Re: Passing parameters to fsolve function - new syntax?

[Michael Creel]

Aivo Jürgenson wrote:

> Hello,
>
> I have a function f(x,a,b,c), which I would like solve for variable x, 
> so that f(x,a,b,c)=0, where a,b,c are parameters, which are known at 
> the time of solving.
>
> However, current fsolve function in Octave only supports one-variable 
> functions. I suppose there is a work-around to declare specific global 
> variables and then giving values to those variables before calling the 
> fsolve function, but this doesn't seem to be a clean way of organizing 
> this.
>
> The fsolve FIX in the octave-forge package seems to expand this 
> functionality, but I'm not able to compile it. Also, the status of 
> this FIX seems to be unclear, as in the development mailing list 
> message http://sourceforge.net/mailarchive/message.php?msg_id=11922522:
>
> Paul Kienzle, in May, 2005:
>
> > I have no interest in maintaining this function.  I am removing it from
> > the build system for now, and will purge it completely if nobody steps
> > forward to update it.
> >
> > IIRC, the purpose of this function is to pass extra arguments to
> > fsolve, but we can do this now with the new syntax:
> >
> >     @(x) f(x,a,b,c)
> >
> > - Paul
>
>
>
> Could somebody explain this new syntax? I would be happy to use this 
> new and official way of passing extra arguments to fsolve, but I'm not 
> been able to find any examples of doing it.
>
> Aivo Jürgenson, address@hidden


You might find bfgsmin in octave-forge to be useful. It will minimize 
wrt any one of multiple arguments, and the argument minimization is done 
over may be a vector rather than a scalar. Running the following code

1; # this is a script

function y = myfunc(x, otherargs)
  a = otherargs{1};
  b = otherargs{2};
  c = otherargs{3};
  y = a + b*x + c*x2;
endfunction

function obj_value = findroot(x, f, otherargs)
  e = feval(f, x, otherargs);      obj_value = e'*e;
endfunction


a = 0;
b = 1;
c = 0.5;
otherargs = {a, b, c};
x = 5;
control = {20, 1, 1, 1};
x = bfgsmin("findroot", {x, "myfunc", otherargs}, control);
x


leads to the output

======================================================
BFGSMIN final results

Used numeric gradient

------------------------------------------------------
STRONG CONVERGENCE
Function conv 1  Param conv 1  Gradient conv 1
------------------------------------------------------
Objective function value 1.38809e-13
Stepsize 1.0007
8 iterations
------------------------------------------------------

param    gradient  change
0.0000   0.0000  -0.0000
x =  3.7257e-07
octave:7>

It is possible to set the convergence tolerance to be tighter, if 
needed. HTH, Michael



-------------------------------------------------------------
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
-------------------------------------------------------------