gistfile1.hs

-- Q3 Write a function that converts a string to a numerical type. The string is in the form
-- $2,345,678.99 and can have leading zeroes.

  -- I'm not going to be doing error checking on the format btw
  
  strToNum :: Fractional a => [Char] -> a
  strToNum str = let
    -- remove the '$' and ','
    plain = filter (\x -> x /= '$' && x /= ',') str 
    whole =  takeWhile (\x -> x /= '.') plain
    fractional = tail (dropWhile (\x -> x /= '.') plain)
    in (strToNumWhole whole) + (strToNumFrac fractional)
  
  -- helper functions
  
  strToNumWhole :: Num a => [Char] -> a
  strToNumWhole str = let
    -- use a zip to combine the individual chars with the index of their position
    -- we will use the index to multiply 10s, 100s, 1000s, etc
    -- produces a list of tuples like [("2", 0), ("1"), 1] from the string "12"
    withIndices = zip (reverse str) [0..]
    in sum (map (\pair -> (digitToNum (fst pair)) * (10 ^ snd pair)) withIndices)
  
  strToNumFrac :: Fractional a => [Char] -> a
  strToNumFrac str = let
    withIndices = zip (str) [1..]
    in sum (map (\pair -> (digitToNum (fst pair)) /  (10 ^ snd pair)) withIndices)
    
  digitToNum :: Num a => Char -> a
  digitToNum '0' = 0
  digitToNum '1' = 1
  digitToNum '2' = 2
  digitToNum '3' = 3
  digitToNum '4' = 4
  digitToNum '5' = 5
  digitToNum '6' = 6
  digitToNum '7' = 7
  digitToNum '8' = 8
  digitToNum '9' = 9