HW9 count_if problems

Hi everyone,

Has anyone had success using the thrust::count_if() function? I'm trying to do this:

  Code:

 struct isGreaterThan5{
    //returns whether an input x is greater than 5
    __host__ __device__ bool operator()(int &x){
      return x > 5;
    }
  };

  int numGreaterThanFive = thrust::count_if(rawMeasurements,rawMeasurements+N, isGreaterThan5());


When I try to compile it I get this error:

  Code:

error: no instance of overloaded function "thrust::count_if" matches the argument list
            argument types are: (int [15], int *, isGreaterThan5)


Any idea what I'm doing wrong? I've even tried the code in the thrust documentation and get the same result: http://docs.thrust.googlecode.com/hg/gr ... nting.html

[Dan Negrut]

I'll ask Andrew to see if there is some issue with the latest CUDA and the version of thrust we've got after the update to 6.0.
Stay tuned,
Dan

Hi Owen,

Try this:

`` __host__ __device__ bool operator()(const int &x)''

and other part of code remains the same.

Though I'm not totally sure why this will make a difference, when we're writing functors, generally speaking the parameter should not be a non-constant reference. It can be a const reference, or simply passing by value (i.e. ``int x'').

(Looks like Ang beat me to it.)

I think that example might not be correct, or at least outdated. I usually just grep/ag the actual code examples for correct usage: https://github.com/thrust/thrust/tree/master/examples Also, I'm not sure if the docs on Google Code are being kept up to date after the migration to Github. Latest docs are at: http://thrust.github.io/doc/group__counting.html (but still looks like it hasn't been updated).


Dan: 6.0 should be publicly available early next year. Only a couple of us have access before then.

Hi Andrew and Ang Li,

I have tried passing by reference and by value, but I still get the same error as Owen.

@Owen: Did the changes solve the problem for you?

Andrew, thanks for the tip about the examples. After cloning that repository and running grep for count_if I found an example in "stream_compaction.cu"

Here's the code that works for me.
Outside of main, declare the test function:

  Code:

template <typename T> struct isGreaterThan5 : public thrust::unary_function<T,bool>{
  //returns whether an input x is greater than 5
  __host__ __device__ bool operator()(T x){
    return x > 5;
  }
};


Then inside your main function you can count the number of elements in rawMeasurements that are greater than five:

  Code:

 int numGreaterThanFive = thrust::count_if(rawMeasurements,rawMeasurements+N, isGreaterThan5<int>());

-Owen

It works even if I don't use templates, which fits the requirements for this homework. I have the struct defined like:

  Code:

 struct count5{
   __host__ __device__
    bool operator()(int x) {
    //magic
    }
  };


And one can directly pass this struct to count_if, no need to create another instance of that type and then pass it.

Omkar, it looks like that only works if you use Compute 1.0. I was compiling with --arch compute_20 which apparently requires the template-based code.

-Owen

I checked by passing flags "-arch=sm_20" and "-arch=compute_20" and seems to be working for me.