# Initialization

A clustering algorithm usually requires initialization before it could be started.

## Seeding

Seeding is a type of clustering initialization, which provides a few seeds – points from a data set that would serve as the initial cluster centers (one for each cluster).

Each seeding algorithm implemented by Clustering.jl is a subtype of SeedingAlgorithm:

Clustering.initseeds!Function
initseeds!(iseeds::AbstractVector{Int}, alg::SeedingAlgorithm,
X::AbstractMatrix) -> iseeds

Initialize iseeds with the indices of cluster seeds for the X data matrix using the alg seeding algorithm.

Clustering.initseeds_by_costs!Function
initseeds_by_costs!(iseeds::AbstractVector{Int}, alg::SeedingAlgorithm,
costs::AbstractMatrix) -> iseeds

Initialize iseeds with the indices of cluster seeds for the costs matrix using the alg seeding algorithm.

Here, costs[i, j] is the cost of assigning points $i$ and $j$ to the same cluster. One may, for example, use the squared Euclidean distance between the points as the cost.

There are several seeding methods described in the literature. Clustering.jl implements three popular ones:

Clustering.KmppAlgType
KmppAlg <: SeedingAlgorithm

Kmeans++ seeding (:kmpp).

Chooses the seeds sequentially. The probability of a point to be chosen is proportional to the minimum cost of assigning it to the existing seeds.

References

D. Arthur and S. Vassilvitskii (2007). k-means++: the advantages of careful seeding. 18th Annual ACM-SIAM symposium on Discrete algorithms, 2007.

Clustering.KmCentralityAlgType
KmCentralityAlg <: SeedingAlgorithm

K-medoids initialization based on centrality (:kmcen).

Choose the $k$ points with the highest centrality as seeds.

References

Hae-Sang Park and Chi-Hyuck Jun. A simple and fast algorithm for K-medoids clustering. doi:10.1016/j.eswa.2008.01.039

Clustering.RandSeedAlgType
RandSeedAlg <: SeedingAlgorithm

Random seeding (:rand).

Chooses an arbitrary subset of $k$ data points as cluster seeds.

In practice, we have found that Kmeans++ is the most effective choice.

For convenience, the package defines the two wrapper functions that accept the short name of the seeding algorithm and the number of clusters and take care of allocating iseeds and applying the proper SeedingAlgorithm:

Clustering.initseedsFunction
initseeds(alg::Union{SeedingAlgorithm, Symbol},
X::AbstractMatrix, k::Integer) -> Vector{Int}

Select k seeds from a $d×n$ data matrix X using the alg algorithm.

alg could be either an instance of SeedingAlgorithm or a symbolic name of the algorithm.

Returns the vector of k seed indices.

Clustering.initseeds_by_costsFunction
initseeds_by_costs(alg::Union{SeedingAlgorithm, Symbol},
costs::AbstractMatrix, k::Integer) -> Vector{Int}

Select k seeds from the $n×n$ costs matrix using algorithm alg.

Here, costs[i, j] is the cost of assigning points iandj to the same cluster. One may, for example, use the squared Euclidean distance between the points as the cost.

Returns the vector of k` seed indices.