Subtracting two histograms with different axis limit

ROOT
1

I made two histograms with different functions having different domain. I tried to subtract the two histograms by doing the following, but it gave me junk. What is the propoer way to subtract two histograms with different axis limit?

TF1 fun1("fun1", "exp(-x)", 0, 5.)
TF1 fun2("fun2", "exp(-2*x)", 0, 3.)

TH1F *hfun1 = fun1.GetHistogram()
TH1F *hfun2 = fun2.GetHistogram()

hfun1->Add(hfun2, -1)

hfun1->Draw() gives me junk result.

Thanks,
asubedi

2

The warning is just a warning. You can bypass it as shown in the example below:

Rene

void reb() {
TCanvas *c1 = new TCanvas(“c1”,“c1”,800,1000);
c1->Divide(1,2);
c1->cd(1);
TF1 *fun1 = new TF1(“fun1”, “exp(-x)”, 0, 5.);
TF1 fun2 = new TF1(“fun2”, "exp(-2x)", 0, 3.);

fun1->Draw();
fun2->Draw(“same”);

TH1F *hfun1 = new TH1F(“hfun1”,“fun1”,100,0,5);
TH1F *hfun2 = new TH1F(“hfun2”,“fun2”,100,0,3);
hfun1->Eval(fun1);
hfun2->Eval(fun2);

c1->cd(2);
hfun1->DrawCopy();
hfun2->Draw(“same”);
//to bypass the warning
hfun2->GetXaxis()->SetLimits(0,5);
hfun1->Add(hfun2, -1);
hfun2->GetXaxis()->SetLimits(0,3);
hfun1->SetLineColor(kBlue);
hfun1->Draw(“same”);
}

3

But compare the resulting histogram with the function:

TF1 fun3 = new TF1(“fun3”, "exp(-x) - exp(-2x)", 0, 5.)

The shape of fun3 and the histogram hfun1->Add(hfun2, -1) should be the same.

I am using version 4.00/06

4

The Warning message is appropriate because the subtraction is bin by bin.
In your case, you should do:

void reb() {
TCanvas *c1 = new TCanvas(“c1”,“c1”,800,1000);
c1->Divide(1,2);
c1->cd(1);
TF1 *fun1 = new TF1(“fun1”, “exp(-x)”, 0, 5.);
TF1 fun2 = new TF1(“fun2”, "exp(-2x)", 0, 3.);
TF1 fun3 = new TF1(“fun3”, "exp(-x) - exp(-2x)", 0, 5.);
fun1->Draw();
fun2->Draw(“same”);
fun3->Draw(“same”);

TH1F *hfun1 = new TH1F(“hfun1”,“fun1”,100,0,5);
TH1F *hfun2 = new TH1F(“hfun2”,“fun2”,100,0,5);
hfun1->Eval(fun1);
hfun2->Eval(fun2);

c1->cd(2);
hfun1->DrawCopy();
hfun2->Draw(“same”);
hfun1->Add(hfun2, -1);
hfun1->SetLineColor(kBlue);
hfun1->Draw(“same”);
}

Rene

5

Thanks rene. But doing hfun2->Eval(fun2) evalutates fun2 over whole axis of hfun2. Is the option r' for Eval() implemented? In the TH1.cxx.html at htmldocs, it looks like only optionsa ’ and `s’ are implemented.

In any case, I solved my problem by making my function return 0 after its actual maximum value of x variable.

6

The documented option “R” is now implemented in the CVS version.

Rene