ra-ra

ra-ra

ra-ra is an expression template / multidimensional array library for C++14. It is a work in progress.

Features

• Dynamic or static array rank. Dynamic or static array shape (all dimensions or none, alas).

• Both memory-owning types and views. You can make array views over any piece of memory.

• Shape agreement rules and rank extension for rank-0 operations of any arity and operands of any rank, any of which can a reference (so you can write on them). These rules are taken from the array language, J.

• Iterators over cells of arbitrary rank.

• A rank conjunction (only for static rank operands and somewhat fragile).

• A proper selection operator with 'beating' of range or scalar subscripts. Beatable subscripts are not beaten if mixed with non-beatable subscripts, which is a defect.

• A TensorIndex object as in Blitz++ (doesn't work in the same exact way).

• Some compatibility with the STL.

Sui generis

• Index and size types are all signed.

• Index base is always 0.

• Default array order is C or row-major (last dimension changes fastest). You can make array views using other orders by transposing or manipulating the strides yourself, but all newly created arrays use C-order.

• The selection operator is (). [] is supported for rank-1 arrays only, where it means the same as ().

• Array constructors have a very regular format. Single argument constructors always take an 'init-expression' which must provide enough shape information to construct the new array (unless the array type has static shape), and is otherwise subject to the regular argument shape agreement rules. Two argument constructors always take a shape argument and a content argument.

• Indices are checked by default. This can be disabled with a compilation flag.

Bugs & wishes

• Be namespace-clean.

• Proper reductions. There are some reduction operations but no general mechanism. This may be the most obvious feature hole.

• Concatenation, search, reshape, and other infinite rank or rank>0 operations.

• Stencils, like in Blitz++.

• More clever/faster traversal of arrays, like in Blitz++.

• Handling of nested arrays; there's no real support for this.

Out of scope

• No GPU / parallelization / calls to external libraries.

• Linear algebra, quaternions, etc. Those things belong in other libraries. The library includes a dual number implementation but it's more of a demo of how to adapt user types to the library.

• Sparse arrays. You'd still want to mix & match with dense arrays, so maybe at some point.

Building

The library is header-only and has no dependencies beyond a C++14 compiler and the standard C++ library. There is a test suite in test/. These tests test internal details and are not meant as demonstrations of how to use the library. There is a directory with examples/, mostly ported from Blitz++.

All tests should pass under g++-5.3 and clang++-3.7/8/9. For clang on OS X you have to remove the -Wa,q option in the SConstruct which is meant for gcc by setting CCFLAGS to something else, for example:

CCFLAGS="-march=native -DRA_OPTIMIZE_SMALLVECTOR=0" CXXFLAGS=-O3 CXX=clang++-3.9 scons -j4

g++-6.1 fails to build the tests because of this bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70942.

I haven't tested on Windows.

Motivation

I do numerical work in C++ so I need a library of this kind. Most C++ array libraries seem to support only vectors and matrices, or small objects for low-dimensional vector algebra. Blitz++ was a great early array library and it hasn't really been replaced as far as I can tell.

I've also been inspired by APL and J, which have led the way in exploring how array operations could be generalized.

This is a simple library. I don't want to second-guess the compiler and I don't stress performance as much as Blitz++ did. However, I am wary of adding features that could become an obstacle if I ever tried to make things fast(er). I believe that improvements such as new traversal methods or the optimization of specific expression patterns should be easy to implement without turning the library inside out.