ROOT Dictionaries
This section is a follow-up to Exercise 15: Data reduction (working with TTrees). In that exercise, we used a dictionary and a file that were created by someone else. 1
While I was preparing a project that was going to do some advanced ROOT I/O, I set up some test programs to teach myself how dictionaries worked in the latest versions of ROOT. After I finished my tests, it occurred to me that they might be useful in the ROOT tutorial, at least as a reference.
Make no mistake, this is fairly advanced ROOT work. I can’t imagine someone being asked to create a ROOT file with complex structure elements as part of a summer project. However, as I said in Exercise 15: Data reduction (working with TTrees), it’s possible that you may be asked to read a file with such structures. That’s why I’ve only included examples of creating a file using a dictionary in C++, but have examples of reading a such a file in both C++ and Python.
To keep this discussion relatively short, I’m going to assume that you’ve at least skimmed C++ Container classes and Exercise 15: Data reduction (working with TTrees); in other words, that you’ve been introduced to the idea of vectors and creating TTrees.
These pages are incomplete
As you look through the example files, you’ll see that the code files (ending in .h, .cxx, and .py) contain lots of comments. The web discussion in the following pages supplements those comments, but does not replace them.
In other words, this appendix mostly discusses “why”. It’s the code that describes “how”.
Figure 131: https://xkcd.com/2326/ by Randall Munroe. You never know when a dictionary might be useful.
Footnotes
- 1
OK, that “someone else” was me. But you get the idea.