Do you mean that you want to say
X = (log(A) + log(B))/(log(A) + log(B) + log(C)) ?
You are free to use the log function anywhere in constraints, though then
you need to send the problem to a solver that can deal with nonlinear
constraints; many such solvers are listed at www.ampl.com/solvers.html. Of
if you do a change of variables A -> e^A, etc. you could move the
nonlinearity to some other part of the problem, perhaps into the objective.
There are actually two log functions in AMPL, log() for the natural
logarithm and log10() for the logarithm base 10.
Bob Fourer
4er@...
> -----Original Message-----
> From:
ampl@... [mailto:
ampl@...]
> On Behalf Of keng [
kengcher@...]
> Sent: Wednesday, September 16, 2009 10:13 AM
> To: AMPL Modeling Language
> Subject: [AMPL 2826] Ratios and exponential
>
> Hi
>
> I'm trying to set a new constraint that as such:
>
> X = (A + B)/(A + B + C)
>
> but the problem is all my variables are in the form of the log of A,
> B, C, etc.
>
> I was wondering if there's a way to still implement such a constraint?
>
> Thanks!
>
> best rgds
>
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