Example to show how to display histograms?

Newbie
1

Hi all,

Is there an example that will show me how to display the histograms in the attached picture?

Thanks in advance

Tim

root_screenshot

2

Hi Tim,

You can double-click on the histogram and it will appear on the right side of the browser.

Cheers,
Jakob

3

Hi Jakob, thanks for your reply.

I suppose I wasn’t sufficiently clear with my request. I want to draw the histograms by code and not through the Root browser so I can do some funky fitting stuff in that code. But I’ve found an example that may help (tree1.C). I’ll play with that first.

Regards

Tim

4 
your_histogram->Draw();
5

Apologies for late reply. I’m a part-timer and don’t get as much time to dedicate to this as I’d like.

Having thought about this for a while I think my confusion stems from the difference between a tree, branch and leaf.

If we look at the picture I posted, there are examples that describe how to read and display histograms on the level immediately below the root file but my histograms are 2 levels below that. May I consider those histograms (in this case) as leaves of the branch PerEntryHistograms which is a branch off Event which is, in turn a branch off the root file?

If I’m correct in my above assumption I suppose I would address the branch Event and, off that, address the branch PerEntryHistograms and, finally, open and draw my histograms?

Regards

Tim

6 
your_file->cd("subdir");
your_histogram->Draw();
7

Hi Olivier,

sorry not at my machine with root installed currently. But do the “branches” appear like directories to root?

Regards

Tim

8

What your picture shows is a TFile with several levels of directories. At the bottom level you have histograms. You just need to cd to that level and do a Draw of your histogram.

9

Thanks Olivier. I found this great tut which explains it well: SLAC link

So I’m on my way…I hope :blush:

Regards

Tim

1 Like
10

Hi all

Oliviers suggestion worked fine but only from the Root command line. If I try do the same in a macro it raises an error when I try to Draw the histogram. Not on my Linux machine currently but will paste the error when I get home.

Regards

Tim

11

In a macro do:

your_file->cd("subdir");
TH1F *h = (TH1F*)your_file->Get("your_histogram");
h->Draw();
12

Sorry for being an idiot but I’m getting a warning when I use your suggestion:

Error in : Trying to dereference null pointer or trying to call routine taking non-null arguments.
Execution of your code was aborted.
In file included from input_line_28:1:
/home/tim/Data/Thesis/bdsim/WP/read_plot_from_file.C:20:2: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
h->Draw();

I tried passing a non-null argument in Draw (“SAME”).

Regards

Tim

read_plot_from_file.C (504 Bytes)
13

Try:

gDirectory->ls();
TH1F *h1; gDirectory->GetObject("Primary_Energy", h1);
14

Nah, sorry. Same error.

15

What do you get from gDirectory->ls();

16

Truncated, terser, version of previous f->ls();

TDirectoryFile* PerEntryHistograms PerEntryHistograms
KEY: TH1D Primary_Energy;1 Primary_Energy
KEY: TH1D EnergyLoss;1 EnergyLoss

17

So, if you have a “TH1D” in your file, why are you surprised that it cannot retrieve a “TH1F”?

18

'cause I’m as thick as :blush: (in my defence I’m a noob).

Cheers mate. That’s got it.

Regards

Tim