(apply-cut-c)=
#  Walkthrough: Apply a cut
**(10 minutes)**

:::{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 in C++ is to use the `if` statement.
This is described in every introductory C and C++ 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 macro:

    [] tree1->MakeSelector("AnalyzeCuts")

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 Definition section of AnalyzeCuts.C:

    Int_t pzCount = 0;

:::{note}
Why `Int_t` and not `Long64_t`? I find that `Int_t` is easier to
remember. I could even "cheat" and just use `int`, which will work for
this example. You would only have to use the type `Long64_t` if you
were counting more than $2^{31}$ entries. I promise you that there aren't
that many entries in this file![^f86]
:::

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

:::{code-block} c++
if ( (*pz) < 145 )
{
    pzCount = pzCount + 1; // you could use "pzCount++;" instead
}
:::

:::{note}
Be careful: it's important that you surround the logical expression
(`*pz) < 145` with parentheses "()", but the "if-clause" must
use curly brackets "{}".
:::

Now we have to display the value. Again, I'm going to defer a complete
description of [formatting text
output](https://www.learncpp.com/cpp-tutorial/introduction-to-iostream-cout-cin-and-endl/)
to a C++ textbook, and simply supply the following statement for your
Wrap-up section:

    std::cout << "The number of events with pz < 145 is "
        << pzCount << std::endl;

:::{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.
:::

[^f86]: Recall that in the lecture I gave at the start of the class, I
    mentioned that other commonly used data-analysis programs couldn't
    handle a large number of events. Can you picture an Excel
    spreadsheet with more than $2^{31}$ rows? ROOT can handle datasets with
    up to $2^{63}$ entries!

    Having trouble visualizing powers of 2? Remember that
    $2^{10} \approx 10^{3}$, so $2^{63} = 2^{3} \times (2^{60}) =  2^{3} \times
    \left(2^{10}\right)^{6} \approx 2^{3} \times
    \left(10^{3}\right)^{6} = 8*10^{18}$ or about eight
    quintillion, roughly the number of grains of sand in the world. My
    claim "ROOT can handle datasets with up to $2^{63}$ entries"
    is theoretical rather than practical.