Package 'genieclust'

Internal Cluster Validity Measures

Description

Implementation of a number of so-called cluster validity indices critically reviewed in (Gagolewski, Bartoszuk, Cena, 2021). See Section 2 therein and (Gagolewski, 2022) for the respective definitions.

The greater the index value, the more valid (whatever that means) the assessed partition. For consistency, the Ball-Hall and Davies-Bouldin indexes as well as the within-cluster sum of squares (WCSS) take negative values.

Usage

calinski_harabasz_index(X, y)

dunnowa_index(
  X,
  y,
  M = 25L,
  owa_numerator = "SMin:5",
  owa_denominator = "Const"
)

generalised_dunn_index(X, y, lowercase_d, uppercase_d)

negated_ball_hall_index(X, y)

negated_davies_bouldin_index(X, y)

negated_wcss_index(X, y)

silhouette_index(X, y)

silhouette_w_index(X, y)

wcnn_index(X, y, M = 25L)

Arguments

X	numeric matrix with n rows and d columns, representing n points in a d-dimensional space
y	vector of n integer labels, representing a partition whose quality is to be assessed; y[i] is the cluster ID of the i-th point, X[i, ]; 1 <= y[i] <= K, where K is the number or clusters
M	number of nearest neighbours
owa_numerator, owa_denominator	single string specifying the OWA operators to use in the definition of the DuNN index; one of: "Mean", "Min", "Max", "Const", "SMin:D", "SMax:D", where D is an integer defining the degree of smoothness
lowercase_d	an integer between 1 and 5, denoting $d_1$, ..., $d_5$ in the definition of the generalised Dunn (Bezdek-Pal) index (numerator: min, max, and mean pairwise intracluster distance, distance between cluster centroids, weighted point-centroid distance, respectively)
uppercase_d	an integer between 1 and 3, denoting $D_1$, ..., $D_3$ in the definition of the generalised Dunn (Bezdek-Pal) index (denominator: max and min pairwise intracluster distance, average point-centroid distance, respectively)

Value

A single numeric value (the more, the better).

Author(s)

Marek Gagolewski and other contributors

References

Ball G.H., Hall D.J., ISODATA: A novel method of data analysis and pattern classification, Technical report No. AD699616, Stanford Research Institute, 1965.

Bezdek J., Pal N., Some new indexes of cluster validity, IEEE Transactions on Systems, Man, and Cybernetics, Part B 28, 1998, 301-315, doi:10.1109/3477.678624.

Calinski T., Harabasz J., A dendrite method for cluster analysis, Communications in Statistics 3(1), 1974, 1-27, doi:10.1080/03610927408827101.

Davies D.L., Bouldin D.W., A Cluster Separation Measure, IEEE Transactions on Pattern Analysis and Machine Intelligence PAMI-1 (2), 1979, 224-227, doi:10.1109/TPAMI.1979.4766909.

Dunn J.C., A Fuzzy Relative of the ISODATA Process and Its Use in Detecting Compact Well-Separated Clusters, Journal of Cybernetics 3(3), 1973, 32-57, doi:10.1080/01969727308546046.

Gagolewski M., Bartoszuk M., Cena A., Are cluster validity measures (in)valid?, Information Sciences 581, 620-636, 2021, doi:10.1016/j.ins.2021.10.004; preprint: https://raw.githubusercontent.com/gagolews/bibliography/master/preprints/2021cvi.pdf.

Gagolewski M., A Framework for Benchmarking Clustering Algorithms, SoftwareX 20, 2022, 101270, doi:10.1016/j.softx.2022.101270, https://clustering-benchmarks.gagolewski.com.

