
From:  Paul Kienzle 
Subject:  Re: polyfit() and Inf 
Date:  Sun, 30 Mar 2003 23:21:17 0500 
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stefan wrote:
I was experimenting with the polyfit() function. It works correct with "normal" functions (y = f(x)). But it seems to compute ages (I never waited until it finished) when there is a 'Inf' in the dataset. x = linspace(0,1,100); y = x * 1 + 4; y(99) = Inf; [p,r] = polyfit(x,y,1); disp(p); This piece of code works fine without the 'y(99) = Inf;' line and stucks with it. I implemented the polyfit() function on my own. I get infinite results but it does not stuck! Is this a real problem/bug with polyfit()? I could send you my implemetation, if interested...
I get the same result (cygwin, octave2.1.42). This is probably a bug in \, which relies on the functions in LAPACK. wpolyfit in octaveforge returns NaN: http://cvs.sourceforge.net/cgibin/viewcvs.cgi/~checkout~/octave/octaveforge/main/optim/wpolyfit.m?rev=HEAD&contenttype=text/plain It uses the slower but more accurate QR decomposition: [Q,R,P] = qr(X,0); p = R\(Q'*y); p(P) = p; You can check it yourself against the NIST Statistical Reference Datasets http://www.itl.nist.gov/div898/strd/ Paul Kienzle address@hidden  Octave is freely available under the terms of the GNU GPL. Octave's home on the web: http://www.octave.org How to fund new projects: http://www.octave.org/funding.html Subscription information: http://www.octave.org/archive.html 
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