Tries to find a linear order for objects using data in the form of a
dissimilarity matrix (two-way one-mode data), a data matrix (two-way
two-mode data), or a data array (k-way k-mode data). The order can then be
used to reorder the dissimilarity matrix/data matrix using
permute().
seriate(x, ...)# S3 method for dist
seriate(x, method = "Spectral", control = NULL, rep = 1L, ...)
# S3 method for matrix
seriate(x, method = "PCA", control = NULL, margin = c(1L, 2L), rep = 1L, ...)
# S3 method for array
seriate(
x,
method = "PCA",
control = NULL,
margin = seq(length(dim(x))),
rep = 1L,
...
)
# S3 method for data.frame
seriate(
x,
method = "Heatmap",
control = NULL,
margin = c(1L, 2L),
rep = 1L,
...
)
# S3 method for table
seriate(x, method = "CA", control = NULL, margin = c(1L, 2L), ...)
Returns an object of class ser_permutation.
the data.
further arguments are added to the control list.
a character string with the name of the seriation method (default: varies by data type).
a list of control options passed on to the seriation algorithm.
number of random restarts for randomized methods.
Uses seriate_rep().
an integer vector giving the margin indices (dimensions) to be
seriated. For example, for a matrix, 1 indicates rows, 2
indicates columns, c(1 ,2) means rows and columns.
Unseriated margins return the identity seriation order for that margin.
Michael Hahsler
Seriation methods are managed via a registry. See
list_seriation_methods() for help. In the following, we focus on
discussing the
built-in methods that are registered automatically by the package seriation.
The available control options, default settings, and
a description for each algorithm
can be retrieved using get_seriation_method(name = "<seriation method>").
Some control parameters are also described in more detail below.
Some methods are very slow, and progress can be printed using the control
parameter verbose = TRUE.
Many seriation methods (heuristically) optimize (minimize or maximize) an
objective function often called seriation criterion.
The value of the seriation criterion for a given order can be
calculated using criterion(). In this manual page, we
include the criterion, which is optimized by each method using bold font.
If no criterion is mentioned, then the method does not directly optimize a criterion.
A definition of the different seriation criteria can be found on the criterion() manual page.
Seriation methods for distance matrices (dist)
One-mode two-way data must be provided as a dist object (not a symmetric matrix). Similarities have to be transformed into dissimilarities. Seriation algorithms fall into different groups based on the approach. In the following, we describe the currently implemented methods. A list with all methods and the available parameters is available here. Hahsler (2017) for a more detailed description and an experimental comparison of the most popular methods.
Dendrogram leaf order
These methods create a dendrogram using hierarchical clustering and then derive the seriation order from the leaf order in the dendrogram. Leaf reordering may be applied.
Hierarchical clustering: "HC", "HC_single", "HC_complete",
"HC_average", "HC_ward"
Uses the order of the leaf nodes in a dendrogram obtained by hierarchical
clustering as a simple seriation technique. This method
applies hierarchical clustering (stats::hclust()) to x. The clustering
method can be given using a "linkage" element in the control
list. If omitted, the default "complete" is used.
For convenience, the other methods are provided as shortcuts.
Reordered by the Gruvaeus and Wainer heuristic: "GW", "GW_single", "GW_average",
"GW_complete", "GW_ward" (Gruvaeus and Wainer, 1972)
Method "GW" uses an algorithm developed by Gruvaeus and Wainer (1972)
as implemented gclus::reorder.hclust() (Hurley 2004). The clusters are
ordered at each level so that the objects at the edge of each cluster are
adjacent to the nearest object outside the cluster. The
method produces a unique order.
The methods start with a dendrogram created by hclust(). As the
"linkage" element in the control list, a clustering method
(default "average") can be specified. Alternatively, an stats::hclust
object can be supplied using an element named "hclust".
A dendrogram (binary tree) has \(2^{n-1}\) internal nodes (subtrees) and the same number of leaf orderings. That is, at each internal node, the left and right subtree (or leaves) can be swapped or, in terms of a dendrogram, be flipped. The leaf-node reordering to minimize
Minimizes the Hamiltonian path length (restricted by the dendrogram).
