# Walkthrough: Apply a cut and a count
**(15 minutes)**

Applying cuts is an important part of any physics analysis.
There'll be some events you want to analyze and others which
are not important to your study. A "cut" is a calculation that
separates the two categories.

In `RDataFrame`, the method that applies a cut is `Filter`. For example, suppose
that we're only interested in events with **`pz`** less than 145 _GeV_. A way
this can be expressed in our example n-tuple is:

    pzcut = dataframe.Filter("pz < 145")

:::{warning}
Again, the string passed on to the `Filter` method is interpreted as a C++ expression,
not a Python expression, even if you're working in Python. You'll get an error
if you try this:

    pzcut = dataframe.Filter("pz lt 145")
:::

You can also apply a cut on any new columns you've defined:

    ptcut = definept.Filter("pt > 50")

:::{note}
There's an important operational difference between `Define` and `Filter`.
`Define` is a _column-wise_ operation; that is, it operates on columns and adds
a new one. `Filter` is a _row-wise_ operation; it essentially removes rows
from the n-tuple that don't pass its criteria. 
:::

You've probably already guessed that you can plot any column from the filtered n-tuple; e.g.,

    pzcut_hist = pzcut.Histo1D("ebeam")

The above line would accumulate a histogram of **`ebeam`** for those rows
with **`pz`** less than 145 _GeV_.

If you just want to know the number of n-tuple rows that pass a cut, the method
to use is `Count`. For example:

    pzcut_count = pzcut.Count()

:::{note}
Unlike `Define` and `Filter`, `Count` never takes an argument. However, you can't
omit the parenthesis, since `Count` is a function; it's always `Count()` and not
`Count` in program code.
:::

This seems a bit counter-intuitive at first: You can't just print out the value of 
`pzcut_count`. That's because it's still an `RDataFrame` variable, in the same sense
that `histchi2` was {ref}`earlier <rdf-make-histogram>`. In the case of `histchi2`,
you had to `Draw` it to see anything. The corresponding method to use with `Count` is `GetValue()`; e.g.,

:::{code-block} c++
:name: cpp-rdf-getvalue
:caption: RDataFrame - get the count after a cut (C++)
pzcut = dataframe.Filter("pz < 145");
pzcount = pzcut.Count();
std::cout << "The number of events with pz < 145 is " 
          << pzcount.GetValue() << std::endl;
:::

:::{code-block} python
:name: python-rdf-getvalue
:caption: RDataFrame - get the count after a cut (Python)
pzcut = dataframe.Filter("pz < 145")
pzcount = pzcut.Count()
print("The number of events with pz < 145 is",pzcount.GetValue())
:::

:::{note}
When I run either of the above code examples, I get

    The number of events with pz < 145 is 14962

Give it a try. Hopefully you'll get the same answer. 
:::

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

<img src="https://imgs.xkcd.com/comics/parking.png" alt="xkcd parking" width="95%">

<https://xkcd.com/562/> by Randall Munroe. This is another way to apply a cut. 
:::