Rousseeuw P.J., Silhouettes: A Graphical Aid to the Interpretation and Validation of Cluster Analysis, Computational and Applied Mathematics 20, 1987, 53-65, doi:10.1016/0377-0427(87)90125-7.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

Examples

X <- as.matrix(iris[,1:4])
X[,] <- jitter(X)  # otherwise we get a non-unique solution
y <- as.integer(iris[[5]])
calinski_harabasz_index(X, y)  # good
calinski_harabasz_index(X, sample(1:3, nrow(X), replace=TRUE))  # bad

---

External Cluster Validity Measures and Pairwise Partition Similarity Scores

Description

The functions described in this section quantify the similarity between two label vectors x and y which represent two partitions of a set of $n$ elements into, respectively, $K$ and $L$ nonempty and pairwise disjoint subsets.

For instance, x and y can represent two clusterings of a dataset with $n$ observations specified by two vectors of labels. The functions described here can be used as external cluster validity measures, where we assume that x is a reference (ground-truth) partition whilst y is the vector of predicted cluster memberships.

All indices except normalized_clustering_accuracy() can act as a pairwise partition similarity score: they are symmetric, i.e., index(x, y) == index(y, x).

Each index except mi_score() (which computes the mutual information score) outputs 1 given two identical partitions. Note that partitions are always defined up to a permutation (bijection) of the set of possible labels, e.g., (1, 1, 2, 1) and (4, 4, 2, 4) represent the same 2-partition.

Usage

normalized_clustering_accuracy(x, y = NULL)

normalized_pivoted_accuracy(x, y = NULL)

pair_sets_index(x, y = NULL, simplified = FALSE, clipped = TRUE)

adjusted_rand_score(x, y = NULL, clipped = FALSE)

rand_score(x, y = NULL)

adjusted_fm_score(x, y = NULL, clipped = FALSE)

fm_score(x, y = NULL)

mi_score(x, y = NULL)

normalized_mi_score(x, y = NULL)

adjusted_mi_score(x, y = NULL, clipped = FALSE)

normalized_confusion_matrix(x, y = NULL)

normalizing_permutation(x, y = NULL)

Arguments

x	an integer vector of length n (or an object coercible to) representing a K-partition of an n-set (e.g., a reference partition), or a confusion matrix with K rows and L columns (see table(x, y))
y	an integer vector of length n (or an object coercible to) representing an L-partition of the same set (e.g., the output of a clustering algorithm we wish to compare with x), or NULL (if x is an K*L confusion matrix)
simplified	whether to assume E=1 in the definition of the pair sets index index, i.e., use Eq. (20) in (Rezaei, Franti, 2016) instead of Eq. (18)
clipped	whether the result should be clipped to the unit interval, i.e., [0, 1]

Details

normalized_clustering_accuracy() (Gagolewski, 2023) is an asymmetric external cluster validity measure which assumes that the label vector x (or rows in the confusion matrix) represents the reference (ground truth) partition. It is an average proportion of correctly classified points in each cluster above the worst case scenario of uniform membership assignment, with cluster ID matching based on the solution to the maximal linear sum assignment problem; see normalized_confusion_matrix). It is given by: $\max_\sigma \frac{1}{K} \sum_{j=1}^K \frac{c_{\sigma(j), j}-c_{\sigma(j),\cdot}/K}{c_{\sigma(j),\cdot}-c_{\sigma(j),\cdot}/K}$, where $C$ is a confusion matrix with $K$ rows and $L$ columns, $\sigma$ is a permutation of the set $\{1,\dots,\max(K,L)\}$, and $c_{i, \cdot}=c_{i, 1}+...+c_{i, L}$ is the i-th row sum, under the assumption that $c_{i,j}=0$ for $i>K$ or $j>L$ and $0/0=0$.

normalized_pivoted_accuracy() is defined as $(\max_\sigma \sum_{j=1}^{\max(K,L)} c_{\sigma(j),j}/n-1/\max(K,L))/(1-1/\max(K,L))$, where $\sigma$ is a permutation of the set $\{1,\dots,\max(K,L)\}$, and $n$ is the sum of all elements in $C$. For non-square matrices, missing rows/columns are assumed to be filled with 0s.

