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## Simplified Luhn Algorithm

 I have the following, and it works, but I am trying teach myself Haskell, and I have the suspicion that my solutions is both inefficient and graceless. Any feedback would be appreciated.Trent.------------------------------------{-8.The Luhn algorithm is used to check bank card numbers for simple errors such as mistyping a digit, and proceeds as follows: * consider each digit as a separate number; * moving left, double every other number from the second last; * subtract 9 from each number that is now greater than 9; * add all the resulting numbers together; * if the total is divisible by 10, the card number is valid. Define a function luhnDouble :: Int -> Int that doubles a digitand subtracts 9 if the result is greater than 9. For example: > luhnDouble 3 6> luhnDouble 6 3 Using luhnDouble and the integer remainder function mod, define a function luhn :: Int -> Int -> Int -> Int -> Bool that decides if a four-digit bank card number is valid. For example: > luhn 1 7 8 4 True> luhn 4 7 8 3 False In the exercises for chapter 7 we will consider a more general version of this function that accepts card numbers of any length.Hutton, Graham. Programming in Haskell (pp. 45-46). Cambridge University Press. Kindle Edition. -}luhnDouble :: Int -> IntluhnDouble x = if (2 * x) > 9    then (2 * x) - 9    else 2 * xluhn :: Int -> Int -> Int -> Int -> Boolluhn x1 x2 x3 x4 = if 0 == sum[luhnDouble x1, x2, luhnDouble x3, x4] `mod` 10    then True    else False       _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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## Re: Simplified Luhn Algorithm

 I guess your issue is how to represent the card numbers of arbitrary length, or? Wouldn't a list work?PatrikDen 31 dec 2017 05:03 skrev "trent shipley" <[hidden email]>:I have the following, and it works, but I am trying teach myself Haskell, and I have the suspicion that my solutions is both inefficient and graceless. Any feedback would be appreciated.Trent.------------------------------------{-8.The Luhn algorithm is used to check bank card numbers for simple errors such as mistyping a digit, and proceeds as follows: * consider each digit as a separate number; * moving left, double every other number from the second last; * subtract 9 from each number that is now greater than 9; * add all the resulting numbers together; * if the total is divisible by 10, the card number is valid. Define a function luhnDouble :: Int -> Int that doubles a digitand subtracts 9 if the result is greater than 9. For example: > luhnDouble 3 6> luhnDouble 6 3 Using luhnDouble and the integer remainder function mod, define a function luhn :: Int -> Int -> Int -> Int -> Bool that decides if a four-digit bank card number is valid. For example: > luhn 1 7 8 4 True> luhn 4 7 8 3 False In the exercises for chapter 7 we will consider a more general version of this function that accepts card numbers of any length.Hutton, Graham. Programming in Haskell (pp. 45-46). Cambridge University Press. Kindle Edition. -}luhnDouble :: Int -> IntluhnDouble x = if (2 * x) > 9    then (2 * x) - 9    else 2 * xluhn :: Int -> Int -> Int -> Int -> Boolluhn x1 x2 x3 x4 = if 0 == sum[luhnDouble x1, x2, luhnDouble x3, x4] `mod` 10    then True    else False       _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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## Re: Simplified Luhn Algorithm

 In reply to this post by trent shipley Looks fine to me. Maybe drop the if-then, and simply return the result of the == ? (Maybe not possible in Haskell (I'm just a duffer myself) but extraneous trues and falses always drive me nuts.)--Sent from my tablet,  which has a funny keyboard. Makes me sound more curt and muted than normal. On Dec 30, 2017 11:03 PM, "trent shipley" <[hidden email]> wrote:I have the following, and it works, but I am trying teach myself Haskell, and I have the suspicion that my solutions is both inefficient and graceless. Any feedback would be appreciated.Trent.------------------------------------{-8.The Luhn algorithm is used to check bank card numbers for simple errors such as mistyping a digit, and proceeds as follows: * consider each digit as a separate number; * moving left, double every other number from the second last; * subtract 9 from each number that is now greater than 9; * add all the resulting numbers together; * if the total is divisible by 10, the card number is valid. Define a function luhnDouble :: Int -> Int that doubles a digitand subtracts 9 if the result is greater than 9. For example: > luhnDouble 3 6> luhnDouble 6 3 Using luhnDouble and the integer remainder function mod, define a function luhn :: Int -> Int -> Int -> Int -> Bool that decides if a four-digit bank card number is valid. For example: > luhn 1 7 8 4 True> luhn 4 7 8 3 False In the exercises for chapter 7 we will consider a more general version of this function that accepts card numbers of any length.Hutton, Graham. Programming in Haskell (pp. 45-46). Cambridge University Press. Kindle Edition. -}luhnDouble :: Int -> IntluhnDouble x = if (2 * x) > 9    then (2 * x) - 9    else 2 * xluhn :: Int -> Int -> Int -> Int -> Boolluhn x1 x2 x3 x4 = if 0 == sum[luhnDouble x1, x2, luhnDouble x3, x4] `mod` 10    then True    else False       _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners