c++ - multi item replacing in cuda thrust -

i have device vector a,b,c following.

a = [1,1,3,3,3,4,4,5,5] b = [1,3,5] c = [2,8,6] 

so want replace each of b in corresponding element in c. eg:

• 1 replaced 2,
• 3 replaced 8,
• 5 replaced 6

so following result

result = [2,2,8,8,8,4,4,6,6] 

how achieve in cuda thrust or way of implementing in cuda c++. found thrust::replace replaces single element @ once. since need replace huge amount of data, becomes bottleneck replace 1 @ time.

this can done efficiently first building map , applying custom functor queries map.

the example code following steps:

1. get largest element of c. assumes data sorted.

2. create map vector of size largest_element. copy new values @ position of old ones.

3. apply mapper functor a. functor reads new_value map vector. if new_value not 0, value in a replaced new value. assumes c never contain 0. if can contain 0, must use condition, e.g. initialize map vector -1 , check if new_value != -1

---

#include <thrust/device_vector.h> #include <thrust/iterator/permutation_iterator.h> #include <thrust/copy.h> #include <thrust/for_each.h> #include <thrust/scatter.h> #include <iostream>   #define printer(name) print(#name, (name)) template <template <typename...> class v, typename t, typename ...args> void print(const char* name, const v<t,args...> & v) {     std::cout << name << ":\t";     thrust::copy(v.begin(), v.end(), std::ostream_iterator<t>(std::cout, "\t"));     std::cout << std::endl; }   template <typename t> struct mapper {     mapper(thrust::device_ptr<const t> map) : map(map)     {     }      __host__ __device__     void operator()(t& value) const     {        const t& new_value = map[value];         if (new_value)        {           value = new_value;        }     }      thrust::device_ptr<const t> map; };  int main() {     using namespace thrust::placeholders;      int a[] = {1,1,3,3,3,4,4,5,5};     int b[] = {1,3,5};     int c[] = {2,8,6};      int size_data    = sizeof(a)/sizeof(a[0]);     int size_replace = sizeof(b)/sizeof(b[0]);      // copy demo data gpu     thrust::device_vector<int> d_a (a, a+size_data);     thrust::device_vector<int> d_b (b, b+size_replace);     thrust::device_vector<int> d_c (c, c+size_replace);      printer(d_a);     printer(d_b);     printer(d_c);      int largest_element = d_c.back();      thrust::device_vector<int> d_map(largest_element);      thrust::scatter(d_c.begin(), d_c.end(), d_b.begin(), d_map.begin());     printer(d_map);      thrust::for_each(d_a.begin(), d_a.end(), mapper<int>(d_map.data()));     printer(d_a);      return 0; } 

output

d_a:    1   1   3   3   3   4   4   5   5    d_b:    1   3   5    d_c:    2   8   6    d_map:  0   2   0   8   0   6    d_a:    2   2   8   8   8   4   4   6   6