pair_sets_index() (PSI) was introduced in (Rezaei, Franti, 2016). The simplified PSI assumes E=1 in the definition of the index, i.e., uses Eq. (20) in the said paper instead of Eq. (18). For non-square matrices, missing rows/columns are assumed to be filled with 0s.

rand_score() gives the Rand score (the "probability" of agreement between the two partitions) and adjusted_rand_score() is its version corrected for chance, see (Hubert, Arabie, 1985): its expected value is 0 given two independent partitions. Due to the adjustment, the resulting index may be negative for some inputs.

Similarly, fm_score() gives the Fowlkes-Mallows (FM) score and adjusted_fm_score() is its adjusted-for-chance version; see (Hubert, Arabie, 1985).

mi_score(), adjusted_mi_score() and normalized_mi_score() are information-theoretic scores, based on mutual information, see the definition of $AMI_{sum}$ and $NMI_{sum}$ in (Vinh et al., 2010).

normalized_confusion_matrix() computes the confusion matrix and permutes its rows and columns so that the sum of the elements of the main diagonal is the largest possible (by solving the maximal assignment problem). The function only accepts $K \leq L$. The reordering of the columns of a confusion matrix can be determined by calling normalizing_permutation().

Also note that the built-in table() determines the standard confusion matrix.

Value

Each cluster validity measure is a single numeric value.

normalized_confusion_matrix() returns a numeric matrix.

normalizing_permutation() returns a vector of indexes.

Author(s)

Marek Gagolewski and other contributors

References

Gagolewski M., A framework for benchmarking clustering algorithms, SoftwareX 20, 2022, 101270, doi:10.1016/j.softx.2022.101270, https://clustering-benchmarks.gagolewski.com.

Gagolewski M., Normalised clustering accuracy: An asymmetric external cluster validity measure, Journal of Classification, 2024, in press, doi:10.1007/s00357-024-09482-2.

Hubert L., Arabie P., Comparing partitions, Journal of Classification 2(1), 1985, 193-218, esp. Eqs. (2) and (4).

Meila M., Heckerman D., An experimental comparison of model-based clustering methods, Machine Learning 42, 2001, pp. 9-29, doi:10.1023/A:1007648401407.

Rezaei M., Franti P., Set matching measures for external cluster validity, IEEE Transactions on Knowledge and Data Mining 28(8), 2016, 2173-2186.

Steinley D., Properties of the Hubert-Arabie adjusted Rand index, Psychological Methods 9(3), 2004, pp. 386-396, doi:10.1037/1082-989X.9.3.386.

Vinh N.X., Epps J., Bailey J., Information theoretic measures for clusterings comparison: Variants, properties, normalization and correction for chance, Journal of Machine Learning Research 11, 2010, 2837-2854.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

Examples

y_true <- iris[[5]]
y_pred <- kmeans(as.matrix(iris[1:4]), 3)$cluster
normalized_clustering_accuracy(y_true, y_pred)
normalized_pivoted_accuracy(y_true, y_pred)
pair_sets_index(y_true, y_pred)
pair_sets_index(y_true, y_pred, simplified=TRUE)
adjusted_rand_score(y_true, y_pred)
rand_score(table(y_true, y_pred)) # the same
adjusted_fm_score(y_true, y_pred)
fm_score(y_true, y_pred)
mi_score(y_true, y_pred)
normalized_mi_score(y_true, y_pred)
adjusted_mi_score(y_true, y_pred)
normalized_confusion_matrix(y_true, y_pred)
normalizing_permutation(y_true, y_pred)

---

Euclidean Minimum Spanning Tree

Description

Provides access to the implementation of the Dual-Tree Boruvka algorithm from the mlpack package (if available). It is based on kd-trees and is fast for (very) low-dimensional Euclidean spaces. For higher dimensional spaces (say, over 5 features) or other metrics, use the parallelised Prim-like algorithm implemented in mst().