Reordered by optimal leaf ordering: "OLO", "OLO_single",
"OLO_average", "OLO_complete", "OLO_ward" (Bar-Joseph et al., 2001)
Starts with a dendrogram and produces an optimal leaf ordering that minimizes the sum of the distances along the (Hamiltonian) path connecting the leaves in the given order. The algorithm's time complexity is \(O(n^3)\). Note that non-finite distance values are not allowed.
Minimizes the Hamiltonian path length (restricted by the dendrogram).
Dendrogram seriation: "DendSer" (Earle and Hurley, 2015)
Use heuristic dendrogram seriation to optimize for various criteria.
The DendSer code has to be first registered. A
detailed description can be found on the manual page for
register_DendSer().
Dimensionality reduction
Find a seriation order by reducing the dimensionality to 1 dimension. This is typically done by minimizing a stress measure or the reconstruction error. Note that dimensionality reduction to a single dimension is a very difficult discrete optimization problem. For example, MDS algorithms used for a single dimension tend to end up in local optima (see Maier and De Leeuw, 2015). However, generally, ordering along a single component of MDS provides good results sufficient for applications like visualization.
Classical metric multidimensional scaling: "MDS"
Orders along the 1D classical metric multidimensional scaling.
control parameters are passed on to stats::cmdscale().
Isometric feature mapping: "isomap" (Tenenbaum, 2000)
Orders along the 1D isometric feature mapping.
control parameters are passed on to vegan::isomap()
Kruskal's non-metric multidimensional scaling: "isoMDS", "monoMDS",
"metaMDS" (Kruskal, 1964)
Orders along the 1D Kruskal's non-metric multidimensional scaling.
Package vegan provides an alternative implementation called monoMDS
and a version that uses random restarts for stability called metaMDS.
control parameters are passed on to MASS::isoMDS(), vegan::monoMDS() or vegan::metaMDS().
Sammon's non-linear mapping: "Sammon_mapping" (Sammon, 1969)
Orders along the 1D Sammon's non-linear mapping.
control parameters are passed on to MASS::sammon().
Angular order of the first two eigenvectors: "MDS_angle"
Finds a 2D configuration using MDS (stats::cmdscale())
to approximate the eigenvectors of the covariance matrix in the
original data matrix.
Orders by the angle in this space and splits the order by the
larges gap between adjacent angles. A similar method was used by
Friendly (2002) to order variables in correlation matrices
by angles of first two eigenvectors.
Smacof: "MDS_smacof" (de Leeuw and Mair, 2009)
Perform seriation using stress majorization with several transformation functions.
This method has to be registered first using register_smacof().
Optimization
These methods try to optimize a seriation criterion directly, typically using a heuristic approach.
Anti-Robinson seriation by simulated annealing: "ARSA" (Brusco et al 2008)
The algorithm automatically finds a suitable start temperature and calculates
the needed number of iterations. The algorithm gets slow for a large number of
objects. The speed can be improved by lowering the cooling parameter "cool"
or increasing the minimum temperature "tmin".
However, this will decrease the seriation quality.
Directly minimizes the linear seriation criterion (LS).
Complete Enumeration: "Enumerate"
This method finds the optimal permutation given a seriation criterion by complete enumeration
of all permutations.
The criterion is specified as the control parameters "criterion".
Default is the weighted gradient measure. Use "verbose = TRUE" to see
the progress.
Note: The number of permutations for \(n\) objects is \(n!\). Complete enumeration is only possible for tiny problems (<10 objects) and is limited on most systems to a problem size of up to 12 objects.
Gradient measure seriation by branch-and-bound: "BBURCG", "BBWRCG" (Brusco and Stahl 2005)
The method uses branch-and-bound to minimize the
unweighted gradient measure ("BBURCG") and the
weighted gradient measure ("BBWRCG").
This type of optimization is only feasible for a small number of objects (< 50 objects).
For BBURCG, the control parameter "eps" can be used to relax the problem by defining
that a distance needs to be eps larger to count as a violation. This relaxation will improve the speed,
but miss some Robinson events. The default value is 0.
Genetic Algorithm: "GA"
The GA code has to be first registered. A detailed description can
be found on the manual page for register_GA().
