thrust::reduce_by_key

Defined in thrust/reduce.h

template<typename DerivedPolicy, typename InputIterator1, typename InputIterator2, typename OutputIterator1, typename OutputIterator2>
thrust::pair<OutputIterator1, OutputIterator2> thrust::reduce_by_key(const thrust::detail::execution_policy_base<DerivedPolicy> &exec, InputIterator1 keys_first, InputIterator1 keys_last, InputIterator2 values_first, OutputIterator1 keys_output, OutputIterator2 values_output)

reduce_by_key is a generalization of reduce to key-value pairs. For each group of consecutive keys in the range [keys_first, keys_last) that are equal, reduce_by_key copies the first element of the group to the keys_output. The corresponding values in the range are reduced using the plus and the result copied to values_output.

This version of reduce_by_key uses the function object equal_to to test for equality and plus to reduce values with equal keys.

The algorithm’s execution is parallelized as determined by exec.

The following code snippet demonstrates how to use reduce_by_key to compact a sequence of key/value pairs and sum values with equal keys using the thrust::host execution policy for parallelization:

#include <thrust/reduce.h>
#include <thrust/execution_policy.h>
...
const int N = 7;
int A[N] = {1, 3, 3, 3, 2, 2, 1}; // input keys
int B[N] = {9, 8, 7, 6, 5, 4, 3}; // input values
int C[N];                         // output keys
int D[N];                         // output values

thrust::pair<int*,int*> new_end;
new_end = thrust::reduce_by_key(thrust::host, A, A + N, B, C, D);

// The first four keys in C are now {1, 3, 2, 1} and new_end.first - C is 4.
// The first four values in D are now {9, 21, 9, 3} and new_end.second - D is 4.

See also

reduce

See also

unique_copy

See also

unique_by_key

See also

unique_by_key_copy

Parameters

  • exec – The execution policy to use for parallelization.

  • keys_first – The beginning of the input key range.

  • keys_last – The end of the input key range.

  • values_first – The beginning of the input value range.

  • keys_output – The beginning of the output key range.

  • values_output – The beginning of the output value range.

Template Parameters

  • DerivedPolicy – The name of the derived execution policy.

  • InputIterator1 – is a model of Input Iterator,

  • InputIterator2 – is a model of Input Iterator,

  • OutputIterator1 – is a model of Output Iterator and and InputIterator1's value_type is convertible to OutputIterator1's value_type.

  • OutputIterator2 – is a model of Output Iterator and and InputIterator2's value_type is convertible to OutputIterator2's value_type.

Returns

A pair of iterators at end of the ranges [keys_output, keys_output_last) and [values_output, values_output_last).

Pre

The input ranges shall not overlap either output range.