# Walkthrough: Calculating our own variables 
**(10 minutes)**

:::::{admonition} Transverse momentum
:class: note

There are other quantities that we may be interested in apart from the
ones already present in the n-tuple. One such quantity
is $p_{T}$ which is defined by:

:::{math}
p_{T} = \sqrt{ p_{x}^{2} + p_{x}^{2}  }
:::

This is the transverse momentum of the particle, that is, the component
of the particle's momentum that's perpendicular to the *z*-axis.

:::::

Let's calculate our own values in an analysis macro. Load a fresh copy
of that script into your notebook:

    %load Analyze.py

In the **Loop** section, put in the following line:

    pt = ROOT.TMath.Sqrt(px*px + py*py)

:::{admonition} Did that not work?
:class: note

To get at the variables **`px`** and **`py`**, you have
fetch them from the n-tuple with something like `mychain.px`. You also
have to have either `import ROOT` or `from ROOT import TMath`.

ROOT comes with a very complete set of math functions. You can browse
them all by looking at the `TMath` class on the ROOT web site, or
Chapter 13 in the [ROOT User's
Guide](https://root.cern/root/htmldoc/guides/users-guide/ROOTUsersGuide.html).
For now, it's enough to know that `ROOT.TMath.Sqrt()` computes the
square root of the expression within the parenthesis "()".[^f95]
:::

Test the script to make sure it runs. You won't see any output, so
we'll fix that in the next exercise.

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

<img src="https://imgs.xkcd.com/comics/location_sharing.png" alt="xkcd location_sharing" width="75%">

<https://xkcd.com/1473/> by Randall Munroe
:::

[^f95]: To be fair, there are Python math packages as well. I could have
    asked you to do something like this:

        import math
        # ... fetch px and py
        pt = math.sqrt(px*px + py*py)

    The reason why I ask you to use ROOT's math packages is that I want
    you to get used to looking up and using ROOT's basic math functions
    (algebra, trig) in preparation for using its advanced routines
    (e.g., fourier transforms, multi-variant analysis).