Walkthrough: Plotting a function
Exercise 1: Detective work
Walkthrough: Working with Histograms
Walkthrough: Saving and printing your work
Walkthrough: The ROOT browser
Walkthrough: Fitting to a Gaussian distribution
Walkthrough: Fitting to a user-defined function
Walkthrough: Saving your work, part 2
Example experiment n-tuple
Using the Treeviewer
The Notebook Server
Decisions
The C++ Path
The Python Path
The RDataframe Path
Intermediate Topics
Advanced Exercises
Expert Exercises
Wrap-up
Appendix
Version History
So now you’ve got a histogram fitted to a complicated function. You can
use
, quit ROOT, restart it, then load canvas
“c1;1” from the file. You’d get your histogram back with the function
superimposed… but it’s not obvious where the function is or how to
access it now.
What if you want to save your work in the same file as the histograms
you just read in? You can do it, but not by using the ROOT browser. The
browser will open .root files in read-only mode. To be able to modify a
file, you have to open it with ROOT commands.
Try the following: Quit ROOT (note that you can select
or
from the canvas). Start ROOT again, then modify
“histogram.root” with the following commands:
[] TFile file1("histogram.root","UPDATE")
It is the “UPDATE” option that will allow you to write new objects to
“histogram.root”.
[] hist2->Draw()
For the following two commands, hit the up-arrow key until you see them
again.
[] TF1 func("user","gaus(0)+gaus(3)")
[] func.SetParameters(5.,2.,1.,1.,10.,1.)
[] hist2->Fit("user")
Now you can do what you couldn’t before: save objects into the ROOT
file:
[] hist2->Write()
[] func.Write()
Close the file to make sure you save your changes:
[] file1.Close()
Quit ROOT, start it again, and use the ROOT browser to open
“histogram.root”. You’ll see a couple of new objects: “hist2;2” and
“user;1”. Double-click on each of them to see what you’ve saved.
You wrote the function with func.Write(), but you saw user;1 in the
file. Do you see why? It has to do with the name you give to objects in
your programming environment, versus the internal name that you give to
ROOT. There’s more about this later in the tutorial.
Though they seem closely connected at times,
the program environment and the ROOT toolkit are two different entities.
Chapter 11 of the ROOT Users
Guide
has more information on using ROOT files.
In case you care: ROOT stores your ROOT commands in the file
.root-hist in your home directory; that’s where it gets the lines
you see with the up-arrow key. Similarly, the UNIX shell stores the
last 5000 commands you’ve typed in .sh-history in your home
directory.
Did you just try to use ls to see these files?
Congratulations on wanting to learn more about UNIX.
You didn’t see them.
The reason why is that, by default, UNIX “hides” filenames that
begin with a . (period), so that you don’t have to look at
work files every time you examine a directory’s contents. To see
these “invisible” files, the command is:
> ls -a
I’ve seen some ROOT documentation that suggests that closing the
file is optional, since ROOT usually closes the file for you when
you quit the program. However, I’ve also seen many ROOT files made
unreadable because they weren’t closed properly. I suggest you
always explicitly close any file you open!
