Matrices {LaplacesDemon} R Documentation

Matrix Utility Functions

Description

These are utility functions for working with matrices.

Usage

as.indicator.matrix(x)
as.inverse(x)
as.parm.matrix(x, k, parm, Data, a=-Inf, b=Inf, restrict=FALSE, chol=FALSE)
as.positive.definite(x)
as.positive.semidefinite(x)
as.symmetric.matrix(x, k=NULL)
is.positive.definite(x)
is.positive.semidefinite(x)
is.square.matrix(x)
is.symmetric.matrix(x)
Cov2Cor(Sigma)
CovEstim(Model, parm, Data, Method="Hessian")
GaussHermiteCubeRule(N, dims, rule)
Hessian(Model, parm, Data, Interval=1e-6, Method="Richardson")
Jacobian(Model, parm, Data, Interval=1e-6, Method="simple")
logdet(x)
lower.triangle(x, diag=FALSE)
SparseGrid(J, K)
TransitionMatrix(theta.y=NULL, y.theta=NULL, p.theta=NULL)
tr(x)
upper.triangle(x, diag=FALSE)

Details

The as.indicator.matrix function creates an indicator matrix from a vector. This function is useful for converting a discrete vector into a matrix in which each column represents one of the discrete values, and each occurence of that value in the related column is indicated by a one, and is otherwise filled with zeroes. This function is similar to the class.ind function in the nnet package.

The as.inverse function returns the matrix inverse of x. The solve function in base R also returns the matrix inverse, but solve can return a matrix that is not symmetric, and can fail due to singularities. The as.inverse function tries to use the solve function to return a matrix inverse, and when it fails due to a singularity, as.inverse uses eigenvalue decomposition (in which eigenvalues below a tolerance are replaced with the tolerance), and coerces the result to a symmetric matrix. This is similar to the solvcov function in the fpc package.

The as.parm.matrix function prepares a correlation, covariance, or precision matrix in two important ways. First, as.parm.matrix obtains the parameters for the matrix specified in the x argument by matching the name of the matrix in the x argument with any parameters in parm, given the parameter names in the Data listed in parm.names. These obtained parameters are organized into a matrix as the elements of the upper-triangular, including the diagonal. A copy is made, without the diagonal, and the lower-triangular is filled in, completing the matrix. Second, as.parm.matrix checks for positive-definiteness. If matrix x is positive-definite, then the matrix is stored as a variable called LaplacesDemonMatrix in a new environment called LDEnv. If matrix x is not positive-definite, then LaplacesDemonMatrix in LDEnv is sought as a replacement. If this variable exists, then it is used to replace the matrix. If not, then the matrix is replaced with an identity matrix. Back in the model specification, after using as.parm.matrix, it is recommended that the user also pass the resulting matrix back into the parm vector, so the sampler or algorithm knows that the elements of the matrix have changed.

The as.positive.definite function returns the nearest positive-definite matrix for a matrix that is square and symmetric (Higham, 2002). This version is intended only for covariance and precision matrices, and has been optimized for speed. A more extensible function is nearPD in the matrixcalc package, which is also able to work with correlation matrices, and matrices that are asymmetric.

The as.positive.semidefinite function iteratively seeks to return a square, symmetric matrix that is at least positive-semidefinite, by replacing each negative eigenvalue and calculating its projection. This is intended only for covariance and precision matrices. A similar function is makePsd in the RTAQ package, though it is not iterative, and returns matrices that fail a logical check with is.positive.semidefinite.

The as.symmetric.matrix function accepts either a vector or matrix, and returns a symmetric matrix. In the case of a vector, it can be either all elements of the matrix, or the lower triangular. In the case of a x being entered as a matrix, this function tolerates non-finite values in one triangle (say, the lower), as long as the corresponding element is finite in the other (say, the upper) triangle.

The Cov2Cor function converts a covariance matrix into a correlation matrix, and accepts the covariance matrix either in matrix or vector form. This function may be useful inside a model specification and also with converting posterior draws of the elements of a covariance matrix to a correlation matrix. Cov2Cor is an expanded form of the cov2cor function in the stats package, where Cov2Cor is also able to accept and return a vectorized matrix.

