Linear fit name

ROOT
1

Why if I declare:

FitFormulaSlope2 = new TF1("FitFormulaSlope2", "1 ++ x ++(- 1./(x*x))", 0.3, 1000);

it says:

Error in <TFormula::TFormula>: The name 'x' is reserved as a TFormula variable name.
This function will not be registered in the list of functions
2

Because of these lines in a TFormula ctor:

//From TFormula
if (strcmp(name,"x")==0 || strcmp(name,"y")==0 ||
       strcmp(name,"z")==0 || strcmp(name,"t")==0 )
   {
      Error("TFormula","The name \'%s\' is reserved as a TFormula variable name.\n"
         "\tThis function will not be registered in the list of functions",name);
   } else {
      gROOT->GetListOfFunctions()->Add(this);
   }

And because of how TFormula::ProcessLinear is implemented.
If I understand the code correctly, for expression with ++, original expression

a ++ b

is parsed into parts and for “sub-expressions” ‘a’ and ‘b’ two TFormula’s with names ‘a’ and ‘b’ are created, so, if you have, 1 ++ x TFormulas with a names ‘1’ and ‘x’ are created. The second is a wrong name.

So, this is simply wrong syntax in your expression?

3

[quote=“tpochep”]Because of these lines in a TFormula ctor:

//From TFormula
if (strcmp(name,"x")==0 || strcmp(name,"y")==0 ||
       strcmp(name,"z")==0 || strcmp(name,"t")==0 )
   {
      Error("TFormula","The name \'%s\' is reserved as a TFormula variable name.\n"
         "\tThis function will not be registered in the list of functions",name);
   } else {
      gROOT->GetListOfFunctions()->Add(this);
   }

And because of how TFormula::ProcessLinear is implemented.
If I understand the code correctly, for expression with ++, original expression

a ++ b

is parsed into parts and for “sub-expressions” ‘a’ and ‘b’ two TFormula’s with names ‘a’ and ‘b’ are created, so, if you have, 1 ++ x TFormulas with a names ‘1’ and ‘x’ are created. The second is a wrong name.

So, this is simply wrong syntax in your expression?[/quote]

It’s not a wrong syntax, I want to use a function like [0] + [1] * x +[2](- 1./(xx)) with linear fitter. If I look at the documentation (root.cern.ch/root/html/TGraph.html#TGraph:Fit):

 Example: to fit the parameters of "[0]*x + [1]*sin(x)", create a
    TF1 *f1=new TF1("f1", "x++sin(x)", xmin, xmax);

I got the same problem. Apart of the error message it seems to works.

4

Ah, ok, I agree. In fact, this is not a problem in your case - simply annoying message and the TFormula for sub-expression ‘x’ is not added into the global list of functions.

5

Hi,

This spurious error message is removed by revision 38247 of the trunk.

Cheers,
Philippe.