# Exercise 11: Stand-alone program (optional) 
**(30 minutes)**

:::{note}
Why would you want to write a stand-alone program instead of using ROOT
interactively?

-   You can't live in a notebook forever.[^f97] Typical analysis scripts
    get so large that you may want to use a regular text editor to work
    with them, instead of the limited editing available in a notebook
    cell.

-   One method of speeding up a Python program is to use Cython, a
    Python optimizing compiler: <http://cython.org/>. You can use [Cython
    within a notebook](https://twiki.nevis.columbia.edu/twiki/bin/view/Main/IPython), but
    you'll get better results if you create a stand-alone program.

-   Stand-alone programs are necessary if you want to submit your Python
    program to a batch system.
:::

So far, you've used ROOT interactively to perform the exercises. Your
task now is to write a stand-alone program that uses ROOT. Start with
the script you created in Exercise 10: you have a notebook cell that
reads an n-tuple, performs a calculation, and writes a plot to a file.
Create a stand-alone program that does the same thing.

:::{hint}
If you tried to do the {ref}`C++ version of this Exercise
<stand-alone-c>`, you may have found it difficult. The Python
equivalent is much easier.  Part of the reason is that all the clues
you need are in the {ref}`condor tutorial <condor-tutorial>` in the
{ref}`appendix <appendices>`.

Look at the instructions for the `.py` files in that tutorial, then
look at the comments in the `.py` files themselves.

Don't forget to use `module load root` (or the equivalent if you're
not using a Nevis particle-physics server) if you expect a stand-alone
Python program to be able to import the ROOT libraries!  
:::

:::{figure-md} reassuring-fig
:class: align-center

<img src="https://imgs.xkcd.com/comics/reassuring.png" alt="xkcd reassuring" width="80%">

<https://xkcd.com/1263/> by Randall Munroe
:::

[^f97]: Whether this is a programming tip or general life advice I leave
    up to you.