[Ipopt] Algorithm flow calling f(x) and g(x), same decision variables?

[Seth Watts]

Hi Martijn -

Using the new_x flag is definitely the correct approach. I have a problem
similar to yours in that evaluating the system state for a given set of
optimization variables is costly, but once this is done, evaluating each of
f, g, grad_f, jac_g is relatively easy.

I include the following as class variables in MyNLP.cpp : my_f, my_g,
my_grad_f, my_jac_g. Then, if new_x is true, I call a function to update my
model and evaluate all of (my_f, my_g, etc.), and then for example the
function eval_f would copy my_f to f. If new_x is false, then the function
just needs to copy what is currently in my_f to f.

In case I wasn't clear above, here is pseudocode of eval_f:

bool MyNLP::eval_f(Index n, const Number* x, bool new_x, Number& obj_value)
{
    if(new_x)
    {
        evaluate(<lots of variables>, my_f, my_g, my_df, my_dg);
    }
    obj_value = my_f;
}

Since these are class variables, they are visible to the class functions,
so you don't need to explicitly create a structure or pass these variables
into the functions. It is a fairly clean implementation.

- Seth

On Mon, Sep 10, 2012 at 3:57 AM, Martijn Disse <M.W.Disse at student.tudelft.nl
> wrote:

>  Hi,
>
> Thank you for your response. Just for sake of completeness on this topic:
> the documentation mentions x_new as:
>
> *"new_x: (in), false if any evaluation method was previously called with
> the same values in x, true otherwise."*
> *
> *
> I will investigate how to implement this for my particular case.
>
>  Best regards,
>
>  -Martijn
> *
> *________________________________________
> Van: Stefan Vigerske [stefan at math.hu-berlin.de]
> Verzonden: maandag 10 september 2012 10:39
> To: Martijn Disse
> Cc: ipopt at list.coin-or.org
> Onderwerp: Re: [Ipopt] Algorithm flow calling f(x) and g(x), same decision
> variables?
>
>
> Hi,
>
> I think Ipopt does not want to give promises on which order functions
> are evaluated.
> You can check the newx flag that is passed with each function evaluation
> to see whether *some* evaluation function has been called for the same
> point already.
>
> Stefan
>
> On 09/10/2012 10:12 AM, Martijn Disse wrote:
> > Dear all,
> >
> > In both my cost and constraint function I simulate the same system with
> an input based on the decision variables and some other non-varying user
> data.
> >
> > In the case when the cost and constraint function are called with the
> same decision variables, to avoid redundant computations, I would would not
> like to simulate the system twice, but only once.
> >
> > I want to use a userdata structure to pass on the simulation results
> from cost to constraints or vice versa. Therefore I am trying to find out
> how the algorithm flow works:
> >
> > Does the IPOPT flow ever call cost and constraint with the same decision
> variables? If so, which one is called first?
> >
> > I have modified the hs071 example and I found out that the constraint
> function is called first and then the cost function is called with the same
> decision variables. Is this generally the case?
> >
> > I have looking into the publications, but was not very succesfull in
> finding an answer to my question. I am guessing it is quite a common
> problem, that why I am asking you :).
> >
> > I appreciate you help,
> >
> > Best regards,
> >
> > Martijn Disse
> >
> >
> >
> > _______________________________________________
> > Ipopt mailing list
> > Ipopt at list.coin-or.org
> > http://list.coin-or.org/mailman/listinfo/ipopt
> >
>
>
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