[Haskell-beginners] can I have a case statement with a pattern matching

[Jonathan Skårstedt]

I'm nitpicking here, but it can be improved a bit

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther [] = []
doubleEveryOther [x] = [x]
doubleEveryOther (x:y:xs) = x : 2 * y : doubleEveryOther (xs)

2015-05-14 8:48 GMT+02:00 Nishant <nishantgeek at gmail.com>:

> This will double very second digit in list.  To do from right, just
> reverse the list first and then apply doubleEverySecond and then reverse
> back.
>
>
>
> doubleEveryOther :: [Integer] -> [Integer]
>
> doubleEveryOther [] = []
> doubleEveryOther (x : []) = [x]
> doubleEveryOther (x : y : []) = x : (2 * y) : []
> doubleEveryOther (x:y:xs) = x : (2 * y) : doubleEveryOther (xs)
>
>
> On Wed, May 13, 2015 at 10:22 PM, Roelof Wobben <r.wobben at home.nl> wrote:
>
>> Hello,
>>
>> I try to make make the assignment of CIS194 where I must multiply every
>> second item from the right by 2
>>
>> So I thought this could be working :
>>
>> -- | Convert a digit to a reversed list for example 123 becomes [3,2,1]
>> revtoDigits :: Integer -> [Integer]
>> revtoDigits number
>>   | number <= 0 = []
>>   | number > 0 = number`mod` 10 : revtoDigits (number `div` 10)
>>
>> -- | Convert a digit to a list for example 123 becomes [1,2,3]
>> toDigits :: Integer -> [Integer]
>> toDigits number
>>   | number <= 0 = []
>>   | number > 0 = toDigitsAcc [] number
>>
>> toDigitsAcc:: [Integer] -> Integer -> [Integer]
>> toDigitsAcc acc number
>>   | number <= 0  = acc
>>   | number > 0 =  toDigitsAcc ((number `mod` 10) : acc) (number `div` 10)
>>
>> doubleEveryOther :: [Integer] -> [Integer]
>> doubleEveryOther list  = case (length list) `mod` 2 of 0 ->
>> doubleEveryOther [] = []
>> doubleEveryOther (x:y:xs) = x*2 : y : xs
>>                               (length list) `mod` 2 of 0 ->
>> doubleEveryOther [] = []
>> doubleEveryOther (x:y:xs) = x : y*2 : xs
>>
>>
>> -- | The main entry point.
>> main = print $ doubleEveryOther [1,2,3]
>>
>>
>> but apperantly you cannot have pattern matching after a case statement
>> because I get a error on the =  of a empty list
>>
>> Roelof
>>
>>
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>
>
>
> --
> Nishant
>
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-- 
Jonathan Skårstedt
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