Hello Roel, Roel van Dijk wrote:
Hello,
I would like to use hmatrix to do some function fitting with the Levenberg Marquardt algorithm. As an example I would like to fit the very simple function "f x = a*x + b" on some data points. The problem is that executing the 'fitModel' function crashes GHC(i) with a segmentation fault.
Can you reproduce it on a 32 bit machine? There is a known problem with some hmatrix GSL wrappers on 64 bit in interpreted mode. This is mentioned at the end of the install page: http://code.haskell.org/hmatrix/install.html In this case the segmentation fault happens inside a foreign GSL function, only in interpreted code, and only in 64 bit. It does not happen in compiled code. (Although I have a report of a similar segmentation fault with compiled code if the output is redirected). I don't really know how to debug this kind of problem. As a first step I will try to obtain a minimal test case without any package dependencies. This makes debugging difficult. Can anyone spot
what I am doing wrong?
Your are doing nothing wrong. I will create a new, more visible page for the known problems. Given all the lists of Double's it seems very
easy to make an error regarding the number of arguments with the model function or the derivative.
Static argument checking would be ideal, but at least they are normal runtime errors and should never crash the program.
Try to evaluate the 'test' function in the small program listed below. I would expect an output of [1, 0] (y = 1*x + 0) instead of a segmentation fault.
It works for me in 32 bit: $ ghci Test.hs GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. [1 of 1] Compiling Test ( Test.hs, interpreted ) Ok, modules loaded: Test. *Test> test Loading package filepath-1.1.0.4 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package old-time-1.0.0.5 ... linking ... done. Loading package unix-2.4.0.2 ... linking ... done. Loading package directory-1.0.1.1 ... linking ... done. Loading package process-1.0.1.3 ... linking ... done. Loading package array-0.3.0.1 ... linking ... done. Loading package storable-complex-0.2.1 ... linking ... done. Loading package hmatrix-0.10.0.1 ... linking ... done. ([1.0,0.0],(1><4) [ 1.0, 0.0, 1.0, 0.0 ])
Relevant versions: - hmatrix-0.10.0.0 - gsl-1.14 - ghc-6.12.3 (64 bit)
Small program:
module Test where
-- from base: import Control.Arrow ( second ) import Control.Applicative ( pure )
-- from hmatrix: import Data.Packed.Matrix ( Matrix ) import Numeric.GSL.Fitting ( FittingMethod(LevenbergMarquardt), fitModel )
-- input list of (x, y) pairs, output coefficients of "f x = a x * b" fitLinear :: [(Double, Double)] -> ([Double], Matrix Double) fitLinear samples = fitModel 1 1 10 (linearModel, linearDer) (map (second pure) samples) [0, 0]
linearModel :: [Double] -> Double -> [Double] linearModel [a, b] x = [a*x + b, 0] linearModel _ x = error "wrong arguments"
linearDer :: [Double] -> Double -> [[Double]] linearDer [_, _] x = [[x, 0]] linearDer _ _ = error "wrong arguments"
test = fitLinear [(0,0), (1,1), (2,2)]