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 :

src/Main.hs@14:8-14:16

Couldn't match expected type ‘Int -> c0’ with actual type

[Integer]

… 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