# Working with folders inside ROOT files

:::{note}
As you worked with the TBrowser, you may have realized that ROOT
organizes its internal resources in the form of "folders," which are
conceptually similar to the hierarchy of directories on a disk. You can
also have folders within a single ROOT file, to {ref}`organize objects within
a file <directories-in-root>`.[^hdf5]
:::

[^hdf5]: You may already be familiar with
    [HDF5](https://www.hdfgroup.org/solutions/hdf5/), which is
    conceptually similar to the approach that ROOT takes for its
    files: containers for a mixed set of different types of data
    within a hierarchy of directories. I haven't worked with HDF5
    (yet) so it's hard for me to compare the two.

Copy the file `folders.root` from my `root-class` directory into your own,
and use the ROOT TBrowser to examine its contents.

:::{note}
You'll see three folders within the file: `example1`, `example2`, and
`example3`. Each of these folders will be the basis of the next three
exercises.

All three of the subsequent exercises will require you to make a plot of data points with
error bars. You'll want to use the
[`TGraphErrors`](https://root.cern/doc/master/classTGraphErrors.html)
class for this.[^f129]
:::

Go to the description of the `TGraphErrors` class. To create a
`TGraphErrors` object, you need to supply some arguments.

:::{note}
These are all different ways to construct a plot with error bars:

-   `TGraphErrors()` -- This is used internally by ROOT when reading a
    `TGraphErrors` object from a file. You won't use this method directly.

-   `TGraphErrors(Int_t n)` -- You use this when you just want to supply
    `TGraphErrors` with the number of points that will be in the graph,
    then use the `SetPoint()` and `SetPointError()` methods to assign values
    and errors to the points.

-   `TGraphErrors(const TGraphErrors& gr)` -- This is called a "copy
    constructor" in C++, and is used when you copy a `TGraphErrors`
    object. You can ignore this.

-   `TGraphErrors(const TH1* h)` -- You use this to create a `TGraphErrors`
    plot based on values in a histogram.

Now that I've given you a guide to four ways to construct a TGraphErrors
object, you can probably figure out what the others are: to create a
graph from the contents of a file, and to create plots from either float
or double-precision... somethings.

Those somethings are containers. We'll learn about those in the
{ref}`next section <containers>`.
:::

[^f129]: For Python programmers: Because I have a generous soul, I'll
    permit you to use matplotlib instead of `TGraphErrors` for your x-y
    plots. The fact that I have no way to stop you has nothing to do
    with it.

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

<img src="https://imgs.xkcd.com/comics/self_description.png" alt="xkcd self_description">

<https://xkcd.com/688/> by Randall Munroe
:::