Quadratic assignment problem seriation:
"QAP_LS", "QAP_2SUM", "QAP_BAR", "QAP_Inertia" (Hahsler, 2017)
Formulates the seriation problem as a quadratic assignment problem and applies a simulated annealing solver to find a good solution. These methods minimize the Linear Seriation Problem (LS) formulation (Hubert and Schultz 1976), the 2-Sum Problem formulation (Barnard, Pothen, and Simon 1993), the banded anti-Robinson form (BAR), or the inertia criterion.
control parameters are passed on to qap::qap().
An important parameter is rep to return the best result from the
given number of repetitions with random restarts. The default is 1, but bigger
numbers result in better and more stable results.
General Simulated Annealing: "GSA"
Implement simulated annealing similar to the ARSA method. However, it
can optimize
for any criterion measure defined in seriation. By default, the
algorithm optimizes for the raw gradient measure, and is warm started with the
result of spectral seriation (2-Sum problem) since Hahsler (2017) shows that
2-Sum solutions are similar to solutions for the gradient measure.
Use warmstart = "random" for no warm start.
The initial temperature t0 and minimum temperature tmin can be set. If
t0 is not set, then it is estimated by sampling uphill moves and setting
t0 such that the median uphill move have a probability
of tinitialaccept.
Using the cooling rate cool, the number of iterations
to go for t0 to tmin is calculated.
Several popular local neighborhood functions are
provided, and new ones can be defined (see LS). Local moves are tried in each
iteration nlocal times the number of objects.
Note that this is an R implementation repeatedly calling the criterion function which is very slow.
Stochastic gradient descent: "SGD"
Starts with a solution and then performs stochastic gradient descent to find a close-by local optimum given a specified criterion.
Important control parameters:
"criterion": the criterion to optimize
"init": initial seriation (an order or the name of a seriation method)
"max_iter": number of trials
Spectral seriation: "Spectral", "Spectral_norm" (Ding and He, 2004)
Spectral seriation uses a relaxation to minimize the 2-Sum Problem (Barnard, Pothen, and Simon, 1993). It uses the order of the Fiedler vector of the similarity matrix's (normalized) Laplacian.
Spectral seriation gives a good trade-off between seriation quality, and scalability (see Hahsler, 2017).
Traveling salesperson problem solver: "TSP"
Uses a traveling salesperson problem solver to minimize the
Hamiltonian path length. The solvers in TSP are used (see
TSP::solve_TSP()). The solver method can be passed on via the control
argument, e.g., control = list(method = "two_opt"). Default is the est
of 10 runs of arbitrary insertion heuristic with 2-opt improvement.
Since a tour returned by a TSP solver is a connected circle and we are looking for a path representing a linear order, we need to find the best cutting point. Climer and Zhang (2006) suggest adding a dummy city with equal distance to each other city before generating the tour. The place of this dummy city in an optimal tour with minimal length is the best cutting point (it lies between the most distant cities).
Other Methods
Identity permutation: `"Identity"
Reverse Identity permutation: `"Reverse"
Random permutation: "Random"
Rank-two ellipse seriation: "R2E" (Chen 2002)
Rank-two ellipse seriation starts with generating a sequence of correlation matrices
\(R^1, R^2, \ldots\). \(R^1\) is the correlation matrix of the original
distance matrix \(D\) (supplied to the function as x), and
$$R^{n+1} = \phi R^n,$$ where \(\phi\) calculates the correlation
matrix.
The rank of the matrix \(R^n\) falls with increasing \(n\). The first \(R^n\) in the sequence, which has a rank of 2 is found. Projecting all points in this matrix on the first two eigenvectors, all points fall on an ellipse. The order of the points on this ellipse is the resulting order.
The ellipse can be cut at the two interception points (top or bottom) of the vertical axis with the ellipse. In this implementation, the topmost cutting point is used.
Sorting Points Into Neighborhoods: "SPIN_STS", "SPIN_NH" (Tsafrir, 2005)
Given a weight matrix \(W\), the SPIN algorithms try to minimize the energy for a permutation (matrix \(P\)) given by $$F(P) = tr(PDP^TW),$$ where \(tr\) denotes the matrix trace.
"SPIN_STS" implements the Side-to-Side algorithm, which tries to push
out large distance values. The default weight matrix suggested in the paper
with \(W=XX^T\) and \(X_i=i-(n+1)/2\) is used. We run the algorithm form
step (25) iteration and restart the algorithm nstart (10) with
random initial permutations (default values in parentheses).