Usage

emst_mlpack(X, leaf_size = 1, naive = FALSE, verbose = FALSE)

Arguments

X	a numeric matrix (or an object coercible to one, e.g., a data frame with numeric-like columns)
leaf_size	size of leaves in the kd-tree, controls the trade-off between speed and memory consumption
naive	logical; whether to use the naive, quadratic-time algorithm
verbose	logical; whether to print diagnostic messages

Value

An object of class mst, see mst() for details.

Author(s)

Marek Gagolewski and other contributors

References

March W.B., Ram P., Gray A.G., Fast Euclidean Minimum Spanning Tree: Algorithm, Analysis, and Applications, Proc. ACM SIGKDD'10, 2010, 603-611, https://mlpack.org/papers/emst.pdf.

Curtin R.R., Edel M., Lozhnikov M., Mentekidis Y., Ghaisas S., Zhang S., mlpack 3: A fast, flexible machine learning library, Journal of Open Source Software 3(26), 2018, 726.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

---

Hierarchical Clustering Algorithm Genie

Description

A reimplementation of Genie - a robust and outlier resistant clustering algorithm (see Gagolewski, Bartoszuk, Cena, 2016). The Genie algorithm is based on a minimum spanning tree (MST) of the pairwise distance graph of a given point set. Just like the single linkage, it consumes the edges of the MST in an increasing order of weights. However, it prevents the formation of clusters of highly imbalanced sizes; once the Gini index (see gini_index()) of the cluster size distribution raises above gini_threshold, a forced merge of a point group of the smallest size is performed. Its appealing simplicity goes hand in hand with its usability; Genie often outperforms other clustering approaches on benchmark data, such as https://github.com/gagolews/clustering-benchmarks.

The clustering can now also be computed with respect to the mutual reachability distance (based, e.g., on the Euclidean metric), which is used in the definition of the HDBSCAN* algorithm (see Campello et al., 2013). If M > 1, then the mutual reachability distance $m(i,j)$ with smoothing factor M is used instead of the chosen "raw" distance $d(i,j)$. It holds $m(i,j)=\max(d(i,j), c(i), c(j))$, where $c(i)$ is $d(i,k)$ with $k$ being the (M-1)-th nearest neighbour of $i$. This makes "noise" and "boundary" points being "pulled away" from each other.

The Genie correction together with the smoothing factor M > 1 (note that M = 2 corresponds to the original distance) gives a robustified version of the HDBSCAN* algorithm that is able to detect a predefined number of clusters. Hence it does not dependent on the DBSCAN's somewhat magical eps parameter or the HDBSCAN's min_cluster_size one.

Usage

gclust(d, ...)

## Default S3 method:
gclust(
  d,
  gini_threshold = 0.3,
  distance = c("euclidean", "l2", "manhattan", "cityblock", "l1", "cosine"),
  cast_float32 = TRUE,
  verbose = FALSE,
  ...
)

## S3 method for class 'dist'
gclust(d, gini_threshold = 0.3, verbose = FALSE, ...)

## S3 method for class 'mst'
gclust(d, gini_threshold = 0.3, verbose = FALSE, ...)

genie(d, ...)

## Default S3 method:
genie(
  d,
  k,
  gini_threshold = 0.3,
  distance = c("euclidean", "l2", "manhattan", "cityblock", "l1", "cosine"),
  M = 1L,
  postprocess = c("boundary", "none", "all"),
  detect_noise = M > 1L,
  cast_float32 = TRUE,
  verbose = FALSE,
  ...
)

## S3 method for class 'dist'
genie(
  d,
  k,
  gini_threshold = 0.3,
  M = 1L,
  postprocess = c("boundary", "none", "all"),
  detect_noise = M > 1L,
  verbose = FALSE,
  ...
)

