thrust::transform_reduce How can I access iterator within unary op?

Question

I am trying to perform a transform reduce on a vector of structs. The struct contains two numbers. I want the unary function to do something with these two numbers and return a single value for each struct in my vector and reduce with a summation of these values. How do I write my functor to access the values in the struct?

for example, whats the proper syntax for mystruct.value1 within the function?:

struct unary_op
{
    float operator()()
    {
        return mystruct.value1 + mystruct.value2; //function is a lot more complex 
    }
}

so I can do:

unary_op uop1;
thrust::transform_reduce(myvec.begin(), myvec.end(), uop1, 0, thrust::add)

Answer 1

Disclaimer: Since you obviously did not post real code and your example looks like a few unrelated code lines, my answer might not be what you are looking for - a SSCCE would have been nice.

If I understand correctly, you want to transform_reduce a vector of MyStructs to the sum of all the structs member values. To do that, you need the binary addition (thrust::add) and a unary op taking a MyStruct and returning the addition of its member values:

struct MyStruct {
  float value1;
  float value2;
};
std::vector<MyStruct> myvec;

/* fill myvec */

//C++11 with lambdas:
auto result = thrust::transform_reduce(begin(myvec), end(myvec), 
  [](MyStruct const& ms) { //unary op for transform
    return ms.value1 + ms.value2;
  },
  0, thrust::add);

//C++03 with a functor:
struct MyStructReducer {
  float operator()(MyStruct const& ms) {
    return ms.value1 + ms.value2;
  }
};

float result = thrust::transform_reduce(myvec.begin, myvec.end(), 
  MyStructReducer(), 0, thrust::add);

You could as well use a free function instead of the Reducer class.

//C++03 with a function:
float reduceMyStruct(MyStruct const& ms) {
  return ms.value1 + ms.value2;
}

/* ... */
float result = thrust::transform_reduce(myvec.begin, myvec.end(), 
  reduceMyStruct, 0, thrust::add);

HTH