Fitting pol2()

ROOT
#1

Hi folks

Finally I get the solution to my problem. I was trying to fit a pol2 into my data but root was fitting a linear function… I was using:

TF1 *myfit;
.
.
.
c1 = new TCanvas(“c1”,“Transmission”,20,50,1200,800);

c1->SetFillColor(kWhite);
c1->SetGrid();
myfit = new TF1(“myfit”,“pol2(0)”, 3170, 3181);
.
.
.
gr->SetTitle(message);
gr->GetXaxis()->SetTitle(“runs”);
gr->GetYaxis()->SetTitle(“Signal Output”);

gr->Fit(“myfit”,“R”);
gr->Draw(“ACP”);
.
.
.

this did not work, but then I wrote this instead:

.
.
.
gr->Fit(“pol2”);

and it is working now. To tell you the truth, this is the first time using root, and after around 4 hours trying to find the error I hit a post that brought me to this solution. I would like to know what the first code that I wrote did not work. I really appreciate your comments.

Second question: how can I extended mi fitting to a wider range … can I use the option “R” in the line

gr->Fit(“pol2”,“R”)

I did like that but root does not show the fitting.

Third and final question: if this is the way to do a plot, then I do not need the object TF1 *myfit ?

Again, thanks for any comments,

Cheers,
Kris[quote][/quote][quote][/quote]

#2

Could you send the shortest possible running script showing this problem?

Rene

#3

This was my program. Please, check line 26 and 46. It is there where I was wondering if I was doing wrong. The text file that I open in the program consist of 2 columns of numbers.

I really appreciate your comments.

{
//THANK YOU TO @DABAGIAN
//MODIFY JULY 8/2004 BY @KRISFRAJER

#include “iostream.h”

gROOT->Reset();
const Int_t n = 5;

char fname[100], message[50];
ifstream in;
Int_t ring=2;
Int_t run=3173;
Int_t chann=21;
Double_t p, q;
Double_t x[n], y[n];
TF1 *myfit;

sprintf(fname,“plotDt%drn5.txt”,chann);
in.open(fname);

c1 = new TCanvas(“c1”,“Transmission”,20,50,1200,800);

c1->SetFillColor(kWhite);
c1->SetGrid();
//line26
myfit = new TF1(“myfit”,"[0]+[1]*x", 3170, 3181);

//I was trying to get a square fit, so a replace the //formula on myfit object to something like “pol2” //and “[0]+[1]*x+[2]*x^2”. None of this work. What
//was I doing wrong?

for (Int_t i=0;i<n;i++){
in >> p >> q;
x[i] = p;
y[i] = q;
printf(" i %i %f %f \n",i,x[i],y[i]);
}

gr = new TGraph(n,x,y);
gr->SetLineColor(1);
gr->SetLineWidth(1);
gr->SetMarkerColor(4);
gr->SetMarkerStyle(25);
sprintf(message,“detector %d”,chann);
gr->SetTitle(message);
gr->GetXaxis()->SetTitle(“runs”);
gr->GetYaxis()->SetTitle(“Signal Output”);

//line46
gr->Fit(“myfit”,“R”);
//I found out that if I write the last line as
//gr->Fit(“pol2”);
//my program worked!
gr->Draw(“ACP”);

// TCanvas::Update() draws the frame, after
// which one can change it
c1->Update();
c1->GetFrame()->SetFillColor(kWhite);
c1->GetFrame()->SetBorderSize(2);
c1->Modified();
in.close();
}

#4

you forgot the data file

Rene

#5

Sorry, I forgot to include the file…

Kris
plotDt21rn5.txt (65 Bytes)

#6

If you want to draw your picture with the function range, you should draw the function first. See your script slightly modified in the attachement

Rene
fpol2.C (1.51 KB)

#7

Thank you rene,

now it is working as I wanted. Now my next challenge is to plot several graphs in the same pad, so I will use TPad. Wish me luck,

Krisfrajer

#8

I have a quick question… can I write this code that i just showed you in c++? for example

int main(int argc, char *argv[])
{
gROOT->Reset();
onst Int_t n = 5;

char fname[100], message[50];
ifstream in;
.
.
.

}

Thanks,

Krisfrajer

#9

Yes but remove

and add the proper #include (at the start of the file).

Cheers,
Philippe.

#10

#include “TH1.h”
#include “TF1.h”
#include “TProfile.h”
#include “TNtuple.h”
#include “TRandom.h”
#include “TGraph.h”
#include “TCanvas.h”
#include “TFrame.h”

this was what I need to de what I wanted,
Thank you,

Krisfrajer