# Haskell Quiz/PP Pascal/Solution Ltriant

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< Haskell Quiz | PP Pascal(Difference between revisions)

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− | [[Category: | + | [[Category:Haskell Quiz solutions|PP Pascal]] |

− | + | 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. | |

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<haskell> | <haskell> | ||

Line 27: | Line 25: | ||

indent :: Integer -> [String] -> [String] | indent :: Integer -> [String] -> [String] | ||

indent n t = | indent n t = | ||

− | ( | + | (replicate (fromInteger $ (n-p) * space_len n) ' '):t |

where p = mylength t | where p = mylength t | ||

space_out :: Integer -> [Integer] -> [String] | space_out :: Integer -> [Integer] -> [String] | ||

space_out n t = | space_out n t = | ||

− | map (\x -> show x ++ ( | + | map (\x -> show x ++ (replicate (num_spaces x) ' ')) t |

where num_spaces x = fromInteger $ (2 * space_len n) - (mylength $ show x) | where num_spaces x = fromInteger $ (2 * space_len n) - (mylength $ show x) | ||

Line 45: | Line 43: | ||

do args <- getArgs | do args <- getArgs | ||

case args of | case args of | ||

− | [v] -> mapM_ putStrLn $ pp_pascal | + | [v] -> mapM_ putStrLn $ pp_pascal $ read v |

_ -> error "No argument specified." | _ -> error "No argument specified." | ||

</haskell> | </haskell> |

## 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."