TChain

TChain is a class in the ROOT data analysis framework that provides a chain of TTrees spread across multiple ROOT files, allowing them to be treated as a single TTree for sequential event processing. It is commonly used when data is produced and stored in many files, each containing a TTree with the same structure.

You create a TChain by specifying the name of the TTree stored in each file, for example a chain with the tree name "treeName". Files are added with methods such as Add or AddFile, for instance chain.Add("data_1.root"); chain.Add("data_.root"); The chain keeps track of the individual trees and their entry counts, enabling a seamless combined view of the data.

During iteration, the chain loads the appropriate file and TTree automatically as you move through entries. GetEntries returns the total number of entries across all trees in the chain. GetEntry loads the data for the current entry, and LoadTree handles the transition between trees when the end of one file is reached. Branch access, drawing, and other standard TTree operations (such as SetBranchStatus and SetBranchAddress) work on the aggregated chain as if it were a single tree.

Usage notes include ensuring that all TTrees in the chain share an identical branch layout (names and types). TChain is particularly useful for analyses that span many files or runs, enabling simple looping over a large dataset without manually handling file boundaries. It is a core tool for scalable data processing in ROOT workflows.