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examples/tutorial/k-means_step2.cpp

This example is part of the k-means tutorial. See Step 2: Pick Random Centers and Classify

/*******************************************************************************
* examples/tutorial/k-means_step2.cpp
*
* Part of Project Thrill - http://project-thrill.org
*
* Copyright (C) 2016 Timo Bingmann <[email protected]>
*
* All rights reserved. Published under the BSD-2 license in the LICENSE file.
******************************************************************************/
//! \example examples/tutorial/k-means_step2.cpp
//!
//! This example is part of the k-means tutorial. See \ref kmeans_tutorial_step2
#include <ostream>
#include <random>
#include <vector>
//! [Point class]
//! 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);
}
};
//! [Point class]
//! make ostream-able for Print()
std::ostream& operator << (std::ostream& os, const Point& p) {
return os << '(' << p.x << ',' << p.y << ')';
}
//! [ClosestCenter class]
//! 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 << ')';
}
//! [ClosestCenter class]
void Process(thrill::Context& ctx) {
std::default_random_engine rng(std::random_device { } ());
std::uniform_real_distribution<double> dist(0.0, 1000.0);
// generate 100 random points using uniform distribution
auto points =
ctx, /* size */ 100,
[&](const size_t&) {
return Point { dist(rng), dist(rng) };
})
.Cache();
// print out the points
points.Print("points");
//! [step2 sample]
// pick some initial random cluster centers
auto centers = points.Sample(/* num_clusters */ 10);
//! [step2 sample]
//! [step2 classify]
// 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[0]);
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");
//! [step2 classify]
}
int main() {
// launch Thrill program: the lambda function will be run on each worker.
return thrill::Run(
[&](thrill::Context& ctx) { Process(ctx); });
}
/******************************************************************************/