Difference between revisions of "Haskell Quiz/PP Pascal/Solution Ltriant"

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(added a brief description of how it does it...)
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[[Category:Code]]
 
[[Category:Code]]
   
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.
 
Edit: doesn't add spaces to the end anymore :)
 
   
 
<haskell>
 
<haskell>
Line 27: Line 25:
 
indent :: Integer -> [String] -> [String]
 
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 :: 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)
   
Line 45: Line 43:
 
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>

Revision as of 03:30, 4 November 2006


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."