RootIO.jl
Introduction
The RootIO package provides an easy interface to write columnar data sets from and into ROOT files using the TTree representation. A TTree is a columnar data representation stored on disk that supports an in-memory buffer for fast data access and automatic writing to disk. This package is based on ROOT.jl, a package that provides Julia bindings to the C++ ROOT API.
The original TTree API is heavily based on pointers; it does not translate well to Julia and is not easy to use. The RootIO module, to be used alongside the ROOT module, provides a higher-level and more user-friendly interface to TTree. RootIO defines its own TTree type to be used in place of ROOT.TTree.
The development of this package was initiated in the context of the 2024 edition of the Google Summer of Code program under the CERN-HSF organization.
Only write support is currently implemented. We recommend UnROOT for an easy read interface to trees. Direct use of ROOT.jl is also possible but requires pointer manipulation.
Contents
Supported types
TTree represents data in two dimensions. Elements of this matrix can be of any type that can be defined in C++, including classes. In the botanical ROOT terminology, a column is called a branch and a row a tree entry.
RootIO includes support for most of the standard Julia primitive types, character strings (String), and vectors of elements of these types. Vectors of Any are not supported. The supported types are summarized in the table below.
| Type | Description | Supported |
|---|---|---|
| String | A character string | ✅ |
| Int8 | An 8-bit signed integer | ✅ |
| UInt8 | An 8-bit unsigned integer | ✅ |
| Int16 | A 16-bit signed integer | ✅ |
| UInt16 | A 16-bit unsigned integer | ✅ |
| Int32 | A 32-bit signed integer | ✅ |
| UInt32 | A 32-bit unsigned integer | ✅ |
| Float32 | A 32-bit floating-point number | ✅ |
| Float64 | A 64-bit floating-point number | ✅ |
| Int64 | A long signed integer, stored as 64-bit | ✅ |
| UInt64 | A long unsigned integer, stored as 64-bit | ✅ |
| Bool | A boolean | ✅ |
| StdVector{T}¹ | A vector of elements of any of the above types stored as std::vector | ✅ |
| Vector{T} | A vector of elements of any of the above types stored as a C-array² | ✅ |
¹ StdVector is a subtype of AbstractVector provided by the CxxWrap package and reexported by RootIO. Use StdVector([1, 2, 3]) to create a vector with elements 1, 2, and 3.
² with a fixed size or a size specified in another column.