# Indices, Segments, and IndexSets¶

Loop variables and their associated iteration spaces are fundamental to writing loop kernels in RAJA. RAJA provides some basic iteration space types that serve as flexible building blocks that can be used to form a variety of loop iteration patterns. These types can be used to define a particular order for loop iterates, aggregate and partition iterates, as well as other configurations. In this section, we introduce RAJA index and iteration space concepts and types.

Note

All RAJA iteration space types described here are located in the namespace RAJA.

Please see the following tutorial sections for detailed examples that use RAJA iteration space concepts:

## Indices¶

Just like traditional C and C++ for-loops, RAJA uses index variables to identify loop iterates. Any lambda expression that represents all or part of a loop body passed to a RAJA::forall or RAJA::kernel method will take at least one loop index variable argument. RAJA iteration space types are templates that allow users to use any integral type for an index variable.

## Segments and IndexSets¶

A RAJA Segment represents a set of indices that one wants to execute as a unit for a kernel. RAJA provides the following Segment types:

• RAJA::TypedRangeSegment represents a stride-1 range
• RAJA::TypedRangeStrideSegment represents a (non-unit) stride range
• RAJA::TypedListSegment represents an arbitrary set of indices

A RAJA::TypedIndexSet is a container that can hold an arbitrary collection of segments to compose iteration patterns in a single kernel invocation.

Segment and IndexSet types are used in RAJA::forall and other RAJA kernel execution mechanisms to define the iteration space for a kernel.

Note

Iterating over the indices of all segments in a RAJA index set requires a two-level execution policy, with two template parameters, as shown above. The first parameter specifies how to iterate over the segments. The second parameter specifies how each segment will execute. See RAJA IndexSet Execution Policies for more information about RAJA index set execution policies.

Note

It is the responsibility of the user to ensure that segments are defined properly when using RAJA index sets. For example, if the same index appears in multiple segments, the corresponding loop iteration will be run multiple times.

Please see Iteration Spaces: Segments and IndexSets for a detailed discussion of how to create and use these segment types.

### Segment Types and Iteration¶

It is worth noting that RAJA segment types model C++ iterable interfaces. In particular, each segment type defines three methods:

• begin()
• end()
• size()

and two types:

• iterator (essentially a random access iterator type)
• value_type

Thus, any iterable type that defines these methods and types appropriately can be used as a segment with RAJA kernel execution templates.