Re: Questions on TTreeFormula

Thank you. This is just what I wanted to do.

The implicit definitions of "pi" and "rndm" are a surprise to me -- is there anyplace that gives the full list of valid functions and aliases known to TTreeFormula?

Correction: The formula for a Gaussian random number is:

     sin(2*pi*rndm)*sqrt(-2*log(rndm))
Note the factor of 2 inside the sin() -- your formula gives only positive values.

Tom Roberts

Rene Brun wrote:
> see TTree::SetAlias
>
> For example, you can define the following alias
> tree.SetAlias("rng","sin(pi*rndm)*sqrt(-2*log(rndm))")
>
> This will create the tree alias named "rng". The value of the rng
> variable will be a gaussian random number with mean=0 and sigma=1. See
> the effect doing
> tree.Draw("rndm")
> tree.Draw("rng")
> tree.Draw("myvar+0.1*rng")
>
> If you want to create an alias returning a random gaussian number with
> mean = -1.5 and sigma=2.5, do
> tree.SetAlias("rng2","-1.5+2.5*sin(pi*rndm)*sqrt(-2*log(rndm))")
> tree.Draw("myvar+rng2")
>
> above "rndm" is a special keyword known by TFormula. It calls the
> current random number generator pointed by gRandom (currently the
> Mersenne Twister algorithm in TRandom3).
>
> Note that using TTree::SetAlias you can define an alias formula
> referencing any combination of variables in the Tree.
>
> Rene Brun
>
>
> 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
>>