Draw a Histogram from within a root Macro

ROOT
1

Okay, so I would like to draw a histogram that I can normally draw by using

After opening the root file that I want to draw.

Now, I would like to draw the same histogram but instead of typing that in on the root command line, I would like to open it with a .C Macro.

I currently have this:

	char newrootpath[];
	*newrootpath = rootpath;

	TFile *f = new TFile(newrootpath);
	T->Draw("c1:t1");

where rootpath is type string, the basic C standard string.

Everytime I run this root crashes. I do not know why. Please help me by god.

Thank you.

Chris

2

Hi Chris,

you cannot access directly the name of the tree variable without having read it before. Try:

Cheers,
D

3

Danilo, you are the kitten’s mittens. Thank you.

I did have to change nullptr to NULL, but that may be because I’m using windows like 1% of the root community.

Thanks Again.

Chris

4

Hi Chris,

the reason for that is that the nullptr literal is available starting from c++11.
This standard is supported only starting from ROOT6: we are working hard to provide this ROOT version natively on windows too.

Cheers,
Danilo

5

Additionally, in this case you do not need to initialize the “T” pointer in advance, as the “GetObject” call will always set a new value (regardless whether the “object” was found or not).

6

Okay so on a similar note, I would like to have my macro display a pedestal histogram that has been created in a root tuple already. I normally do this by just double clicking on the pedestal histogram in the root object browser

RootObjectBrowserPedestal.PNG806×536 19.9 KB

Similarly to c1:t1 histogram, I’d like to open this histogram as well.

Here’s my function so far. I’ve verified that up to the cout statement everything works properly.

void MyMainFrame::ViewPedClicked()
{
	string rootpath = FilePath->GetText();
	rootpath = rootpath.substr(0, rootpath.length() - 4);
	rootpath += "_Analyzed.root";
	cout << rootpath << endl; // For Debugging

	TString newrootpath;
	newrootpath = rootpath;


		TFile f(newrootpath);
		TTree* T = NULL;
		f.GetObject("pedestal", T);
		f.Draw("pedestal");

}

Thanks for the support.

Chris

7

Hi Chris,

you should specify the correct type of object you are reading from the file. In addition, you need to invoke the Draw method of TH1F/D: please refer to root.cern.ch/doc/master/classTH1F.html
Assuming that the histogram is a TH1F (it could be a TH1D, it depends on who wrote it):

      TFile f(newrootpath);
      TH1F* T = NULL;
      f.GetObject("pedestal", T);
      T.Draw();

Cheers,
D

8

Okay after changing T.Draw() to T->Draw(), This code runs, but it just produces a blank canvas, without the histogram drawn on it.

Suggestions?

Thanks

Chris

9

Hi,

one thing which could be done is to go through the ROOT introductory material we suggest to get started with the Framework: root.cern.ch/getting-started
The histogram goes out of scope and its graphical representation too. Try DrawClone.