Intersection of TGraph and TF1

ROOT
1

Hello, I am trying to find the point of intersection between a TGraph and an analytic TF1. I have an example from Rene where he uses a C-style function to create a TF1 from a TGraph object, and I am trying to use this approach, but I am getting an error when trying to define the object to find a difference between the TF1 defined the normal way and the TF1 defined from the TGraph.

ROOT Version: 6.20/04

Here is a minimal example using a simple sin function:

using namespace std;

TGraph* trueStress;
double myfunc(double *x, double *){ return trueStress->Eval(x[0]);}

void minimalProblem(){

    //populate TGraph with a sin function
    Double_t x[100], y[100];
    Int_t m = 20;
    for (Int_t i=0;i<m;i++) {
	x[i] = i*0.1;
	y[i] = 10*sin(x[i]+0.2);
    }
    trueStress = new TGraph(m,x,y);
    trueStress->Draw();

    //simple TF1 of a line
    TF1* line = new TF1("line","10*x");
    gROOT->GetListOfFunctions()->Add(line);
    //verify that line and sin function intersect somewhere, want to find the point of intersection
    line->Draw("same");

    //create a TF1 from my TGraph using C style function (from Rene Brun example)
    TCanvas* c4 = new TCanvas("c4");
    TF1* f1 = new TF1("f1",myfunc,0,2);
    gROOT->GetListOfFunctions()->Add(f1);
    f1->Draw();
    
    //create new TF1 to find intersection of two other TF1s
    TF1* diff = new TF1("diff", Form("abs(%s-%s)",f1->GetName(),line->GetName()),0,0.2);
}

The errors I get are:

input_line_111:2:77: error: use of undeclared identifier 'f1'
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
                                                                            ^
input_line_112:2:80: error: expected ')'
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
                                                                               ^
input_line_112:2:75: note: to match this '('
Double_t TFormula____id5398377958029293329(Double_t *x){ return TMath::Abs({f1}-(10*x[0])) ; }
                                                                          ^
Error in <prepareMethod>: Can't compile function TFormula____id5398377958029293329 prototype with arguments Double_t*
Error in <TFormula::InputFormulaIntoCling>: Error compiling formula expression in Cling
Error in <TFormula::ProcessFormula>: "f1" has not been matched in the formula expression
Error in <TFormula::ProcessFormula>: Formula "abs(f1-(10*x))" is invalid !

Thank you for any assistance!

2

The last line of your macro produces this error. It is equivalent to:

   auto diff = new TF1("diff", "abs(f1-line)",0,0.2);

Which also produces the same error. May be @moneta will know.

3

Hi,
The TF1 function f1 is based on a C++ function pointer myfunc.
It cannot then used to build another TF1 based on an expression.
See ROOT: TF1 Class Reference

Cheers

Lorenzo

1 Like
4

Ah, I was afraid that would be the answer.
Is there by chance another way to find an intersection point between a TGraph and an analytical TF1?

Thank you!

5

Hi,
You can use the example above but you need to use a lambda for creating the TF1 diff .
You can do as following:

auto diff = new TF1("diff",[&](double *x, double *p){ return std::abs(f1->EvalPar(x,p)-line->EvalPar(x,p)); },0,0.2, f1->GetNpar() );

Lorenzo

6

Lorenzo, thank you! I never would have figured that out.

This is not that important, but is there a reason ROOT crashes when I try to diff->Draw()?

 Generating stack trace...
 0x00007f88a8234d3b in <unknown function>
 0x00007f88961bb970 in TF1::EvalPar(double const*, double const*) + 0x1b0 from /home/jbouffar/root-6.20.04-install/lib/libHist.so
 0x00007f88961c46b2 in TF1::DoCreateHistogram(double, double, bool) + 0x512 from /home/jbouffar/root-6.20.04-install/lib/libHist.so
 0x00007f88961c3b98 in TF1::Paint(char const*) + 0x1e8 from /home/jbouffar/root-6.20.04-install/lib/libHist.so
 0x00007f8895a5b69b in TPad::PaintModified() + 0x23b from /home/jbouffar/root-6.20.04-install/lib/libGpad.so
 0x00007f8895a18986 in TCanvas::Update() + 0x136 from /home/jbouffar/root-6.20.04-install/lib/libGpad.so
 0x00007f88a222587c in <unknown> from /home/jbouffar/root-6.20.04-install/lib/libCling.so
 0x00007f88a222bc2a in <unknown> from /home/jbouffar/root-6.20.04-install/lib/libCling.so
 0x00007f88a222d120 in <unknown> from /home/jbouffar/root-6.20.04-install/lib/libCling.so
 0x00007f88a7de186f in TRint::Run(bool) + 0x1bf from /home/jbouffar/root-6.20.04-install/lib/libRint.so
 0x00007f88a8400a0c in <unknown> from /home/jbouffar/root-6.20.04-install/bin/root.exe
 0x00007f88a6cc1b97 in __libc_start_main + 0xe7 from /lib/x86_64-linux-gnu/libc.so.6
 0x00007f88a8400a6a in _start + 0x2a from /home/jbouffar/root-6.20.04-install/bin/root.exe
terminate called after throwing an instance of 'cling::InvalidDerefException'
  what():  Trying to access a pointer that points to an invalid memory address.
7

Hi,

You have to make sure that the objects referenced by the function in diff through the lambda do not disappear (are deleted) when drawing

Lorenzo

8

Thanks for update and quick reply, its work for me. really appreciate.

9

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