[Haskell-cafe] Haskell transformation of C code

[mukesh tiwari]

Thank you for your reply.

On Aug 17, 7:48 pm, Ryan Yates <fryguy... at gmail.com> wrote:
> I had to stare at this for a while to see the difference.  In the C++
> version the comparison's in loop B and loop C are always between areas
> of triangles that are off by one in a single index.  In the haskell
> calArea though, comparisons are between the passed area and triangles
> off by one from the passed coordinates.  Recursive calls under calArea
> hold the invariant found in the C++ code, but in looPing area' is the
> area of a' b' c', but the recursive call is to looPing a'' b'' c''
> with area' so calArea is passed the indexes for a different triangle
> then the area it is given.  We can solve this by explicitly passing
> around the best result so far:
>
> looPing :: ( Num a , Ord a , Eq a , Floating a ) => Int -> Int -> Int
> -> Int -> a -> a -> Array Int ( Point a ) -> a
> looPing a b c n area best arr
>  | a == n = max best area
>  | otherwise = looPing a'' b'' c'' n area'' (max area' best) arr where
>         ( a' , b' , c' , area' ) = calArea a b c n area arr
>         a'' = a' + 1
>         b'' = if a'' == b' then mod ( b' + 1 ) n else b'
>         c'' = if b'' == c' then mod ( c' + 1 ) n else c'
>         area'' = caltmp (a'' `mod` n) b'' c'' arr
>
> and solve now applies area twice:  looPing 0 1 2  n area area arr'
>
> This now matches the result from C++ for me.
>
> On Wed, Aug 17, 2011 at 8:09 AM, mukesh tiwari
>
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> <mukeshtiwari.ii... at gmail.com> wrote:
> > Hello all
> > I am trying implement this algorithm  [
> >http://stackoverflow.com/questions/1621364/how-to-find-largest-triang...
> > ] in Haskell but i am getting wrong answer for some test cases. A c++
> > implementation which is accepted for this problem [http://www.spoj.pl/problems/MTRIAREA
> > ] and i   rewrote  the C++ code in Haskell . Running both programs on
> > some  random test cases produce different result . For Haskell
> > implementation , convex hull part is correct . Both codes [ c++ and
> > Haskell ] produced same convex hull so i am skeptical about
> > looPing :: ( Num a , Ord a , Eq a , Floating a ) => Int -> Int -> Int -
> >> Int -> a -> Array Int ( Point a ) -> a  and calArea :: ( Num a , Ord
> > a , Floating a ) => Int -> Int -> Int  -> Int  -> a -> Array Int
> > ( Point a ) -> ( Int , Int , Int , a ) function . Could some one
> > please tell me what is wrong with these functions. I have posted both
> > code on ideone . Haskell code [http://ideone.com/IlwBv] and accepted
> > c++ for SPOJ problem [http://ideone.com/vgNnt] . For test case
> > 20
> > 886 9383
> > 6915 2777
> > 8335 7793
> > 492 5386
> > 1421 6649
> > 27 2362
> > 59 8690
> > 3926 7763
> > 3426 540
> > 5736 9172
> > 5368 5211
> > 6429 2567
> > 1530 5782
> > 5123 2862
> > 3135 4067
> > 9802 3929
> > 3058 4022
> > 8167 3069
> > 8456 1393
> > 8042 5011
> > -1
> > Haskell produced the convex hull [P 27.0 2362.0,P 3426.0 540.0,P
> > 8456.0 1393.0,P 9802.0 3929.0,P 8335.0 7793.0,P 5736.0 9172.0,P 886.0
> > 9383.0,P 59.0 8690.0] and maximum triangle area 31466755.50 while C++
> > code produced the convex hull
> > 27 2362
> > 3426 540
> > 8456 1393
> > 9802 3929
> > 8335 7793
> > 5736 9172
> > 886 9383
> > 59 8690
> > and maximum area 33642111.00 . Based on these observation , I think my
> > convex hull part is correct and I am stuck with looPing and calArea
> > function . I tried to write Haskell code many ways but no success so
> > it would be great if some one can tell me what is wrong with these two
> > functions .
>
> > Thank you
>
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