thrust::discard_iterator

Defined in thrust/iterator/discard_iterator.h

template<typename System = use_default> class discard_iterator : public detail::discard_iterator_base::type<use_default>

discard_iterator is an iterator which represents a special kind of pointer that ignores values written to it upon dereference. This iterator is useful for ignoring the output of certain algorithms without wasting memory capacity or bandwidth. discard_iterator may also be used to count the size of an algorithm’s output which may not be known a priori.

The following code snippet demonstrates how to use discard_iterator to ignore one of the output ranges of reduce_by_key

#include <thrust/iterator/discard_iterator.h>
#include <thrust/reduce.h>
#include <thrust/device_vector.h>

int main()
{
  thrust::device_vector<int> keys(7), values(7);

  keys[0] = 1;
  keys[1] = 3;
  keys[2] = 3;
  keys[3] = 3;
  keys[4] = 2;
  keys[5] = 2;
  keys[6] = 1;

  values[0] = 9;
  values[1] = 8;
  values[2] = 7;
  values[3] = 6;
  values[4] = 5;
  values[5] = 4;
  values[6] = 3;

  thrust::device_vector<int> result(4);

  // we are only interested in the reduced values
  // use discard_iterator to ignore the output keys
  thrust::reduce_by_key(keys.begin(), keys.end(),
                        values.begin(),
                        thrust::make_discard_iterator(),
                        result.begin());

  // result is now [9, 21, 9, 3]

  return 0;
}