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Haskell Quiz/PP Pascal/Solution Ltriant

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< Haskell Quiz | PP Pascal(Difference between revisions)
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(sharpen cat)
 
(2 intermediate revisions by one user not shown)
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[[Category:Code]]
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[[Category:Haskell Quiz solutions|PP Pascal]]
   
Works...kind of; adds a space or two (or three) to the end of each line.
+
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.
   
 
<haskell>
 
<haskell>
Line 8: Line 8:
 
import System.Environment ( getArgs )
 
import System.Environment ( getArgs )
   
  +
fac :: Integer -> Integer
 
fac n = product [1..n]
 
fac n = product [1..n]
  +
  +
nck :: Integer -> Integer -> Integer
 
nck n k = (fac n) `div` ((fac $ n - k) * (fac k))
 
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)
 
space_len n = mylength $ show $ (n-1) `nck` ((n-1) `div` 2)
  +
  +
mylength :: [a] -> Integer
 
mylength = toInteger . length
 
mylength = toInteger . length
   
  +
indent :: Integer -> [String] -> [String]
 
indent n t =
 
indent n t =
(concat $ replicate (fromInteger $ (n-p) * space_len 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 n t =
 
space_out n t =
map (\x -> show x ++ (concat $ replicate (num_spaces x) " ")) t
+
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)
   
  +
pp_pascal :: Integer -> [String]
 
pp_pascal n =
 
pp_pascal n =
map (concat . indent n . space_out n) $ f 0 n
+
map (rstrip ' ' . concat . indent n . space_out n) $ f 0 n
 
where f acc n | acc == n = []
 
where f acc n | acc == n = []
 
| otherwise = (map (nck acc) [0..acc]):(f (acc+1) n)
 
| otherwise = (map (nck acc) [0..acc]):(f (acc+1) n)
   
  +
main :: IO ()
 
main =
 
main =
 
do args <- getArgs
 
do args <- getArgs
 
case args of
 
case args of
[v] -> mapM_ putStrLn $ pp_pascal (read v :: Integer)
+
[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."