# Walkthrough: A simple analysis using the Draw command 
**(10 minutes)**

:::{note}
It may be that all the analysis tasks that your supervisor will ask you
to do can be performed using the tools you learned about in {ref}`the basics`:
the Draw command, the `Treeviewer`, the **FitPanel**, and other simple
techniques discussed in the early chapters of the [ROOT Users
Guide](https://root.cern/root/htmldoc/guides/users-guide/ROOTUsersGuide.html).

However, it's more likely that these simple commands will only be useful
when you get started; for example, you can draw a histogram of just one
variable to see what the histogram limits might be in C++. Let's start
with the same tasks you did with `Treeviewer`.[^f73]
:::

If you didn't copy the example n-tuple file, do so now:

    > cp ~seligman/root-class/experiment.root $PWD

If you don't already have the sample ROOT `TTree` file open, open it with
the following command:

    [] TFile myFile("experiment.root")

You can use the `Scan` command to look at the contents of the tree,
instead of using the `TBrowser`:

    [] tree1->Scan()

:::{note}
If you take a moment to think about it (a habit I strongly encourage),
you may ask how ROOT knows that there's a variable named **`tree1`**,
when you didn't type a command to create it.

The answer is that when you read a file containing ROOT objects (remember
{ref}`the second part of Saving your work <savework2>`?) in an interactive
ROOT session, ROOT automatically looks at the objects in the file and
creates variables with the same name as the objects.

This is *not* standard behavior in C++; it isn't even standard behavior
when you're working with ROOT macros. Don't become too used to it!
:::

You can also display the `TTree` in a different way that doesn't show the
data, but displays the names of the variables and the size of the `TTree`:

    [] tree1->Print()

Either way, you can see that the variables stored in the `TTree` are
**`event`**, **`ebeam`**, **`px`**, **`py`**, **`pz`**, **`zv`**, and **`chi2`**.

Create a histogram of one of the variables. For example:

    [] tree1->Draw("ebeam")

Using the Draw command, make histograms of the other variables.

[^f73]: I duplicate some of the descriptive material from the Treeviewer
    section, in case you decided to skip the quickie tools and get right
    into the programming.