"SPIN_NH" implements the neighborhood algorithm (concentrate low
distance values around the diagonal) with a Gaussian weight matrix
\(W_{ij} = exp(-(i-j)^2/n\sigma)\), where \(n\) is the size of the
dissimilarity matrix and \(\sigma\) is the variance around the diagonal
that control the influence of global (large \(\sigma\)) or local (small
\(\sigma\)) structure.
We use the heuristic suggested in the paper for the linear assignment
problem. We do not terminate as indicated in the algorithm but run all the
iterations since the heuristic does not guarantee that the energy is
strictly decreasing. We also implement the heuristic "annealing" scheme
where \(\sigma\) is successively reduced. The parameters in control
are sigma which can be a single value or a decreasing sequence
(default: 20 to 1 in 10 steps), and step, which defines how many update
steps are performed before for each value of alpha. Via
W_function a custom function to create \(W\) with the function
signature function(n, sigma, verbose) can be specified.
Visual Assessment of (Clustering) Tendency: "VAT" (Bezdek and Hathaway, 2002).
Creates an order based on Prim's algorithm for finding a minimum spanning tree (MST) in a weighted connected graph representing the distance matrix. The order is given by the order in which the nodes (objects) are added to the MST.
Seriation methods for matrices (matrix)
Two-mode two-way data are general matrices. Some methods also require that the matrix is positive. Data frames and contingency tables (base::table) are converted into a matrix. However, the default methods are different.
Some methods find the row and column order simultaneously,
while others calculate them independently.
Currently, the
following methods are implemented for matrix:
Seriating rows and columns simultaneously
Row and column order influence each other.
Bond Energy Algorithm: "BEA" (McCormick, 1972).
The algorithm tries to maximize a non-negative matrix's Measure of Effectiveness. Due to the definition of this measure, the tasks of ordering rows and columns are separable and can be solved independently.
BEA consists of the following three steps:
Place one randomly chosen column.
Try to place each remaining column at each possible position left, right and between the already placed columns and calculate every time the increase in ME. Choose the column and position which gives the largest increase in ME and place the column. Repeat till all columns are placed.
Repeat procedure with rows.
The overall procedure amounts to two approximate traveling salesperson problems (TSP) where the distance is the -1 times the ME increase. The BEA algorithm is equivalent to a simple suboptimal TSP heuristic called 'cheapest insertion'. Several consecutive runs of the algorithm might improve the energy if a better initial column/row is chosen.
Arabie and Hubert (1990) question its use with non-binary data if the objective is to find a seriation or one-dimensional orderings of rows and columns.
TSP to optimize the Measure of Effectiveness: "BEA_TSP" (Lenstra 1974).
Since BEA is equivalent to a simple TSP heuristic, we can use better TSP solvers to get better results. Distances between rows are calculated for a \(M \times N\) data matrix as $$d_{jk} = - \sum_{i=1}^{i=M} x_{ij}x_{ik}\ (j,k=0,1,...,N).$$
Distances between columns are calculated the same way from the transposed data matrix.
Solving the two TSP using these distances optimizes the measure of effectiveness. With an exact TSP solver, the optimal solution can be found.
control parameter:
"method": a TSP solver method (see TSP::solve_TSP()).
Unconstrained Brower and Kyle seriation: "BK_unconstrained" (Brower and Kyle 1988).
Reorderes 0-1 matrices to create a block structure along the diagonal. It iteratively reorders by the mean row indices of 1s and mean column indices of 1s till the orders become stable.
control parameter: None
Correspondence analysis "CA" (Friendly, 2023)
This function is designed to help simplify a mosaic plot or other displays of a matrix of frequencies. It calculates a correspondence analysis of the matrix and an order for rows and columns according to the scores on a correspondence analysis dimension.
This is the default method for contingency tables.
control parameters:
"dim": CA dimension used for reordering.
"ca_param": List with parameters for the call to ca::ca().
Seriating rows and columns separately using dissimilarities
Heatmap seriation: "Heatmap"
Calculates distances between rows and between columns and then applies seriation so each. This is the default method for data frames.
control parameter:
"seriation_method": a list with row and column seriation methods.
The special method "HC_Mean" is available to use hierarchical clustering
with reordering the leaves by the row/column means (see stats::heatmap()).
Defaults to optimal leaf ordering "OLO".
"seriation_control": a list with control parameters for row and column
seriation methods.
