◆ transform_reduce() [2/2]

template<typename InputIterator , typename UnaryFunction , typename OutputType , typename BinaryFunction >
OutputType thrust::transform_reduce ( InputIterator  first,
InputIterator  last,
UnaryFunction  unary_op,
OutputType  init,
BinaryFunction  binary_op 

transform_reduce fuses the transform and reduce operations. transform_reduce is equivalent to performing a transformation defined by unary_op into a temporary sequence and then performing reduce on the transformed sequence. In most cases, fusing these two operations together is more efficient, since fewer memory reads and writes are required.

transform_reduce performs a reduction on the transformation of the sequence [first, last) according to unary_op. Specifically, unary_op is applied to each element of the sequence and then the result is reduced to a single value with binary_op using the initial value init. Note that the transformation unary_op is not applied to the initial value init. The order of reduction is not specified, so binary_op must be both commutative and associative.

firstThe beginning of the sequence.
lastThe end of the sequence.
unary_opThe function to apply to each element of the input sequence.
initThe result is initialized to this value.
binary_opThe reduction operation.
The result of the transformed reduction.
Template Parameters
InputIteratoris a model of Input Iterator, and InputIterator's value_type is convertible to UnaryFunction's argument_type.
UnaryFunctionis a model of Unary Function, and UnaryFunction's result_type is convertible to OutputType.
OutputTypeis a model of Assignable, and is convertible to BinaryFunction's first_argument_type and second_argument_type.
BinaryFunctionis a model of Binary Function, and BinaryFunction's result_type is convertible to OutputType.

The following code snippet demonstrates how to use transform_reduce to compute the maximum value of the absolute value of the elements of a range.

template<typename T>
struct absolute_value : public unary_function<T,T>
__host__ __device__ T operator()(const T &x) const
return x < T(0) ? -x : x;
int data[6] = {-1, 0, -2, -2, 1, -3};
int result = thrust::transform_reduce(data, data + 6,
// result == 3
See also