] File I/O: reading from, writing to,
by Doug McIlroy 05 Feb '15

05 Feb '15

same file in one function > I strongly urge you to use the proper way with temp file and renaming, even making it a function if you need to repeat this several times The "proper way" is fraught with pitfalls: potentially changes in file ownership and permissions, loss of linkage when the file is known by multiple names, and even total failure if you don't have The proper way does have one safety advantage. The original file is highly likely to survive should the writing of the new contents stop prematurely for some external reason. Doug

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