You're right. I broke them when experimenting to see where the cycles were going. I've fixed them, but the other pair of functions, named gaussElim2, were correct and the issue remains that unsafeRead appears slower than readArray. I've included your implementation for comparison, as well as a simple pure matrix multiplication function, but I'm having trouble tracking them down in the profiling. I think all of their cycles are being counted under main, but I'm not sure how to stop this. They certainly "feel" faster when I sit there and watch the program run, which is great. Why would lists-of-lists be faster than unboxed arrays? No indexing arithmetic? Deforestation? I'm very curious. The first advice I got from #haskell when trying to speed up my original code was "get rid of all those lists." Scott On 5/29/07, Mirko Rahn <rahn@ira.uka.de> wrote:
instead of (!) (or readArray in stateful code) helped a lot. So then I went to optimize my gaussian elimination function and found just the opposite. unsafeRead is slower than readArray. This struck me as very
The functions gaussElimSafe and gaussElimUnsafe are different in their outputs (*). I guess you made a mistake during conversion from one version to the other.
BTW, the attached file contains an outline of a pure functional and indexless implementation. It calculates the inverse of a quadratic matrix in time O(n^3), so maybe you can get rid of all this exclamation marks and IO stuff...
BR,
(*) Try:
newStdGen >>= \ g -> flip mapM_ [gaussElimSafe,gaussElimUnsafe] $ \ f -> makeMatrix g >>= \ m -> printMatrix m >> f m >> printMatrix m
-- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---