TTreeFormula

TTreeFormula is a class in the ROOT data analysis framework that provides a lightweight expression engine for data stored in a TTree or TChain. It parses a user-supplied formula and evaluates it against the values of tree branches. The primary use is to compute derived variables or to specify event selections without writing explicit C++ code for every entry.

The expression language supports standard arithmetic operators (+, -, *, /, %), logical and relational operators, and a variety of mathematical functions such as sin, cos, exp, and sqrt. It can reference branch names directly, including elements of vector or array branches, and can combine several branches into a single computed value. The class resolves the truth value of boolean expressions for filtering and can compute numeric results for further analysis. It is designed to work with per-entry evaluation as trees are iterated, and it uses internal caching to speed repeated evaluations when possible.

Usage typically involves constructing a TTreeFormula with a formula string and a TTree or TChain, then evaluating the expression for a given entry, for example within a manual loop or via ROOT’s convenient integration with TTree::Draw or TTree::Project. This enables users to implement cuts, compute new variables on the fly, or project distributions without modifying the underlying data structure.

Performance considerations: TTreeFormula is interpreted rather than compiled, so it may be slower than hand-written code, especially for large datasets. However, it provides flexibility and tight integration with ROOT I/O, auto-resolving branch addresses and handling different branch types through a uniform interface. It is suitable for rapid analysis, prototyping, and scripting within ROOT.