Exercise 9: Writing the n-tuple

(10 minutes)

If you’ve made changes to your n-tuple (defining new columns, applying filters) and you want to write the revised n-tuple to a file, use the Snapshot() method.

Here’s an example

Listing 29: An example using the Snapshot method (C++)

// Open a dataframe from an n-tuple within a file.
auto dataframe = ROOT::RDataFrame("tree1","experiment.root");

// Make a change to the n-tuple.
auto pzcut = dataframe.Filter("pz < 145");

// Write the revised n-tuple with the name "pzcut" to file
// "analysis.root".
pzcut.Snapshot("pzcut","analysis.root");

Here’s the Exercise

In the past few pages, we’ve built a collection of new variables (\(p_{T}\), \(\theta\), \(E_{meas}\), \(E_{loss}\)) and useful analysis cuts (\(\chi^{2} < 1.5\), \(\theta < 0.15\)). Derive an n-tuple with all these new columns and cuts, and write it as n-tuple analyzed to file analysis.root.

Tip

If you look up Snapshot() on the RDataFrame web page you’ll see that it’s classified as an instant action. This means that the event loop will be executed when the Snapshot() is executed.

If, for some reason, you want a lazy evaluation of Snapshot(), click on the highlighted word Snapshot() on that page. It will take you to a different page; search for fLazy on that new page.

xkcd effect_size — Figure 58: https://xkcd.com/2755/ by Randall Munroe