Thrust scan of just one class member

Question

I have a custom class myClass which has members weight and config. I'd like to run an inclusive scan on a bunch of myClasses, but only on the weights. Basically what I want is to take:

[ {configA, weightA}, {configB, weightB}, {configC, weightC}, ...]

to:

[ {configA, weightA}, {configB, weight A + weightB}, {configC, weight A + weight B + weightC}, ...]

Is there a simple way to do this using Thrust's fancy iterators? Since the binaryOp is required to be associative, I don't see how to do this with just overloading operator+.

Answer 1

inclusive_scan requires an associative operator, but it needn't be commutative. If you create a binary function which copies the config member of its second parameter to the result, it should work out:

#include <iostream>
#include <thrust/device_vector.h>
#include <thrust/scan.h>

struct my_struct
{
  __host__ __device__
  my_struct() {}

  __host__ __device__
  my_struct(const my_struct &other)
    : config(other.config), weight(other.weight)
  {}

  __host__ __device__
  my_struct(char c, double w)
    : config(c), weight(w)
  {}

  char config;
  double weight;
};


struct functor
{
  __host__ __device__
  my_struct operator()(my_struct a, my_struct b)
  {
    my_struct result;
    result.config = b.config;
    result.weight = a.weight + b.weight;
    return result;
  }
};

int main()
{
  thrust::device_vector<my_struct> vec(3);

  vec[0] = my_struct('a', 1);
  vec[1] = my_struct('b', 2);
  vec[2] = my_struct('c', 3);

  std::cout << "input: ";
  for(int i = 0; i < vec.size(); ++i)
  {
    my_struct x = vec[i];
    std::cout << "{" << x.config << ", " << x.weight << "} ";
  }
  std::cout << std::endl;

  thrust::inclusive_scan(vec.begin(), vec.end(), vec.begin(), functor());

  std::cout << "result: ";
  for(int i = 0; i < vec.size(); ++i)
  {
    my_struct x = vec[i];
    std::cout << "{" << x.config << ", " << x.weight << "} ";
  }
  std::cout << std::endl;

  return 0;
}

The output:

$ nvcc -arch=sm_20 test.cu -run
input: {a, 1} {b, 2} {c, 3} 
result: {a, 1} {b, 3} {c, 6}