(histogram-analyze-reader-c)=
# Walkthrough: Making a histogram with the Reader macro
**(15 minutes)**

Edit the file {file}`AnalyzeReader.C`. In the Definitions section, insert the
following code:

    TH1* chi2Hist = nullptr;

:::{admonition} Why use a pointer?
:class: note

This means "define a new histogram pointer and call it `chi2Hist`."
Why define this as a pointer when plain ol' variables are easier to use?
The short answer is that ROOT uses pointers all the time; for example,
if you want to read something from a file, you must always use pointers.
The sooner you get used to pointers, the better.[^f79]

Don't forget the semi-colons ";" at the ends of the lines! You can omit
them in interactive commands, but not in macros.
:::

In the Initialization section, insert the following code:

    chi2Hist = new TH1D("chi2","Histogram of Chi2",100,0,20);


:::{admonition} Why here?
:class: note

This means "set this pointer to a new histogram object." We're doing
this here, instead of the Definitions section, because sometimes you
want quantities like histogram limits to be variable rather than fixed;
e.g., they depend on user input.
:::

In the Loop section, put this in:

    chi2Hist->Fill(*chi2);

:::{admonition} Entry getting and filling
:class: note

This line means "in the histogram **`chi2Hist`** add 1 to a bin that
corresponds to the value of **`*chi2`**." **Important:** Any variable that you
define using `TTreeReaderValue` behaves like a pointer. 

:::

This goes in the Wrap-up section:

    chi2Hist->Draw();

:::{admonition} Reminder
:class: note

You already know what this does; you've used it before!
:::

Save the file, then enter the same command as before:

    [] .x AnalyzeReader.C

:::{admonition} Finally!
:class: note

Finally, we've made our first histogram with a C++ analysis macro. In
the Initialization section, we defined a histogram; in the Loop section,
we filled the histogram with values; in the Wrap-up section, we drew the
histogram.

"What histogram? I don't see anything!" Don't forget: if you have the
TBrowser open, you may need to click on the **Canvas 1** tab.

How did I know which bin limits to use on **`chi2Hist`**? Before I wrote
the code, I drew a test histogram with the command:

    [] tree1->Draw("chi2")

:::{admonition} Adding axis labels
:class: note

Hmm, the histogram's axes aren't labeled. How do I put the labels in the
macro? Here's how I figured it out: I labeled the axes on the test
histogram by right-clicking on them and selecting {menuselection}`SetTitle`. I saved
the canvas by selecting {menuselection}`File --> Save --> c1.C`. I looked
at c1.C and saw these commands in the file:

    chi2->GetXaxis()->SetTitle("chi2");
    chi2->GetYaxis()->SetTitle("number of events");

I scrolled up and saw that ROOT had used the variable `chi2` for the
name of the histogram pointer. I copied the lines into AnalyzeReader.C, but
used the name of my histogram instead:

    chi2Hist->GetXaxis()->SetTitle("chi2");
    chi2Hist->GetYaxis()->SetTitle("number of events");
:::

Try this yourself: add the two lines above to the Initialization
section, right after the line that defines the histogram. Test the
revised AnalyzeReader class.

[^f79]: Why are we defining a pointer then setting it equal to `nullptr`?
    I'm teaching you to avoid a common problem in programming:
    uninitialized variables. If we didn't set `chi2Hist` to `nullptr`,
    what would its value be? I don't know. It would likely be set to
    zero, which is also the typical value of `nullptr`. But this behavior
    varies between different C++ compilers. It's better to be sure.

    This is not an issue in the code we're writing now, but in the
    future you'll discover that uninitialized variables cause lots of
    crashes. Let's get into good programming habits and avoid them from
    the start.