sample(3) Simple Sample MLPACK Programs

## Introduction

On this page, several simple MLPACK examples are contained, in increasing order of complexity.

## Covariance Computation

A simple program to compute the covariance of a data matrix ('data.csv'), assuming that the data is already centered, and save it to file.

```// Includes all relevant components of MLPACK.
#include <mlpack/core.hpp>
// Convenience.
using namespace mlpack;
int main()
{
arma::mat data;
// Use data::Load() which transposes the matrix.
// Now compute the covariance.  We assume that the data is already centered.
// Remember, because the matrix is column-major, the covariance operation is
// transposed.
arma::mat cov = data * trans(data) / data.n_cols;
// Save the output.
data::Save("cov.csv", cov, true);
}
```

## Nearest Neighbor

This simple program uses the mlpack::neighbor::NeighborSearch object to find the nearest neighbor of each point in a dataset using the L1 metric, and then print the index of the neighbor and the distance of it to stdout.

```#include <mlpack/core.hpp>
#include <mlpack/methods/neighbor_search/neighbor_search.hpp>
using namespace mlpack;
using namespace mlpack::neighbor; // NeighborSearch and NearestNeighborSort
using namespace mlpack::metric; // ManhattanDistance
int main()
{
// Load the data from data.csv (hard-coded).  Use CLI for simple command-line
// parameter handling.
arma::mat data;
// Use templates to specify that we want a NeighborSearch object which uses
// the Manhattan distance.
NeighborSearch<NearestNeighborSort, ManhattanDistance> nn(data);
// Create the object we will store the nearest neighbors in.
arma::Col<size_t> neighbors;
arma::vec distances; // We need to store the distance too.
// Compute the neighbors.
nn.Search(1, neighbors, distances);
// Write each neighbor and distance using Log.
for (size_t i = 0; i < neighbors.n_elem; ++i)
{
Log::Info << "Nearest neighbor of point " << i << " is point "
<< neighbors[i] << " and the distance is " << distances[i] << ".;
}
}
```

## Other examples

For more complex examples, it is useful to refer to the main executables:

• methods/neighbor_search/allknn_main.cpp
• methods/neighbor_search/allkfn_main.cpp
• methods/emst/emst_main.cpp