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Haskell Quiz/PP Pascal/Solution Burton

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(added my solution)
 
m
 
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[[Category:Haskell Quiz solutions|PP Pascal]]
 
<haskell>
 
<haskell>
 
{--
 
{--
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--}
 
--}
 
--Pascal's triangle from http://www.haskell.org/haskellwiki/Blow_your_mind
 
--Pascal's triangle from http://www.haskell.org/haskellwiki/Blow_your_mind
pascal :: [[Int]]
+
pascal :: [[Integer]]
 
pascal = iterate (\row -> zipWith (+) ([0] ++ row) (row ++ [0])) [1]
 
pascal = iterate (\row -> zipWith (+) ([0] ++ row) (row ++ [0])) [1]
   
   
 
ppPascal :: Int -> IO ()
 
ppPascal :: Int -> IO ()
ppPascal n = putStrLn $ unlines $ map ppRow tr
+
ppPascal n = mapM_ (putStrLn . ppRow) tr
 
where tr = take n pascal
 
where tr = take n pascal
 
mRow = last tr
 
mRow = last tr
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cols n = if n < 10 then 1 else 1 + cols (n `div` 10)
 
cols n = if n < 10 then 1 else 1 + cols (n `div` 10)
 
ppRow r@(x:xs) = padRow ++ (showFirst x)
 
ppRow r@(x:xs) = padRow ++ (showFirst x)
++ (concat $ map showRest xs) ++ padRow
+
++ (concatMap showRest xs) ++ padRow
 
where showFirst e = (show e) ++ (padR e)
 
where showFirst e = (show e) ++ (padR e)
 
showRest e = padL ++ (show e) ++ padR e
 
showRest e = padL ++ (show e) ++ padR e
 
padRow = replicate ((length mRow - length r)*pad) ' '
 
padRow = replicate ((length mRow - length r)*pad) ' '
padL = replicate (pad-1) ' '
+
padL = replicate pad ' '
padR d = replicate (pad-(cols d)+1) ' '
+
padR d = replicate (pad-(cols d)) ' '
  +
</haskell>
  +
  +
------------
  +
A bit shorter than the above, though certainly not prettier. (reverse._.reverse) makes it easier to calculate the leading padding for each line. -- AlsonKemp
  +
<haskell>
  +
pascal :: [[Integer]]
  +
pascal = iterate (\row -> zipWith (+) ([0] ++ row) (row ++ [0])) [1]
  +
  +
ppPascal :: Int -> IO ()
  +
ppPascal n = mapM_ putStrLn $ reverse $ ppRow 0 (reverse tr)
  +
where ppRow _ [] = []
  +
ppRow lpads (x:xs) = (replicate lpads ' ' ++ concatMap (padNum pad) x) : (ppRow (lpads+pad) xs)
  +
tr = take n pascal
  +
padNum pads n = take (2*pads) (show n ++ (repeat ' '))
  +
pad = 1 + (length $ show $ maximum $ last tr) `div` 2
 
</haskell>
 
</haskell>

Latest revision as of 11:36, 13 December 2009

{--
*Main> ppPascal 10
                           1                              
                        1     1                           
                     1     2     1                        
                  1     3     3     1                     
               1     4     6     4     1                  
            1     5     10    10    5     1               
         1     6     15    20    15    6     1            
      1     7     21    35    35    21    7     1         
   1     8     28    56    70    56    28    8     1      
1     9     36    84    126   126   84    36    9     1 
 
--}
--Pascal's triangle from http://www.haskell.org/haskellwiki/Blow_your_mind
pascal :: [[Integer]]
pascal = iterate (\row -> zipWith (+) ([0] ++ row) (row ++ [0])) [1]
 
 
ppPascal :: Int -> IO ()
ppPascal n = mapM_ (putStrLn . ppRow) tr
    where tr     = take n pascal
          mRow   = last tr
          pad    = cols $ maximum $ mRow
          cols n = if n < 10 then 1 else 1 + cols (n `div` 10)
          ppRow r@(x:xs) = padRow ++ (showFirst x)
                         ++ (concatMap showRest xs) ++ padRow
                    where showFirst e = (show e) ++ (padR e)
                          showRest  e = padL ++ (show e) ++ padR e
                          padRow      = replicate ((length mRow - length r)*pad) ' '
                          padL        = replicate pad ' '
                          padR d      = replicate (pad-(cols d)) ' '

A bit shorter than the above, though certainly not prettier. (reverse._.reverse) makes it easier to calculate the leading padding for each line. -- AlsonKemp

pascal :: [[Integer]]
pascal = iterate (\row -> zipWith (+) ([0] ++ row) (row ++ [0])) [1]
 
ppPascal :: Int -> IO ()
ppPascal n = mapM_ putStrLn $ reverse $ ppRow 0 (reverse tr)
    where ppRow _     []     = []
          ppRow lpads (x:xs) = (replicate lpads ' ' ++ concatMap (padNum pad) x) : (ppRow (lpads+pad) xs)
          tr            = take n pascal
          padNum pads n = take (2*pads) (show n ++ (repeat ' ')) 
          pad           = 1 + (length $ show $ maximum $ last tr) `div` 2