thrust::reduce

Defined in thrust/reduce.h

template<typename DerivedPolicy, typename InputIterator, typename T, typename BinaryFunction> T thrust::reduce(const thrust::detail::execution_policy_base<DerivedPolicy> &exec, InputIterator first, InputIterator last, T init, BinaryFunction binary_op)

reduce is a generalization of summation: it computes the sum (or some other binary operation) of all the elements in the range [first, last). This version of reduce uses init as the initial value of the reduction and binary_op as the binary function used for summation. reduce is similar to the C++ Standard Template Library’s std::accumulate. The primary difference between the two functions is that std::accumulate guarantees the order of summation, while reduce requires associativity of binary_op to parallelize the reduction.

Note that reduce also assumes that the binary reduction operator (in this case binary_op) is commutative. If the reduction operator is not commutative then thrust::reduce should not be used. Instead, one could use inclusive_scan (which does not require commutativity) and select the last element of the output array.

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

The following code snippet demonstrates how to use reduce to compute the maximum value of a sequence of integers using the thrust::host execution policy for parallelization:

#include <thrust/reduce.h>
#include <thrust/functional.h>
#include <thrust/execution_policy.h>
...
int data[6] = {1, 0, 2, 2, 1, 3};
int result = thrust::reduce(thrust::host,
                            data, data + 6,
                            -1,
                            thrust::maximum<int>());
// result == 3