TF ErrAna InClassLab

Start ROOT and Draw Histogram

Step 1

In this first lab your goal will be to start root and draw a histogram.

If you are on a Windows machine start the ROOT interpreter by double clicking on the Desctop icon which looks like a tree and says ROOT underneath it If you are on a Unix machine set the environmental variable ROOTSYS to point to the base subdirectory containing the ROOT files and start root with the command $ROOTSYS/bin/root. You can set the ROOTSYS environmental variable under the bash chell with the command

export ROOTSYS=path

where path identifies the location of the root base subdirectory. My ROOT base subdirectory is located at ~/src/ROOT/root so I would execture the following shell command

export ROOTSYS=~/src/ROOT/root

Step 2

Define a variable to contain the desired histogram

TH1F *Hist1=new TH1F("Hist1","Hist1",50,-0.5,49.5);

The above function "TH1F" has 5 parameters and is a member of the class TH1.

The first parameter "Hist1" create a name for the histogram. The second parameter gives the histogram a title. I usually set the name and title to the same variable name I use to store the histogram. This allows me to look up the variable name of the histogram when I double click on the histogram from the browser listing. The third parameter (50) is an integer which identifies the number of "bins" the histogram will use to store information. The Fourth parameter (-0.5) is a real number identifying the numerical value to be used for the lowest bin and the fifth parameter is the value for the highest bin.

Step 3

The class TH1 contains a function "Fill" which you can you to insert data into the histogram. In the above example the variable "Hist1" was defined in terms of the class TH1. To use the Fill function which is defined within the class TH1 and associated with the histogram named Hist1 you use the command

 Hist1->Fill(10);

The above Fill function will increment the counter for the bin in Hist1 which is associated with the numerical value "10"

Step 4

The TH1 class also has a function to graphically display the created histogram. The function is executed with the command

 Hist1->Draw();

You should now see a histogram on your computer screen if ROOT is able to create a graphics window.

Lab 2

We are going to repeat Lab 1 but this time we will make a script to do the typing for us.

Step 1.

Use an edit to create a text file.

In windows this could be the program "notepad" under the utilities menu (ie : Start->All Programs->Accessories->Notepad)

The important point is that you want to create and ASCII text file.

As a test, write the following in the text file

MyProgram()
{
new TBrowser();
}

Now save the file as "test.C" in the root sub directory.

In Windows the root program launches such that it is within the subdirectory "C:\root". You should save your text file under the subdirectory "Local Disk (C:) root" using the "save as" menu.

If you did things correctly you can use the command completion function on the ROOT command line to load in your program

If you type the partial name

.L te

then hit the tab key, ROOT will complete the command or offer a list of commands you can use.

root[1] .L test
test/
test.C

If you are having trouble then just type the following within root

.L test.C
MyProgram();

You should see the ROOT browser open in a new window.

Step 2

Now let's create a program to do create and fill a histogram like we did in Lab 1.

Using Notepad or another editor create and ASCII file with the following contents

MyProgram()
{
  TH1F *Hist1=new TH1F("Hist1","Hist1",50,-0.5,49.5);
  Hist1->Fill(10);
  Hist1->Draw();
}

Save the file under the name "MyProgram.C"

Now launch root and type into the interpreter window

.x MyProgram.C

alternatively you can type

.L MyProgram.C

and then

MyProgram();

The above represent two different ways to do the same thing. One advantage of the

.L MyProgram.C

sequence is that when you are creating or editing a program you can reload the program into the interpreter after saving changes to a file. So if you create a program that doesn't work as expected you debug it, save the changes, reload it into the interpreter, and then execute it again.

[1] Forest_Error_Analysis_for_the_Physical_Sciences