Thrill  0.1
Step 2: Pick Random Centers and Classify

From step 1 of this tutorial we have a DIA<Point> point containing random points. In this step, we will pick 10 random point as initial centers. Then we will iterate over all points to determine the closest cluster centers.

Selecting 10 random cluster centers is easy, since Thrill provides a Sample() DIA operation, which selects a fixed number of items uniformly at random.

// pick some initial random cluster centers
auto centers = points.Sample(/* num_clusters */ 10);

Next we want to classify all points using the centers. This can be done in Thrill using a Map(), which takes a point and outputs its associated cluster id.

For the classification, this Map() operation requires the cluster centers. The easiest way to accomplish this is to use C++ lambda captures to copy them into the lambda context.

However, first we have to collect the center points on all workers, since they are currently stored in a DIA, which cannot be accessed directly. This broadcast of the centers is called AllGather() in Thrill (after the corresponding MPI collective), which delivers the same std::vector<Point> on each worker thread.

The final classification Map() reads as follows:

// collect centers in a local vector on each worker
std::vector<Point> local_centers = centers.AllGather();
// calculate the closest center for each point
auto closest = points.Map(
[local_centers](const Point& p) {
double min_dist = p.DistanceSquare(local_centers);
size_t cluster_id = 0;
for (size_t i = 1; i < local_centers.size(); ++i) {
double dist = p.DistanceSquare(local_centers[i]);
if (dist < min_dist)
min_dist = dist, cluster_id = i;
}
return ClosestCenter { cluster_id, p };
});
closest.Print("closest");

To perform the actual classification loop over all centers, we need to calculate the distance of the point to each center. With high-flying object-oriented spirits, we decided to add a .DistanceSquare() method to our Point. For the classification the squared distance is sufficient, since square root is a monotone increasing function.

//! A 2-dimensional point with double precision
struct Point {
//! point coordinates
double x, y;
double DistanceSquare(const Point& b) const {
return (x - b.x) * (x - b.x) + (y - b.y) * (y - b.y);
}
};

The question of what kind of items the classification Map() shall return is still open. For step 2 in the tutorial, we decided it is best to just output the result of the classification to check that it is working correctly.

So we have to create a struct which contains the point and its resulting cluster id. This struct should have an operator << such that it can be printed easily.

//! Assignment of a point to a cluster.
struct ClosestCenter {
size_t cluster_id;
Point point;
};
//! make ostream-able for Print()
std::ostream& operator << (std::ostream& os, const ClosestCenter& cc) {
return os << '(' << cc.cluster_id << ':' << cc.point << ')';
}

The actual calculation loop is already shown above. It simply iterates over all centers and picks the closest.

See the complete example code examples/tutorial/k-means_step2.cpp

The output of our program so far is something like the following:

[... as before ...]
points: (721.08,599.95)
points --- End DIA.Print() --- size=100
[host 0 worker 0 000005] PushData() stage Cache.2 with targets [Sample.4]
[host 0 worker 0 000006] Execute()  stage Sample.4
[host 0 worker 0 000007] PushData() stage Sample.4 with targets [AllGather.5]
[host 0 worker 0 000008] Execute()  stage AllGather.5
[host 0 worker 0 000009] PushData() stage Cache.2 with targets [Print.7]
[host 0 worker 0 000010] Execute()  stage Print.7
closest --- Begin DIA.Print() --- size=100
closest: (2:(173.567,374.421))
closest: (6:(827.163,471.481))
closest: (3:(796.444,955.701))
[... more closest ...]
closest: (7:(532.274,41.1314))
closest: (5:(474.302,201.813))
closest: (3:(619.357,889.185))
closest --- End DIA.Print() --- size=100
[...]