Question: Is it possible to sort objects using the Thrust library?
I have the following struct:

```
struct OB{
  int N;
  Cls *C; //CLS is another struct.
}

```

Is it possible to use thrust in order to sort an array of OB according to N? Can you provide a simple example on using thrust to sort objects? If thrust is not able to do so, is there any other CUDA libraries that allows me to do so?

Answer: 
The docs for [thrust::sort](http://wiki.thrust.googlecode.com/hg/html/group__sorting.html) show it accepts a comparison operator. See in their [example](http://code.google.com/p/thrust/source/browse/examples/sort.cu) how those are defined and used. I haven't tested this, but based on the example, all you would need is a struct that looks something like this:

```
struct OBCmp {
  __host__ __device__
  bool operator()(const OB& o1, const OB& o2) {
      return o1.N < o2.N;
  }
};

```

and then just invoke `thrust::sort(obs.begin(), obs.end(), OBCmp())`.
I have not tried **Thrust** yet, but there is a similar sort function in [CUDPP](http://code.google.com/p/cudpp/) called **cudppSort**. You cannot directly sort structures using cudppSort, it can only handle integers or floats.

So, one way to sort array of structures is to sort the keys (of your structure) and an index array of values along with it. Later, use the sorted index array to move the structures to their final sorted locations. I have described how to do this for the cudppCompact compaction algorithm in a blog post [here](http://choorucode.wordpress.com/2011/03/09/cudpp-compacting-a-structure-array/). The technique should be similar for cudppSort too.