Re: Octave-forge: you get what you pay for

[Stefan van der Walt]

Hi Vic

I think your comments in the first paragraph are a bit harsh.  The
Octave Forge community go to great length to ensure the quality of the
code they produce.  Sure, sometimes code gets included that is flawed,
but because it is Open Source, you have the opportunity to correct
these mistakes.

Reporting a problem isn't always enough -- fixing it is much more
helpful.  The people who work on Octave Forge donate their time (as
you mentioned, free of charge) -- but often they don't have any
motivation to fix certain problems.  And that is not because of a lack
of interest or commitment: simply a very fine balancing act for people
whose time is strained to start with.

You should always do some sanity checking on your results -- and that
is why we often include unit tests with the algorithms these days.  I
personally used Octave and Octave Forge in all the calculations for my
MSc thesis, and I have great admiration and respect for all the
individuals who donated their time to allow me to do this: free of
charge, and free to share.

Clearly, mileage varies.  But your attitude is not cool.

Regards
Stéfan

On Wed, Jun 15, 2005 at 09:36:56AM -0400, Vic Norton wrote:
> A warning about Octave-forge: you get what you pay for.
> 
> I don't use Octave-forge. I don't generally need it for my "work".
> Still, the only function on Octave-forge that I have really tested
> failed to work on certain elementary problems. Obviously the creator
> of the function was extremely naive about the subtleties of numerical
> computation.
> 
> I am speaking of the linear programming routine lp. This function can
> fail completely on relatively trivial but realistic problems. The
> following test case illustrates the deficiency of lp.
> 
>    ## Problem 6
>    %  Linear Programming 1: Introduction
>    %  Dantzig, George B. & Thapa, Mukund N.
>    %  Springer-Verlag, New York, 1997
>    %  Example 1.4 (A Blending Problem)
>    A6 = [
>      1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
>      0.2 0.5 0.3 0.3 0.3 0.6 0.4 0.1 0.1
>      0.3 0.4 0.2 0.4 0.3 0.3 0.5 0.3 0.1
>      0.5 0.1 0.5 0.3 0.4 0.1 0.1 0.6 0.8
>    ];
>    B6 = [1.0; 0.3; 0.3; 0.4];
>    F6 = [4.1 4.3 5.8 6.0 7.6 7.5 7.3 6.9 7.3];
>    LB6 = zeros(9, 1);
>    UB6 = ones(9, 1)*Inf;
>    N6 = 4;
>    sol6 = lp(F6, A6, B6, LB6, UB6, N6);
>    x6 = [0.5; 0.25; 0.25; 0; 0; 0; 0; 0; 0];
>    assert(x6, sol6, 10*eps);
> 
> It is trivial to check that x6 is a feasible solution to Problem 6:
>    A6 * x6 = B6,
>    LB6 <= x <= UB6.
> This computation doesn't show that x6 is optimal, that
>    F6 * x6 = 4.575
> is the minimum of F6 * x subject to the constraints on x. However it
> does imply that Problem 6 has an optimal solution.
> 
> The last line of Problem 6, as presented above, is supposed to verify
> that x6 is the optimal solution. In fact you never get to the last
> line. The
>    sol6 = lp(F6, A6, B6, LB6, UB6, N6)
> line simply bombs. I get stuff like
>    warning: lp is unreliable: the returned solution does not always
>       lie within the bounds
>    error: T& Array<T>::checkelem (0, 10): range error
>    error: T& Array<T>::checkelem (1, 10): range error
>    ...
>    error: T& Array<T>::checkelem (1, 10): range error
>    error: evaluating assignment expression near line 17, column 9
>    error: near line 17 of file
>       `/Users/vic/octave/m/tests-work/lp_prob6.m'
> on my Macintosh. (Line 17, column 9 refers to the "=" in
> "sol6 = lp(...);").
> 
> I reported this problem with lp to the powers that be over 4 years ago.
> Nothing has been done about it. So my warning to you stands. In so far
> as the Octave-forge routines go, don't believe everything you compute.
> 
> Regards,
> 
> Vic
> 



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