# Haskell Quiz/PP Pascal/Solution Ltriant

### From HaskellWiki

< Haskell Quiz | PP Pascal(Difference between revisions)

m |
(sharpen cat) |

(One intermediate revision by one user not shown) |

## Latest revision as of 10:52, 13 January 2007

Basically it builds the list of Pascal's triangle, then spaces the integers out (by adding trailing spaces to each integer), then indents each line by adding spaces onto the head of each list, strips off any trailing spaces then prints it all out.

module Main where import System.Environment ( getArgs ) fac :: Integer -> Integer fac n = product [1..n] nck :: Integer -> Integer -> Integer nck n k = (fac n) `div` ((fac $ n - k) * (fac k)) rstrip :: Eq a => a -> [a] -> [a] rstrip n t = reverse $ dropWhile (== n) $ reverse t space_len :: Integer -> Integer space_len n = mylength $ show $ (n-1) `nck` ((n-1) `div` 2) mylength :: [a] -> Integer mylength = toInteger . length indent :: Integer -> [String] -> [String] indent n t = (replicate (fromInteger $ (n-p) * space_len n) ' '):t where p = mylength t space_out :: Integer -> [Integer] -> [String] space_out n t = map (\x -> show x ++ (replicate (num_spaces x) ' ')) t where num_spaces x = fromInteger $ (2 * space_len n) - (mylength $ show x) pp_pascal :: Integer -> [String] pp_pascal n = map (rstrip ' ' . concat . indent n . space_out n) $ f 0 n where f acc n | acc == n = [] | otherwise = (map (nck acc) [0..acc]):(f (acc+1) n) main :: IO () main = do args <- getArgs case args of [v] -> mapM_ putStrLn $ pp_pascal $ read v _ -> error "No argument specified."