"dist_fun": specify the distance calculation as a function.
"scale": "none", "row", or "col".
Seriate rows using the data matrix
These methods need access to the data matrix instead of dissimilarities to reorder objects (rows). Columns can also be reorderd by applying the same technique to the transposed data matrix.
Order along the 1D locally linear embedding: "LLE"
Performs 1D the non-linear dimensionality reduction method locally linear embedding
(see lle()).
Order along the first principal component: "PCA"
Uses the projection of the data on its first principal component (using
stats::princomp()) to
determine the order of rows. Performs the same procedure on the transposed
matrix to obtain the column order.
Note that for a distance matrix calculated from x with Euclidean
distance, this method minimizes the least square criterion.
Angular order of the first two PCA components: "PCA_angle"
For rows, projects the data on the first two principal components and then orders by the angle in this space. The order is split by the larges gap between adjacent angles. A similar method was suggested by Friendly (2002) to order variables in correlation matrices by angles of first two eigenvectors. PCA also computes the eigenvectors of the covariance matrix of the data.
Performs the same process on the transposed matrix for the column order.
Other methods
Angular order of the first two eigenvectors: "AOE" (Friendly 2002)
This method reordered correlation matrices by the angle in the space spanned by the two largest eigenvectors of the matrix. The order is split by the largest angle gap. This is the original method proposed by Friendly (2002).
By row/column mean: "Mean"
A transformation can be applied before calculating the means.
The function is specified as control
parameter "transformation". Any function that takes as an input a
matrix and returns the transformed matrix can be used. Examples
are scale or \(x) x^.5.
Identity permutation: "Identity"
Reverse Identity permutation: "Reverse"
Random permutation: "Random"
For general arrays no built-in methods are currently available.
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Other seriation:
register_DendSer(),
register_GA(),
register_optics(),
register_smacof(),
register_tsne(),
register_umap(),
registry_for_seriation_methods,
seriate_best()
# Show available seriation methods (for dist and matrix)
list_seriation_methods()
# show the description for ARSA
get_seriation_method("dist", name = "ARSA")
### Seriate as distance matrix (for 50 flowers from the iris dataset)
data("iris")
x <- as.matrix(iris[-5])
x <- x[sample(nrow(x), size = 50), ]
d <- dist(x)
order <- seriate(d)
order
pimage(d, main = "Distances (Random Order)")
pimage(d, order, main = "Distances (Reordered)")
# Compare seriation quality
rbind(
random = criterion(d),
reordered = criterion(d, order)
)
# Reorder the distance matrix
d_reordered <- permute(d, order)
pimage(d_reordered, main = "Distances (Reordered)")
### Seriate a matrix (50 flowers from iris)
# To make the variables comparable, we scale the data
x <- scale(x, center = FALSE)
# The iris flowers are ordered by species in the data set
pimage(x, main = "original data", prop = FALSE)
criterion(x)
# Apply some methods
order <- seriate(x, method = "BEA_TSP")
pimage(x, order, main = "TSP to optimize ME", prop = FALSE)
criterion(x, order)
order <- seriate(x, method = "PCA")
pimage(x, order, main = "First principal component", prop = FALSE)
criterion(x, order)
order <- seriate(x, method = "heatmap")
pimage(x, order, main = "Heatmap seriation", prop = FALSE)
criterion(x, order)
# reorder the matrix
x_reordered <- permute(x, order)
# create a heatmap seriation manually by calculating
# distances between rows and between columns
order <- c(
seriate(dist(x), method = "OLO"),
seriate(dist(t(x)), method = "OLO")
)
pimage(x, order, main = "Heatmap seriation", prop = FALSE)
criterion(x, order)
### Seriate a correlation matrix
corr <- cor(x)
# plot in original order
pimage(corr, main = "Correlation matrix")
# reorder the correlation matrix using the angle of eigenvectors
pimage(corr, order = "AOE", main = "Correlation matrix (AOE)")
# we can also define a distance (we used d = sqrt(1 - r)) and
# then reorder the matrix (rows and columns) using any seriation method.
d <- as.dist(sqrt(1 - corr))
o <- seriate(d, method = "R2E")
corr_reordered <- permute(corr, order = c(o, o))
pimage(corr_reordered, main = "Correlation matrix (R2E)")
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