Compiler error when using thrust placeholders with the ternary operator ?: - cuda

Question

I am using Thrust 1.8 and I get two compiler errors when I try to compile the below code :
#include <thrust/device_vector.h>
#include <thrust/functional.h>
int main(int argc, char* argv[])
{
thrust::device_vector<bool> condition(100);
thrust::device_vector<int> input(100);
thrust::device_vector<float> result(100);
float mean = 10.4f;
thrust::transform(condition.begin(),condition.end(),input.begin(),result.begin(), ( (thrust::placeholders::_1 ) ? ( thrust::placeholders::_2) : (mean) ) );
}
When I try to compile, I get the following compiler time errors :
(for placeholders::_1)
Error : Expression must be of bool type (or convertible to bool)
(for placeholders::_2)
Error : operand types are incompatible ("const thrust::detail::functional::actor < thrust::detail::functional::argument<1U>>" and "float")
How to correct this?

Answer 1

You cannot use the placeholders like you tried to, i.e. in combination with a ternary operator.
Instead, you could create your own functor:
struct my_fun : public thrust::binary_function<bool,float,float>
{
float mean;
my_fun(float mean) : mean(mean) {}
__host__ __device__
float operator()(bool condition, float input) const
{
float result = mean;
if (condition)
{
result = input;
}
return result;
}
};
...
thrust::transform(condition.begin(),
condition.end(),
input.begin(),
result.begin(),
my_fun(mean));