Neman - 8 months ago 52

C++ Question

Using VexCL in C++ I am trying to count all values in a vector above a certain minimum and I would like to perform this count on the device. The default Reductors only provide methods for MIN, MAX and SUM and the examples do not show very clear how to perform such a operation. This code is slow as it is probably executed on the host instead of the device:

`int amount = 0;`

int minimum = 5;

for (vex::vector<int>::iterator i = vector.begin(); i != vector.end(); ++i)

{

if (*i >= minimum)

{

amount++;

}

}

The vector I am using will consists of a large amount of values, say millions and mostly zero's. Besides the amount of values that are above the minimum, I also would like to retrieve a list of vector-ID's which contains these values. Is this possible?

Answer

If you only needed to count elements above the minimum, this would be as simple as

```
vex::Reductor<int, vex::SUM> sum(ctx);
int amount = sum( vec >= minimum );
```

The `vec >= minimum`

expression results in a sequence of ones and zeros, and `sum`

then counts ones.

Now, since you also need to get the positions of the elements above the minimum, it gets a bit more complicated:

```
#include <iostream>
#include <vexcl/vexcl.hpp>
int main() {
vex::Context ctx(vex::Filter::Env && vex::Filter::Count(1));
// Input vector
vex::vector<int> vec(ctx, {1, 3, 5, 2, 6, 8, 0, 2, 4, 7});
int n = vec.size();
int minimum = 5;
// Put result of (vec >= minimum) into key, and element indices into pos:
vex::vector<int> key(ctx, n);
vex::vector<int> pos(ctx, n);
key = (vec >= minimum);
pos = vex::element_index();
// Get number of interesting elements in vec.
vex::Reductor<int, vex::SUM> sum(ctx);
int amount = sum(key);
// Sort pos by key in descending order.
vex::sort_by_key(key, pos, vex::greater<int>());
// First 'amount' of elements in pos now hold indices of interesting
// elements. Lets use slicer to extract them:
vex::vector<int> indices(ctx, amount);
vex::slicer<1> slice(vex::extents[n]);
indices = slice[vex::range(0, amount)](pos);
std::cout << "indices: " << indices << std::endl;
}
```

This gives the following output:

```
indices: {
0: 2 4 5 9
}
```