Re: Questions on TTreeFormula

It is not clear if your question is related to TTreeFormula or not. In case it is, see my previous mail about TTree::SetAlias. In case it is not, see TRandom::Gaus, etc

Note that rand is a very bad random number generator.

Rene Brun

Tom Roberts wrote:
> I can get a Gaussian random number using
> TMath::ErfInverse(rand()/0x3fffffff-1)
> Three problems:
> 1) rand() is known to be a poor random number generator
> 2) for 2 million calls it never exceeds +-3.4
> 3) its RMS is 0.707 rather than 1.0 (did I forget a sqrt(2)?)
>
> Still, this is good enough for my immediate needs (dividing by 0.707
> to get sigma right).
>
>
> Tom Roberts
>
>
>
> Tom Roberts wrote:
>> Questions on TTreeFormula:
>>
>> 1) What functions are permitted? Specifically: are there any
>> random-numbers? I would specifically like GaussRan(mean,sigma).
>> (I want to add a measurement resolution in, but am in a place
>> where C++ is hard but can type an expression into a TTreeFormula).
>>
>> 2) Is it possible for me to define new functions? If so, how?
>>
>> 3) What is the relationship between TFormula and TTreeFormula? yes,
>> I know the latter inherits from the former, but what is the
>> relationship for evaluation? This might make (4) below moot.
>>
>> 4) Is it possible for me to define new variables? specifically,
>> I would like to define something like "Ptot=sqrt(Px*Px+Py*Py+Pz*Pz)"
>> where Px,Py,Pz are float-s in the TTree. It should be evaluated
>> before the TTreeFormula is evaluated (as the formula might use Ptot).
>>
>> This might be implementable as a friend, but that's complicated, and
>> I don't really want to create a whole new TFile for a transient
>> thing like this; it would also require a scan of the original
>> TTree to fill the friend.
>>
>> 4a) If it is possible to define a new function, is it possible to
>> define one without an argument or parens? this would let me do (4)
>> in C++.
>>
>> If neither (4) nor (4a) is possible, I may implement (4) as a text
>> macro that expands defined variable names before creating the
>> TTreeFormula.
>>
>>
>> Tom Roberts
>>
>
Received on Tue Mar 25 2008 - 08:07:11 CET