(cpp-run-analyze)=
# Walkthrough: Running the Analyze macro 
**(10 minutes)**

As it stands, the Analyze macro does nothing, but let's learn how to run
it anyway. Quit ROOT, start it again, and enter the following lines:

    [] TFile myFile("experiment.root")
    [] tree1->Process("Analyze.C")

:::{note}
Get used to these commands. You'll be executing them over and over again
for the next several exercises. Remember, the up-arrow and tab keys are
your friends![^f78]

Let's examine each of those commands:

-   `TFile myFile("experiment.root")` -- tells ROOT to load the file
    {file}`experiment.root` into memory. This saves you from creating the
    `TBrowser` and double-clicking on the file name every time you start
    ROOT (and you'll be restarting it a lot!).

-   `tree1->Process("Analyze.C")` -- load Analyze.C and run
    its analysis code on the contents of the tree. This means:

    -   load your definitions;

    -   execute your initializations;

    -   execute the loop code for each entry in the tree;

    -   execute your wrap-up code.
:::

After the second command, ROOT will pause as it reads through all the
events in the tree. Since we haven't included any analysis code yet, you
won't see anything happen.

:::{note}
Take another look at `Analyze.h`, also called a "header file." (Analyze.C
is the "implementation file.") If you scan through it, you'll see C++
commands that do something with "branches," "chains," and loading the
variables from a tree. Fancy stuff, but you don't have to know about any
of the nitty-gritty details. Now go back and look at the top of
Analyze.C. You'll see the line

    #include "Analyze.h"

This means ROOT will include the contents of `Analyze.h` when it loads
`Analyze.C`. This takes care of defining the C++ variables for the
contents of the tree.
:::

[^f78]: If you're a real ROOT honcho (and I know you want to be), there's
    an even faster way to do this. When I work through the exercises in
    this course, I start ROOT with this command:

        > root experiment.root

    This means to run ROOT and to open file {file}`experiment.root` right away.
    I can omit the `TFile` command and get to work.

    If you want to be even faster (and who doesn't want to be?) you can 
    sometimes skip messing with the `TBrowser`. After you've opened a
    file, try the ROOT interpreter command:

        [] .ls

    This will list the contents of the file on the terminal.