## S3 method for class 'mst'
genie(
  d,
  k,
  gini_threshold = 0.3,
  postprocess = c("boundary", "none", "all"),
  detect_noise = FALSE,
  verbose = FALSE,
  ...
)

Arguments

d	a numeric matrix (or an object coercible to one, e.g., a data frame with numeric-like columns) or an object of class dist, see dist or an object of class mst, see mst().
...	further arguments passed to other methods.
gini_threshold	threshold for the Genie correction, i.e., the Gini index of the cluster size distribution; Threshold of 1.0 disables the correction. Low thresholds highly penalise the formation of small clusters.
distance	metric used to compute the linkage, one of: "euclidean" (synonym: "l2"), "manhattan" (a.k.a. "l1" and "cityblock"), "cosine".
cast_float32	logical; whether to compute the distances using 32-bit instead of 64-bit precision floating-point arithmetic (up to 2x faster).
verbose	logical; whether to print diagnostic messages and progress information.
k	the desired number of clusters to detect, k = 1 with M > 1 acts as a noise point detector.
M	smoothing factor; M <= 2 gives the selected distance; otherwise, the mutual reachability distance is used.
postprocess	one of "boundary" (default), "none" or "all"; in effect only if M > 1. By default, only "boundary" points are merged with their nearest "core" points (A point is a boundary point if it is a noise point and it's amongst its adjacent vertex's M-1 nearest neighbours). To force a classical k-partition of a data set (with no notion of noise), choose "all".
detect_noise	whether the minimum spanning tree's leaves should be marked as noise points, defaults to TRUE if M > 1 for compatibility with HDBSCAN*.

Details

Note that, as in the case of all the distance-based methods, the standardisation of the input features is definitely worth giving a try.

If d is a numeric matrix or an object of class dist, mst() will be called to compute an MST, which generally takes at most $O(n^2)$ time (the algorithm we provide is parallelised, environment variable OMP_NUM_THREADS controls the number of threads in use). However, see emst_mlpack() for a very fast alternative in the case of Euclidean spaces of (very) low dimensionality and M = 1.

Given an minimum spanning tree, the algorithm runs in $O(n \sqrt{n})$ time. Therefore, if you want to test different gini_thresholds, (or ks), it is best to explicitly compute the MST first.

According to the algorithm's original definition, the resulting partition tree (dendrogram) might violate the ultrametricity property (merges might occur at levels that are not increasing w.r.t. a between-cluster distance). gclust() automatically corrects departures from ultrametricity by applying height = rev(cummin(rev(height))).

Value

gclust() computes the whole clustering hierarchy; it returns a list of class hclust, see hclust. Use cutree to obtain an arbitrary k-partition.

genie() returns a k-partition - a vector with elements in 1,...,k, whose i-th element denotes the i-th input point's cluster identifier. Missing values (NA) denote noise points (if detect_noise is TRUE).

Author(s)

Marek Gagolewski and other contributors

References

Gagolewski M., Bartoszuk M., Cena A., Genie: A new, fast, and outlier-resistant hierarchical clustering algorithm, Information Sciences 363, 2016, 8-23, doi:10.1016/j.ins.2016.05.003.

Campello R.J.G.B., Moulavi D., Sander J., Density-based clustering based on hierarchical density estimates, Lecture Notes in Computer Science 7819, 2013, 160-172, doi:10.1007/978-3-642-37456-2_14.

Gagolewski M., Cena A., Bartoszuk M., Brzozowski L., Clustering with minimum spanning trees: How good can it be?, Journal of Classification, 2024, in press, doi:10.1007/s00357-024-09483-1.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

mst() for the minimum spanning tree routines.

adjusted_rand_score() (amongst others) for external cluster validity measures (partition similarity scores).

