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

[Alexey Shmalko]

Then it could be:

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

It could also be generalized to Num's ;)

On Thu, May 14, 2015 at 7:28 PM Jonathan Skårstedt <
jonathan.skarstedt at gmail.com> wrote:

> 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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