(apply-cut-python)=
# Walkthrough: Applying a cut 
**(10 minutes)**

:::{admonition} A final analysis concept
:class: note

The last "trick" you need to learn is how to apply a cut in an analysis
macro. Once you've absorbed this, you'll know enough about ROOT to start
using it for a real physics analysis.

The simplest way to apply a cut is to use the `if` statement. This is
described in every introductory Python text, and I won't go into detail
here. Instead I'll provide an example to get you started.
:::

Once again, let's start with a fresh Analyze script:

    %load Analyze.py

Our goal is to count the number of events for which `pz` is less than
145 *GeV*. Since we're going to count the events, we're going to need
a counter. Put the following in the **Set-up** section:

    pzCount = 0

For every event that passes the cut, we want to add one to the count.
Put the following in the **Loop** section:

:::{code-block} python
    if ( pz < 145 ):
        pzCount = pzCount + 1
:::

:::{admonition} Be careful
:class: note

Remember that indentation is important. The next statement
after `pzCount=pzCount+1` must not be indented the same amount, or it
will be considered part of the `if` statement.

:::

Now we have to display the value. Include the following statement in
your **Wrap-up** section:

    print ("The number of events with pz < 145 is", pzCount)

:::{admonition} My results
:class: note

When I run this macro, I get the following output:

    The number of events with pz < 145 is 14962

Hopefully you'll get the same answer.
:::