Examples

library("datasets")
data("iris")
X <- iris[1:4]
h <- gclust(X)
y_pred <- cutree(h, 3)
y_test <- iris[,5]
plot(iris[,2], iris[,3], col=y_pred,
   pch=as.integer(iris[,5]), asp=1, las=1)
adjusted_rand_score(y_test, y_pred)
pair_sets_index(y_test, y_pred)

# Fast for low-dimensional Euclidean spaces:
# h <- gclust(emst_mlpack(X))

---

Inequality Measures

Description

gini_index() gives the normalised Gini index, bonferroni_index() implements the Bonferroni index, and devergottini_index() implements the De Vergottini index.

Usage

gini_index(x)

bonferroni_index(x)

devergottini_index(x)

Arguments

x	numeric vector of non-negative values

Details

These indices can be used to quantify the "inequality" of a numeric sample. They can be conceived as normalised measures of data dispersion. For constant vectors (perfect equity), the indices yield values of 0. Vectors with all elements but one equal to 0 (perfect inequality), are assigned scores of 1. They follow the Pigou-Dalton principle (are Schur-convex): setting $x_i = x_i - h$ and $x_j = x_j + h$ with $h > 0$ and $x_i - h \geq x_j + h$ (taking from the "rich" and giving to the "poor") decreases the inequality

These indices have applications in economics, amongst others. The Genie clustering algorithm uses the Gini index as a measure of the inequality of cluster sizes.

The normalised Gini index is given by:

$G(x_1,\dots,x_n) = \frac{ \sum_{i=1}^{n} (n-2i+1) x_{\sigma(n-i+1)} }{ (n-1) \sum_{i=1}^n x_i },$

The normalised Bonferroni index is given by:

$B(x_1,\dots,x_n) = \frac{ \sum_{i=1}^{n} (n-\sum_{j=1}^i \frac{n}{n-j+1}) x_{\sigma(n-i+1)} }{ (n-1) \sum_{i=1}^n x_i }.$

The normalised De Vergottini index is given by:

$V(x_1,\dots,x_n) = \frac{1}{\sum_{i=2}^n \frac{1}{i}} \left( \frac{ \sum_{i=1}^n \left( \sum_{j=i}^{n} \frac{1}{j}\right) x_{\sigma(n-i+1)} }{\sum_{i=1}^{n} x_i} - 1 \right).$

Here, $\sigma$ is an ordering permutation of $(x_1,\dots,x_n)$.

Time complexity: $O(n)$ for sorted (increasingly) data. Otherwise, the vector will be sorted.

Value

The value of the inequality index, a number in $[0, 1]$.

Author(s)

Marek Gagolewski and other contributors

References

Bonferroni C., Elementi di Statistica Generale, Libreria Seber, Firenze, 1930.

Gagolewski M., Bartoszuk M., Cena A., Genie: A new, fast, and outlier-resistant hierarchical clustering algorithm, Information Sciences 363, 2016, pp. 8-23. doi:10.1016/j.ins.2016.05.003

Gini C., Variabilita e Mutabilita, Tipografia di Paolo Cuppini, Bologna, 1912.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

Examples

gini_index(c(2, 2, 2, 2, 2))   # no inequality
gini_index(c(0, 0, 10, 0, 0))  # one has it all
gini_index(c(7, 0, 3, 0, 0))   # give to the poor, take away from the rich
gini_index(c(6, 0, 3, 1, 0))   # (a.k.a. Pigou-Dalton principle)
bonferroni_index(c(2, 2, 2, 2, 2))
bonferroni_index(c(0, 0, 10, 0, 0))
bonferroni_index(c(7, 0, 3, 0, 0))
bonferroni_index(c(6, 0, 3, 1, 0))
devergottini_index(c(2, 2, 2, 2, 2))
devergottini_index(c(0, 0, 10, 0, 0))
devergottini_index(c(7, 0, 3, 0, 0))
devergottini_index(c(6, 0, 3, 1, 0))

---

Minimum Spanning Tree of the Pairwise Distance Graph

Description

An parallelised implementation of a Jarnik (Prim/Dijkstra)-like algorithm for determining a(*) minimum spanning tree (MST) of a complete undirected graph representing a set of n points with weights given by a pairwise distance matrix.

