[Ns-developers] Random Variables API changes

Gustavo Carneiro gjcarneiro at gmail.com
Tue Jan 20 10:09:11 PST 2009

2009/1/20 Tom Henderson <tomh at tomh.org>

> >-----Original Message-----
> >From: Gustavo Carneiro [mailto:gjcarneiro at gmail.com]
> >Sent: Tuesday, January 20, 2009 09:19 AM
> >To: 'Raj Bhattacharjea'
> >Cc: 'Tom Henderson', 'NS3 Developers Mailing List'
> >Subject: Re: [Ns-developers] Random Variables API changes
> >
> >2009/1/20 Raj Bhattacharjea <raj.b at gatech.edu>
> >
> >> On Tue, Jan 20, 2009 at 11:40 AM, Raj Bhattacharjea <raj.b at gatech.edu>
> >> wrote:
> >> > quick ideas that need to be looked at.  The biggest of these is a
> >> > change like the following:
> >> >
> >> > - UniformVariable::GetSingleValue (0, N)
> >> > + UniformVariable().GetValue (0, N)
> >> >
> >> > I had to do this in one place to make things compile, but seems a bit
> >>
> >> I forgot I had to do this in several places, or things similar to
> >> this.  The affected files are:
> >>
> >> src/devices/csma/backoff.cc
> >> src/mobility/random-direction-2d-mobility-model.cc
> >> src/routing/olsr/olsr-agent-impl.cc
> >>
> >> I'm sure the maintainers of these would have their preferred
> >> implementation.
> >
> >
> >I wonder what is the rationale for the removal of GetSingleValue?  It
> seems
> >to me that keeping a single static UniformVariable global variable is just
> a
> >pointless API change, as it is surely equivalent to previous code, while
> >keeping one UniformVariable per OLSR instance sounds like a memory waste
> if
> >no reason for the change is known.
> >
> >I think there is probably a good reason for the API change and, not that I
> >don't trust you, but I would like to know what that reason is.  Was the
> >previous GetSingleValue usage pattern incorrect?
> Here is the background on this proposed change:
> http://mailman.isi.edu/pipermail/ns-developers/2008-September/004732.html
> My understanding is that the recommended way would be for users to create
> their own random variables, but if there is some reason they don't want to
> do that, they could draw from a common stream; the first stream generated by
> the RNG.  This stream would only provide Uniform(a,b) random variables,
> presumably via static functions or else via some global object.

>From the email:

* remove the GetSingleValue function from RandomVariable classes

The GetSingleValue function allows for potential non-independence as
it draws values from a single RNG stream for different random
variables.  Each random variable should have its own RNG stream.

>From that text it sounds like either a temporary RV instance or a single
global RV for all OLSR instances is not recommended.  But I am partially
guessing.  Under which conditions does "Each random variable should have its
own RNG stream" hold true?  Without reading the source code I can't be 100%

Basically I believe the Changes.html files should contain an answer to the
following question: "how to migrate code using the old GetSingleValue to new
API?".  The sooner that question is answered, the sooner ns-3's own code can
be correctly fixed, let alone 3rd party code.

So, in the OLSR example, OLSR agent would have the choice of a per-agent
> UniformVariable generator, or could access the (global) first stream.  But,
> I didn't think that we were talking about using anonymous temporary
> variables for this purpose.

I have no idea what a "stream" is and I don't care :-)  I thought we were
supposed to do everything via XxxVariable classes, not via RNG streams.


Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert

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