How to use thrust::remove_if to check and remove blocks 2×i and 2×i+1 together

Question

I am using Cuda C++, and I have a big array Arr including 64-bit unsigned integers in a form like the below:

Arr = {a1, b1, a2, b2, ..., an, bn}

The number of items in Arr is 2n which is an even number. Now, given a boolean function f(int a, int b), I wonder if I can use thrust::remove_if to check f(a1,b1), f(a2, b2), ..., f(an, bn) and remove both consecutive numbers (ai, bi) together if needed?

Answer 1

Rather than zip_iterator, I think a simpler approach is just to reinterpret the array of 64-bit integers as an array of thrust::pair. here is an example:

$ cat t2157.cu
#include <iostream>
#include <thrust/device_vector.h>
#include <thrust/remove.h>
#include <thrust/sequence.h>
#include <thrust/copy.h>

struct my_remove
{
  template <typename T>
  __host__ __device__
  bool operator()(T t){
    return (thrust::get<0>(t) > thrust::get<1>(t));
  }
};

const size_t n = 32768;
const size_t s = 2*n;
using mt=unsigned long long;
using dt=thrust::pair<mt,mt>;

int main(){

  thrust::device_vector<mt> A(s);
  thrust::sequence(A.begin(), A.end());
  A[0] = 2;  // expecting removal of the first pair
  thrust::copy_n(A.begin(), 6, std::ostream_iterator<mt>(std::cout, ","));
  std::cout << std::endl;
  auto D = thrust::device_pointer_cast<dt>(reinterpret_cast<dt *>(thrust::raw_pointer_cast(A.data())));
  thrust::remove_if(D, D+n, my_remove());
  thrust::copy_n(A.begin(), 6, std::ostream_iterator<mt>(std::cout, ","));
  std::cout << std::endl;
}
$ nvcc -o t2157 t2157.cu
$ compute-sanitizer ./t2157
========= COMPUTE-SANITIZER
2,1,2,3,4,5,
2,3,4,5,6,7,
========= ERROR SUMMARY: 0 errors
$