(*) Note that there might be multiple minimum trees spanning a given graph.

Usage

mst(d, ...)

## Default S3 method:
mst(
  d,
  distance = c("euclidean", "l2", "manhattan", "cityblock", "l1", "cosine"),
  M = 1L,
  cast_float32 = TRUE,
  verbose = FALSE,
  ...
)

## S3 method for class 'dist'
mst(d, M = 1L, verbose = FALSE, ...)

Arguments

d	either a numeric matrix (or an object coercible to one, e.g., a data frame with numeric-like columns) or an object of class dist, see dist
...	further arguments passed to or from other methods
distance	metric used to compute the linkage, one of: "euclidean" (synonym: "l2"), "manhattan" (a.k.a. "l1" and "cityblock"), "cosine"
M	smoothing factor; M = 1 gives the selected distance; otherwise, the mutual reachability distance is used
cast_float32	logical; whether to compute the distances using 32-bit instead of 64-bit precision floating-point arithmetic (up to 2x faster)
verbose	logical; whether to print diagnostic messages and progress information

Details

If d is a numeric matrix of size $n p$, the $n (n-1)/2$ distances are computed on the fly, so that $O(n M)$ memory is used.

The algorithm is parallelised; set the OMP_NUM_THREADS environment variable Sys.setenv to control the number of threads used.

Time complexity is $O(n^2)$ for the method accepting an object of class dist and $O(p n^2)$ otherwise.

If M >= 2, then the mutual reachability distance $m(i,j)$ with smoothing factor M (see Campello et al. 2013) is used instead of the chosen "raw" distance $d(i,j)$. It holds $m(i, j)=\max(d(i,j), c(i), c(j))$, where $c(i)$ is $d(i, k)$ with $k$ being the (M-1)-th nearest neighbour of $i$. This makes "noise" and "boundary" points being "pulled away" from each other. Genie++ clustering algorithm (see gclust) with respect to the mutual reachability distance gains the ability to identify some observations are noise points.

Note that the case M = 2 corresponds to the original distance, but we determine the 1-nearest neighbours separately as well, which is a bit suboptimal; you can file a feature request if this makes your data analysis tasks too slow.

Value

Matrix of class mst with n-1 rows and 3 columns: from, to and dist. It holds from < to. Moreover, dist is sorted nondecreasingly. The i-th row gives the i-th edge of the MST. (from[i], to[i]) defines the vertices (in 1,...,n) and dist[i] gives the weight, i.e., the distance between the corresponding points.

The method attribute gives the name of the distance used. The Labels attribute gives the labels of all the input points.

If M > 1, the nn attribute gives the indices of the M-1 nearest neighbours of each point.

Author(s)

Marek Gagolewski and other contributors

References

Jarnik V., O jistem problemu minimalnim, Prace Moravske Prirodovedecke Spolecnosti 6, 1930, 57-63.

Olson C.F., Parallel algorithms for hierarchical clustering, Parallel Comput. 21, 1995, 1313-1325.

Prim R., Shortest connection networks and some generalisations, Bell Syst. Tech. J. 36, 1957, 1389-1401.

Campello R.J.G.B., Moulavi D., Sander J., Density-based clustering based on hierarchical density estimates, Lecture Notes in Computer Science 7819, 2013, 160-172, doi:10.1007/978-3-642-37456-2_14.

See Also

The official online manual of genieclust at https://genieclust.gagolewski.com/

Gagolewski M., genieclust: Fast and robust hierarchical clustering, SoftwareX 15:100722, 2021, doi:10.1016/j.softx.2021.100722.

emst_mlpack() for a very fast alternative in case of (very) low-dimensional Euclidean spaces (and M = 1).

Examples

library("datasets")
data("iris")
X <- iris[1:4]
tree <- mst(X)

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