Question

Recently I am trying to solve a problem using Foldr. The task is following:

In:[5,1,3,8,2,4,7,1]
Out:[16,8]

It means, I will double those element of the input list which is in the odd index position and even digit. I wrote the program without using foldr which is following:(It shows pattern match failure: head[])

findPos list elt =
  map fst $ filter ((elt==).snd) $ zip [0..] list

doublePos [] = []
doublePos (x:xs)
  | ((head(findPos xs x)`mod` 2) /= 0) && (x `mod` 2 == 0) =
      [2*x] ++ doublePos xs
  | otherwise = doublePos xs

How do I write this program using foldr?

Answer 1

foldr isn’t really a good choice for this function, as you need to pass the parity of the index of each element from the front of the list.

A list comprehension is probably the cleanest:

doublePos xs = [2*x | (i,x) <- zip [0..] xs, even x, odd i] 

or you could use plain old recursion:

doublePos' (_:x:xs)
  | even x    = (2*x) : doublePos' xs
  | otherwise =         doublePos' xs
doublePos' _ = []

Though, if you must use foldr, you can do it by having the accumulator be a function
which takes the parity of the current index as an argument:

doublePos'' xs = foldr step (const []) xs False where
  step x next p
    | p && even x = 2*x : next (not p)
    | otherwise   = next (not p)