[Ns-developers] Random Variables API changes

[Tom Henderson]

>> One other API question I had was concerning the granularity of 
>> controlling the seeding of the RandomVariables.  Presently, there 
>> seems to be no way to decouple some RandomVariables and hold their 
>> generators constant while varying others.  For instance, in a given 
>> simulation scenario, there might be a lot of random variables in use.  
>> A user might want to fix (e.g., use a static seed) the randomness of 
>> nearly all of those variables while allowing one random variable to 
>> vary across run numbers, in order to isolate the contribution of a 
>> single random variable.  I thought that this decoupling was a design 
>> goal at some point.  The current API seems to amount to requiring all 
>> random variables to be fixed across different simulation runs, or all 
>> to be reseeded (independently) across runs.
> 
> This is definitely something that would be useful.
> 
> Here's what I would propose:
> * create a runNumber variable in RandomVariable
> * create a setRun function in RandomVariable
> * upon initialization of the random variable, copy the value in 
> SeedManager::runNumber to runNumber
> 
> Then, for each random variable, the user can override the runNumber in 
> SeedManager. We'd have to change the calls when initializing new RNGs 
> (for instance, in GetValue) to use the RandomVariable version of 
> runNumber instead of SeedManager::runNumber.  We'd also want to ensure 
> that once the RandomVariable was initialized (or used for the first 
> time) that the run number couldn't change.
> 
> Is this something we want to go ahead and add now while we're messing 
> with the API, or something to add later?

I think now is better than later for any API changes, and the proposal 
above.

- Tom