Creating multiple histograms

ROOT
1

Hello Rooters,

I am currently trying to write a code that will read in a general data set. I can get it read into a general array, such that the number of lines and columns are arbitrary.
The next step however is to create histograms of each read in column. In trying to do this I ran into a problem, namely I cannot create the histograms until I know how many columns of data there are, but I have to write the code not knowing how many columns of data. So I tried creating a general histogram so that I could create a loop and thus create as many as needed. However I cannot get it to work. Any thoughts would be useful. I also need to be able to do this with canvas’.

Thanks
-Paul

Here is a sample code

{
       gROOT->Reset();

       // these arrays would become 2 dimensional for multiple names
       char h0[] = "hist";  
	char histogram_name[] = "histogram";
    
       // this would go in a loop to create as many as needed
	TH1F h0(histogram_name, histogram_name, 100, -3, 3);
	
       //there would also need to be any given number of these
	TCanvas *Canvas1= new TCanvas("Canvas1", "Canvas 1", 1);
       Canvas1->Divide(1,2);

       Canvas1->cd(1);
       h0->Draw();
}
2

It is not clear what you are trying to do!
Do you want to read an ascii file?
If yes, may be have a look at TTree::readFile or TNtuple::ReadFile
see: root.cern.ch/root/htmldoc/TTree. … e:ReadFile

Rene

3

Hi Again,

no I understand how to read in a ascii file. My question relates to creating TH1F’s
I read in an ascii file and I get an nxm array of type Float where n is the number of rows and m is the number of columns. Then I want to create a loop (for, while…) to create m TH1F s all with diffrent names so that each one corresponds to a column. However I cannot simply start creating the TH1F’s as follows:

TH1F *h0 = new TH1F(“Histo 0”, “Histo 0”, 10, 0, 100);
TH1F *h1 = new TH1F(“Histo 1”, “Histo 1”, 10, 0, 100);
TH1F *h2 = new TH1F(“Histo 2”, “Histo 2”, 10, 0, 100);
.
.
.
TH1F *hm = new TH1F(“Histo m”, “Histo m”, 10, 0, 100);

I cannot do it that way because I do not know how many I want to create until after I have read in the text file as the number changes from file to file. so what I tried doing was something more along the lines of:

char h[] = “h”;

for( i=0;
i <=m;
i = i +1)
{
h[i] = “h”;
strcat(h[i], (char)m); // so now h[i] would be hm
TH1F *h[i] = new TH1F(Histo[i], Histo[i], 10, 0, 100);
}

this way I could vary m and Histo[i]
my problem comes in when I try to vary *h[i] no matter how I try it always gives me an error of some sort.

I hope that makes things a little more clear. Sorry for the confusion earlier.

-Paul

4

Create a dynamic array of pointers

  TH1F** h = new TH1F*[m];
  for (int i=0;i<m;i++) {
     h[i] = new TH1F(name,title...

Rene

5

Dear brun,
This code does not work at all.
Hai

6

Hi,

Could you be more precise? What doesn’t work? Could you tell us what you’re trying to do, and if possible with a piece of code showing the problem?

Cheers, Bertrand.

7

I tried the code written as follows:

 int m=5;
 Th1F** h=new TH1F*[m];
 for (int i=0; i<m;i++){
  h[i] = new TH1F("h0","event",1000,0,1000);
}


Running code in Root, it returns:

Warning in <ROOT::Append>: Replacing existing TH1: h0 (Potential memory leak).
Warning in <ROOT::Append>: Replacing existing TH1: h0 (Potential memory leak).
Warning in <ROOT::Append>: Replacing existing TH1: h0 (Potential memory leak).
Warning in <ROOT::Append>: Replacing existing TH1: h0 (Potential memory leak).

Could you let me know the problem?
Thanks!
Hai

8

Try:

h[i] = new TH1F(TString::Format("h0_%d", i), "event", 1000, 0., 1000.);
3 Likes
9

It works well! Thank you very much!