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Haskell Quiz/PP Pascal/Solution Kelan

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Revision as of 21:10, 3 November 2006 by Kelan (Talk | contribs)

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Pretty basic solution. Find the biggest number the triangle will contain given the number of rows, use that number to determine column widths. Then just print out each row with the correct columns and spacing.

import System.Environment
 
nCr n r =
    ( product [ ( s + 1 ) .. n ] ) `div` ( product [ 2 .. t ] )
    where
    s = max r ( n - r )
    t = min r ( n - r )
 
-- maximum number in a pascal triangle
-- with given number of rows
trimax rows =
    nCr ( rows - 1 ) ( ( rows - 1 ) `div` 2 )
 
main = do
    args <- getArgs
 
    let rows = read . head $ args
        digits = 1 + ( truncate . logBase 10 . fromIntegral . trimax $ rows )
        pad s = ( replicate ( digits - length s ) ' ' ) ++ s
        for_ = flip mapM_
 
    for_ [ 0 .. ( rows - 1 ) ] $ \ n -> do
        putStr . concat . replicate ( rows - n - 1 ) . pad $ ""
        for_ [ 0 .. n ] $ \ r -> do
            putStr . pad . show . nCr n $ r
            putStr . pad $ ""
        putStrLn ""