TheRock/rocm-libraries/projects/rocthrust/examples/counting_iterator.cu

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#include <thrust/copy.h>
#include <thrust/device_vector.h>
#include <thrust/functional.h>
#include <thrust/iterator/counting_iterator.h>

#include <iostream>
#include <iterator>

int main()
{
  // this example computes indices for all the nonzero values in a sequence

  // sequence of zero and nonzero values
  thrust::device_vector<int> stencil(8);
  stencil[0] = 0;
  stencil[1] = 1;
  stencil[2] = 1;
  stencil[3] = 0;
  stencil[4] = 0;
  stencil[5] = 1;
  stencil[6] = 0;
  stencil[7] = 1;

  // storage for the nonzero indices
  thrust::device_vector<int> indices(8);

  // counting iterators define a sequence [0, 8)
  thrust::counting_iterator<int> first(0);
  thrust::counting_iterator<int> last = first + 8;

  // compute indices of nonzero elements
  using IndexIterator = thrust::device_vector<int>::iterator;

  IndexIterator indices_end = thrust::copy_if(first, last, stencil.begin(), indices.begin(), ::internal::identity{});
  // indices now contains [1,2,5,7]

  // print result
  std::cout << "found " << (indices_end - indices.begin()) << " nonzero values at indices:\n";
  thrust::copy(indices.begin(), indices_end, std::ostream_iterator<int>(std::cout, "\n"));

  return 0;
}