thrust::counting_iterator

Defined in thrust/iterator/counting_iterator.h

template<typename Incrementable, typename System = use_default, typename Traversal = use_default, typename Difference = use_default> class counting_iterator

counting_iterator is an iterator which represents a pointer into a range of sequentially changing values. This iterator is useful for creating a range filled with a sequence without explicitly storing it in memory. Using counting_iterator saves memory capacity and bandwidth.

The following code snippet demonstrates how to create a counting_iterator whose value_type is int and which sequentially increments by 1.

#include <thrust/iterator/counting_iterator.h>
...
// create iterators
thrust::counting_iterator<int> first(10);
thrust::counting_iterator<int> last = first + 3;

first[0]   // returns 10
first[1]   // returns 11
first[100] // returns 110

// sum of [first, last)
thrust::reduce(first, last);   // returns 33 (i.e. 10 + 11 + 12)

// initialize vector to [0,1,2,..]
thrust::counting_iterator<int> iter(0);
thrust::device_vector<int> vec(500);
thrust::copy(iter, iter + vec.size(), vec.begin());

This next example demonstrates how to use a counting_iterator with the thrust::copy_if function to compute the indices of the non-zero elements of a device_vector. In this example, we use the make_counting_iterator function to avoid specifying the type of the counting_iterator.

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

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);

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

 // use make_counting_iterator to define the sequence [0, 8)
 IndexIterator indices_end = thrust::copy_if(thrust::make_counting_iterator(0),
                                             thrust::make_counting_iterator(8),
                                             stencil.begin(),
                                             indices.begin(),
                                             ::cuda::std::identity{});
 // indices now contains [1,2,5,7]

 return 0;
}