TTree::SetAlias with TMath::Min, TMath::Max issue

ROOT
1

Dear experts
I am observing some odd behaviour using TMath::Min/TMath::Max function in Alias definition.

The minimal reproduction of it is to take any TTree (or TChain)
and do something like this

tree->SetAlias("MyForm" , "TMath::Min(A,B)")
tree->Draw("MyForm")

doing so throws :

Error in <TTreeFormula::DefinedVariable>: The substitution of the alias "MyForm" by "TMath::Min(A,B)" failed.

Altough if i do the direct Draw it works.
A fall back is :

tree->SetAlias("MyForm" , "A<B?A:B")
tree->Draw("MyForm")

and it works.

I wonder if there is anything special on the TMath::Min and TMath::Max when used via Aliases .

Thanks in advance,

Renato

_ROOT Version: 6.16.0
_Platform: Any
Compiler: Not Provided

2

If I try to do
SetAlias(“myForm”, “TMath::Min<Double_t>(A,B)”);
I can draw it.

I am wondering what is going on and if it is expected and how one can identify the TMath functions which needs this special treatment

3

@pcanal what are the limitations of an alias - does it expand as a TTreeFormula itself?

4

I have found another weird behaviour.
If i do

tree->SetAlias( "cosAlpha", "TMath::Cos(alpha)")

and i do tree->Draw("xx","..... && cosAlpha>2")

it doesn’t work

If before doing the Draw i expand cosAlpha to the Alias defined it does.

5

@pcanal still waiting for your wisdom here :slight_smile:

6

There is a deficiency in TTreeFormula related to function calls (actually to the use of paranthesis), it does not detect (yet) that this should trigger the avoidance of a shortcut in the code and this lead to use the alias value as a variable. To work around the problem you can use:

tree->SetAlias("MyForm" , "TMath::Min(A,B)+0");

Cheers,
Philippe.

7

See also https://github.com/root-project/root/pull/3797

8

Thanks a lot,
In which root release this will be fixed?

9

It will be fixed in v6.18/00 (to be release soon) and could easily be backported to older release patch branches if needed.