**Problem:**

Write a function that converts excel column IDs to corresponding integers with “A” corresponding to 1 and so on.

**Solution:**

We will modify the solution in EPI 5.6 slightly so that we have:

indexOf :: Eq a => a -> [a] -> Int indexOf item xs | length firstMatched == 0 = -1 | otherwise = fst (head firstMatched) where matchesItem x = snd x /= item firstMatched = dropWhile matchesItem (zip [0 .. (length xs) - 1] xs)

digitIndices :: [Char] -> [Int] digitIndices digits = [ indexOf (chr i) digits | i <- [0 .. 255]]

spreadsheet_column_encoding :: [Char] -> [Int] spreadsheet_column_encoding xs = sum [ digit_to_value digit pos | (pos, digit) <- dig_positions xs] where reverse_indexes xs = [(length xs) - 1, (length xs) - 2 .. 0] dig_positions xs = zip (reverse_indexes xs) xs symbols = ['A' .. 'Z'] ourIndexes = digitIndices symbols digit_to_value digit pos = (1 + (ourIndexes !! (ord digit))) * floor (26 ** fromIntegral pos)