controlparameters.html.svn-base

OPT++

/** \page ControlParameters  Algorithmic Parameters

Below, we describe the common parameters for our optimization algorithms and 
list their default values.  For information about additional parameters
for a specific algorithm,  please review the documentation for that algorithm.

<UL>
<LI> <a href="#Tolerances">Stopping Tolerances</a>
<p>
 <UL>
<LI><b>setFTol</b><p>
The setFTol method assigns a stopping tolerance for an optimization algorithm.
Please assign tolerances that make sense given the accuracy of your function.
For example, including the following code fragment, 
\code
objfcn.setFTol(1.e-4)
\endcode
in your problem means the optimization algorithm converges when 
the function value from one iteration to the next changes by 1.e-4 or less. 
<p>
Default value: 1.49012e-8
<p> 
<LI><b>setGradTol</b><p>
The setGradTol method assigns a stopping tolerance for an optimization 
algorithm.
Please assign tolerances that make sense given your function accuracy.
For example, including the following code fragment, 
\code
objfcn.setGradTol(1.e-6)
\endcode
in your problem means the optimization algorithm converges when 
the absolute or relative norm of the gradient is 1.e-6 or less. 
<p>
Default value: 6.05545e-6
<p>
<LI><b>setStepTol</b><p>
The setStepTol method assigns a stopping tolerance for the optimization 
algorithm.
Please set tolerances that make sense, given the accuracy of your function. 
For example, including the following code fragment, 
\code
objfcn.setStepTol(1.e-2)
\endcode
in your problem means the optimization algorithm converges when the relative
steplength is 1.e-2 or less.
<p>
Default value: 1.49012e-8
<p>
<LI><b>setMaxIter</b><p>
The setMaxIter method places a limit on the number of iterations of the
optimization algorithm.  The method is useful when your function is 
computationally expensive or you are debugging the optimization algorithm. 
<p>
\code
objfcn.setMaxIter(50)
\endcode
<p>
In the example above, when 50 iterations evaluations have been completed, 
the optimization algorithm will stop and report the solution it has reached at 
that point. It may not be the optimal solution, but it will be 
the best it could provide given the limit on the number of iterations.
<p>
Default value: 100
<p>
<LI> <b> setMaxFeval</b><p>
The setMaxFeval method places an upper bound on the number of function evaluations.  
The method is useful when your function is computationally expensive and you only 
have time to perform a limited number of evaluations.  
<p>
\code
objfcn.setMaxFeval(200)
\endcode
<p>
In the example above, when 200 function evaluations have been completed, 
the optimization algorithm will stop and report the solution it has reached at 
that point. It may not be the optimal solution, but it will be 
the best it could provide given the limit on the number of function evaluations.
<p>
Default value: 1000
 </UL>
<p> 
<LI> <a href="#Steplength">Steplength Control</a>
<p>
<UL>
<LI><b>setMaxStep</b><p>
The setMaxStep method places an upper bound on the length of the
step that can be taken at each iteration of the optimization algorithm.
If the scale of your optimization parameters exceeds the bound, adjust
accordingly.  If you want to be conservative in your search, you may
want to set MaxStep to a smaller value than the default.  In our experience,
the default value is generally fine.
<p> 
Default value: 1.0e3
<p> 
<LI><b>setMinStep</b><p>
The setMinStep method places a lower bound on the length of the
step that can be taken at each iteration of the optimization algorithm.
If the scale of your optimization parameters exceeds the bound, adjust
accordingly.  If you expect the optimization algorithm to navigate some
tricky areas, set MinStep to a smaller value than the default. In our experience,
the default value is generally fine.
<p> 
Default value: 1.49012e-8
<p> 
</UL>
<LI> <a href="#Globalization">Globalization Strategy Parameters</a>
<p>
<UL>
<LI><b>setLSTol</b><p>
In practice, the linesearch tolerance is set to a small value, so that almost
any decrease in the function value results in an acceptable step.  Suggested 
values are 1.e-4 for Newton methods and 1.e-1 for more exact line searches.
<p>
Default value: 1.e-4
<p>
<LI><b>setMaxBacktrackIter</b><p>
The setMaxBacktrackIter method is only relevant when you use a algorithm
with a linesearch search strategy.  The value places a limit on the number of 
iterations in the linesearch routine of the optimization algorithm.  
If the limit is reached before computing a step with acceptable decrease,
the algorithm terminates with an error message.  The reported solution
is not optimal, but the best one given the number of linesearch iterations. 
Increasing the number of linesearch iterations may lead to an acceptable step,
but it also results in more function evaluations and a shorter steplength.
The formula for computing the steplength is
\f[ \alpha = \hat{\alpha}^L, \mbox{where}~ 
\hat{\alpha} = \frac{1}{2}  \mbox{and}~ 
L \mbox{ is the number of backtrack iterations.} \f] 
<p>
Default value: L = 5 
<p>
<LI><b>setTRSize</b><p>
The setTRSize method is only relevant when you are using an algorithm
with a trust-region or a trustpds search strategy.  The value
initializes the size of the trust region.  
<p>
Default value:  \f$ 0.1* \| \nabla f(x) \| \f$
<p>
If your problem is quadratic or close to it, you may want to initialize the
size of the trust region to a larger value.
<p>
</UL>
</UL>
Last revised <em> June 30, 2006</em>

*/