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Re: Baffled by FindMinimum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124911] Re: Baffled by FindMinimum
  • From: Sam Takoy <sam.takoy at yahoo.com>
  • Date: Sun, 12 Feb 2012 04:59:36 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201202101055.FAA00702@smc.vnet.net> <jh6kbb$jsf$1@smc.vnet.net>

On Feb 11, 3:50 pm, Bob Hanlon <hanlonr... at gmail.com> wrote:
> >From the documentaion for FindMinimum: "FindMinimumfirst localizes the
>
> values of all variables, then evaluates f with the variables being
> symbolic, and then repeatedly evaluates the result numerically."
>
> Since you don't want ObjectiveFunction to try evaluating with a
> symbolic argument, restrict its definition to a vector of numeric
> arguments.
>
> In general, restrict the definition of any function to numeric
> arguments when the function uses numeric techniques.
>
> Clear[ObjectiveFunction]
>
> ObjectiveFunction[p : {__?NumericQ}] :=
>  (p[[3]] - 1)^2 + Norm[p] /; Length[p] > 2
>
> p0 = {1, 2, 2, 1, 2};
>
> FindMinimum[ObjectiveFunction[xx], {xx, {1, 2, 3, 2, 1, 2}}]
>
> {0.75, {xx -> {-5.48284*10^-9, -7.29576*10^-9,
>     0.5, -7.02789*10^-9, -5.48284*10^-9, -7.02789*10^-9}}}
>
> Bob Hanlon
>
>
>
>
>
>
>
> On Fri, Feb 10, 2012 at 5:55 AM, Sam Takoy <sam.ta... at yahoo.com> wrote:
> > Hi,
>
> > Please take a look at this simple code.
>
> > ObjectiveFunction[p_] := Module[{},
> >  Print["Why is this p: ", p];
> >  (p[[3]] - 1)^2 + Norm[p]]
> > p0 = {1, 2, 2, 1, 2};
> > FindMinimum[ObjectiveFunction[xx], {xx, {1, 2, 3, 2, 1, 2}}]
>
> > If you run it, you will find that ObjectiveFunction is called with the
> > argument "xx" rather than the numerical vector. How come?
> > Also, judging by the lack of the Print statement firing,
> > ObjectiveFunction is not called again. Yet, it eventually (after
> > complaining of understandable errors) produces the correct answer.
> > What's going on here? I would expect if to be called repeated in the
> > course of numerical minimization. I'm completely baffled by what's
> > going on here.
>
> > Here's the output:
>
> > During evaluation of In[377]:= Why is this p: xx
>
> > During evaluation of In[377]:= Part::partd: Part specification xx[[3]]
> > is longer than depth of object. >>
>
> > During evaluation of In[377]:= Part::partd: Part specification xx[[3]]
> > is longer than depth of object. >>
>
> > Out[380]= {0.75, {xx -> {-5.48284*10^-9, -7.29576*10^-9,
> >    0.5, -7.02789*10^-9, -5.48284*10^-9, -7.02789*10^-9}}}
>
> > Many thanks in advance,
>
> > Sam

Thank you so much!



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