Sometimes you need to fit a function to a histogram in ROOT. Attached is code for how to do that in the simple case of a power law fit.

To run the example, you should type "root fitSnr.C" in the command line. The code will access the source histogram (hstrip1nsr.root, which is actually ANITA-2 satellite data). The result is stripe1snrfit.png.

#include "TF1.h"

void fitSnr();

void fitSnr()

{
	gStyle->SetLineWidth(4); //set some style parameters
	TFile *stripe1file = new TFile("hstripe1snr.root"); //import the file containing the histogram to be fit
	TH1D *hstripe1 = (TH1D*)stripe1file->Get("stripe1snr"); //get the histogram to be fit
	TCanvas c1("c1","c1",1000,800); //make a canvas
	hstripe1->Draw(""); //draw it
	c1.SetLogy(); //set a log axis

	//need to declare an equation
	//I want to fit for two paramters, in the equation these are [0] and [1]
	//so, you'll need to re-write the equation to whatever you're trying to fit for
	//but ROOT wants the variables to find to be given as [0], [1], [2], etc.
	
	char equation1[150]; //declare a container for the equation
	sprintf(equation1,"([0]*(x^[1]))"); //declare the equation
	TF1 *fit1 = new TF1("PowerFit",equation1,20,50); //create a function to fit with, with the range being 20 to 50

	//now, we need to set the initial parameters of the fit
	//fit->SetParameter(0,H->GetRMS()); //this should be a good starting place for a standard deviation like variable
	//fit->SetParameter(1,H->GetMaximum()); //this should be a good starting place for amplitude like variable
	fit1->SetParameter(0,60000.); //for our example, we will manually choose this
	fit1->SetParameter(1,-3.);

	hstripe1->Fit("PowerFit","R"); //actually do the fit;
	fit1->Draw("same"); //draw the fit

	//now, we want to print out some parameters to see how good the fit was
	cout << "par0 " << fit1->GetParameter(0) << " par1 " << fit1->GetParameter(1) << endl;
	cout<<"chisquare "<<fit1->GetChisquare()<<endl;
	cout<<"Ndf "<<fit1->GetNDF()<<endl;
	cout<<"reduced chisquare "<<double(fit1->GetChisquare())/double(fit1->GetNDF())<<endl;
	cout<<"   "<<endl;
	c1.SaveAs("stripe1snrfit.png");

}