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POThrust: how to get the number of elements copied by the copy_if function when using device_ptr
 

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  1. POThrust: how to get the number of elements copied by the copy_if function when using device_ptr
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    <p>I am using the thrust::copy_if function of the Thrust library coupled with counting iterators to get the indices of nonzero elements in an array. I also need to get the number of copied elements.</p> <p>I am using the code from the 'counting_iterator.cu' example, except that in my application I need to reuse pre-allocated arrays, so I wrap them with thrust::device_ptr and then pass them to the thrust::copy_if function. This is the code:</p> <pre><code>using namespace thrust; int output[5]; thrust::device_ptr&lt;int&gt; tp_output = device_pointer_cast(output); float stencil[5]; stencil[0] = 0; stencil[1] = 0; stencil[2] = 1; stencil[3] = 0; stencil[4] = 1; device_ptr&lt;float&gt; tp_stencil = device_pointer_cast(stencil); device_vector&lt;int&gt;::iterator output_end = copy_if(make_counting_iterator&lt;int&gt;(0), make_counting_iterator&lt;int&gt;(5), tp_stencil, tp_output, _1 == 1); int number_of_ones = output_end - tp_output; </code></pre> <p>If I comment the last line of code, the function fills correctly the output array. However, when I uncomment it, I get the following compilation error:</p> <p>1>C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v5.5\include\thrust/iterator/iterator_adaptor.h(223): error : no operator "-" matches these operands</p> <p>1> operand types are: int *const - const thrust::device_ptr</p> <p>1> detected during: 1> instantiation of "thrust::iterator_adaptor::difference_type thrust::iterator_adaptor::distance_to(const thrust::iterator_adaptor &amp;) const [with Derived=thrust::detail::normal_iterator>, Base=thrust::device_ptr, Value=thrust::use_default, System=thrust::use_default, Traversal=thrust::use_default, Reference=thrust::use_default, Difference=thrust::use_default, OtherDerived=thrust::device_ptr, OtherIterator=int *, V=signed int, S=thrust::device_system_tag, T=thrust::random_access_traversal_tag, R=thrust::device_reference, D=ptrdiff_t]" 1> C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v5.5\include\thrust/iterator/iterator_facade.h(181): here 1> instantiation of "Facade1::difference_type thrust::iterator_core_access::distance_from(const Facade1 &amp;, const Facade2 &amp;, thrust::detail::true_type) [with Facade1=thrust::detail::normal_iterator>, Facade2=thrust::device_ptr]" 1> C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v5.5\include\thrust/iterator/iterator_facade.h(202): here 1> instantiation of "thrust::detail::distance_from_result::type thrust::iterator_core_access::distance_from(const Facade1 &amp;, const Facade2 &amp;) [with Facade1=thrust::detail::normal_iterator>, Facade2=thrust::device_ptr]" 1> C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v5.5\include\thrust/iterator/iterator_facade.h(506): here 1> instantiation of "thrust::detail::distance_from_result, thrust::iterator_facade>::type thrust::operator-(const thrust::iterator_facade &amp;, const thrust::iterator_facade &amp;) [with Derived1=thrust::detail::normal_iterator>, Value1=signed int, System1=thrust::device_system_tag, Traversal1=thrust::random_access_traversal_tag, Reference1=thrust::device_reference, Difference1=signed int, Derived2=thrust::device_ptr, Value2=signed int, System2=thrust::device_system_tag, Traversal2=thrust::random_access_traversal_tag, Reference2=thrust::device_reference, Difference2=signed int]" 1> C:/ProgramData/NVIDIA Corporation/CUDA Samples/v5.5/7_CUDALibraries/nsgaIIparallelo_23ott/rank_cuda.cu(70): here </p> <p>If I use thrust::device_vector for the output array instead, everything is okay:</p> <pre><code>using namespace thrust; thrust::device_vector&lt;int&gt; output(5); float stencil[5]; stencil[0] = 0; stencil[1] = 0; stencil[2] = 1; stencil[3] = 0; stencil[4] = 1; device_ptr&lt;float&gt; tp_stencil = device_pointer_cast(stencil); device_vector&lt;int&gt;::iterator output_end = copy_if(make_counting_iterator&lt;int&gt;(0), make_counting_iterator&lt;int&gt;(5), tp_stencil, output.begin(), _1 == 1); int number_of_ones = output_end - output.begin(); </code></pre> <p>Can you suggest any solution to this problem? Thank you.</p>
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