# Sorts¶

RAJA provides portable parallel sort operations, which are basic parallel algorithm building blocks. They are described in this section.

A few important notes:

Note

• All RAJA sort operations are in the namespace RAJA.
• Each RAJA sort operation is a template on an execution policy parameter. The same policy types used for RAJA::forall methods may be used for RAJA sorts.
• RAJA sort operations accept an optional comparator argument so users can perform different types of sort operations. If no operator is given, the default is a less than operation and the result is non-decreasing.

Also:

Note

• For sorts using the CUDA back-end, RAJA uses the implementations provided by the NVIDIA cub library. For information please see build-external-tpl.
• The RAJA CUDA and HIP back-ends only support sorting arithmetic types using RAJA operators ‘less than’ and ‘greater than’.

Please see the Parallel Sort Operations tutorial section for usage examples of RAJA sort operations.

## Sort Operations¶

In general, a sort operation takes a sequence of numbers x and a binary comparison operator op that forms a strict weak ordering of elements in input sequence x and produces a sequence of numbers y as output. The output sequence is a permutation of the input sequence where each pair of elements a and b, where a is before b in the output sequence, satisfies !(b op a). Sorts are stable if they always preserve the order of equivalent elements, where equivalent elements satisfy !(a op b) && !(b op a).

A stable sort takes an input sequence x where ai appears before aj if i < j when ai and aj are equivalent for any i != j.

x = { a0, b0, a1, … }

and calculates the stably sorted output sequence y that preserves the order of equivalent elements. That is, the sorted sequence where element ai appears before the equivalent element aj if i < j:

y = { a0, a1, b0, … }

An unstable sort may not preserve the order of equivalent elements and may produce either of the following output sequences:

y = { a0, a1, b0, … }

or

y = { a1, a0, b0, … }

## RAJA Unstable Sorts¶

RAJA unstable sort operations look like the following:

• RAJA::sort< exec_policy >(container)
• RAJA::sort< exec_policy >(container, comparator)
• RAJA::sort< exec_policy >(iter, iter + N)
• RAJA::sort< exec_policy >(iter, iter + N, comparator)

For example, sorting an array with this sequence of values:

6 7 2 1 0 9 4 8 5 3 4 9 6 3 7 0 1 8 2 5


with a sequential unstable sort operation:

  RAJA::sort<RAJA::seq_exec>(out, out + N);


produces the out array with this sequence of values:

0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9


Note that the syntax is essentially the same as Parallel Scan Operations. Here, container is a range of elements and iter is a random access iterator to a range of elements. container and iter provide access to the input sequence and contain the output sequence at the end of sort. The first and third sort operations listed above will be non-decreasing sorts since there is no comparator argument given; i.e., the sequences will be reordered in-place using operator::less. The second and fourth sorts will apply the comparator that is passed into the function.

RAJA also provides sort operations that operate on key-value pairs stored separately:

• RAJA::sort_pairs< exec_policy >(keys_container, vals_container)
• RAJA::sort_pairs< exec_policy >(keys_container, vals_container, comparator)
• RAJA::sort_pairs< exec_policy >(keys_iter, keys_iter + N, vals_iter)
• RAJA::sort_pairs< exec_policy >(keys_iter, keys_iter + N, vals_iter, comparator)

RAJA::sort_pairs methods generate the same output sequence of keys in keys_container or keys_iter as RAJA::sort does in container or iter and reorders the sequence of values in vals_container or vals_iter by permuting the sequence of values in the same manner as the sequence of keys; i.e. the sequence of pairs is sorted based on comparing their keys.

Note

The comparator used in RAJA::sort_pairs only compares keys.

## RAJA Stable Sorts¶

RAJA stable sorts are essentially the same as unstable sorts:

• RAJA::stable_sort< exec_policy >(container)
• RAJA::stable_sort< exec_policy >(container, comparator)
• RAJA::stable_sort< exec_policy >(iter, iter + N)
• RAJA::stable_sort< exec_policy >(iter, iter + N, comparator)

RAJA also provides stable sort pairs that operate on key-value pairs stored separately:

• RAJA::stable_sort_pairs< exec_policy >(keys_container, vals_container)
• RAJA::stable_sort_pairs< exec_policy >(keys_container, vals_container, comparator)
• RAJA::stable_sort_pairs< exec_policy >(keys_iter, keys_iter + N, vals_iter)
• RAJA::stable_sort_pairs< exec_policy >(keys_iter, keys_iter + N, vals_iter, comparator)

## RAJA Comparison Operators¶

RAJA provides two operators that can be used to produce different ordered sorts:

• RAJA::operators::less<T>
• RAJA::operators::greater<T>

Note

All RAJA comparison operators are in the namespace RAJA::operators.

## Sort Policies¶

For information about RAJA execution policies to use with sort operations, please see Policies.