The CovEstim function estimates a covariance matrix with one of several methods. This is mainly used by LaplaceApproximation, where the parm argument receives the posterior modes. See the CovEst argument for more details.

The GaussHermiteCubeRule function returns a matrix of nodes and a vector of weights for a dims-dimensional integral given N univariate nodes. The number of multivariate nodes will differ from the number of univariate nodes. This function is for use with multivariate quadrature, often called cubature. This has been adapted from the multiquad function in the NominalLogisticBiplot package. The GaussHermiteQuadRule function is a univariate version. A customized univariate rule may be supplied when constraints necessitate that one or more nodes and weights had to be altered.

The Hessian returns a symmetric, Hessian matrix, which is a matrix of second partial derivatives. The estimation of the Hessian matrix is approximated numerically using Richardson extrapolation by default. This is a slow function. This function is not intended to be called by the user, but is made available here. This is essentially the hessian function from the numDeriv package, adapted to Laplace's Demon.

The is.positive.definite function is a logical test of whether or not a matrix is positive-definite. A k \times k symmetric matrix \textbf{X} is positive-definite if all of its eigenvalues are positive (\lambda_i > 0, i \in k). All main-diagonal elements must be positive. The determinant of a positive-definite matrix is always positive, so a positive-definite matrix is always nonsingular. Non-symmetric, positive-definite matrices exist, but are not considered here.

The is.positive.semidefinite function is a logical test of whether or not a matrix is positive-semidefinite. A k x k symmetric matrix \textbf{X} is positive-semidefinite if all of its eigenvalues are non-negative (\lambda_i \ge 0, i \in k).

The is.square.matrix function is a logical test of whether or not a matrix is square. A square matrix is a matrix with the same number of rows and columns, and is usually represented as a k \times k matrix \textbf{X}.

The is.symmetric.matrix function is a logical test of whether or not a matrix is symmetric. A symmetric matrix is a square matrix that is equal to its transpose, \textbf{X} = \textbf{X}^T. For example, where i indexes rows and j indexes columns, \textbf{X}_{i,j} = \textbf{X}_{j,i}. This differs from the isSymmetric function in base R that is inexact, using all.equal.

The Jacobian function estimates the Jacobian matrix, which is a matrix of all first-order partial derivatives of the Model. The Jacobian matrix is estimated by default with forward finite-differencing, or optionally with Richardson extrapolation. This function is not intended to be called by the user, but is made available here. This is essentially the jacobian function from the numDeriv package, adapted to LaplacesDemon.

The logdet function returns the logarithm of the determinant of a positive-definite matrix via the Cholesky decomposition. The determinant is a value associated with a square matrix, and was used historically to determine if a system of linear equations has a unique solution. The term determinant was introduced by Gauss, where Laplace referred to it as the resultant. When the determinant is zero, the matrix is singular and non-invertible; there are either no solutions or many solutions. A unique solution exists when the determinant is non-zero. The det function in base R works well for small matrices, but can return erroneously return zero in larger matrices. It is better to work with the log-determinant.

The lower.triangle function returns a vector of the lower triangular elements of a matrix, and the diagonal is included when diag=TRUE.

The read.matrix function is provided here as one of many convenient ways to read a numeric matrix into R. The most common method of storing data in R is the data frame, because it is versatile. For example, a data frame may contain character, factor, and numeric variables together. For iterative estimation, common in Bayesian inference, the data frame is much slower than the numeric matrix. For this reason, the LaplacesDemon package does not use data frames, and has not traditionally accepted character or factor data. The read.matrix function returns either an entire numeric matrix, or row-wise samples from a numeric matrix. Samples may be taken from a matrix that is too large for available computer memory (RAM), such as with big data.

The SparseGrid function returns a sparse grid for a J-dimensional integral with accuracy K, given Gauss-Hermite quadrature rules. A grid of order eqnK provides an exact result for a polynomial of total order of 2K - 1 or less. SparseGrid returns a matrix of nodes and a vector of weights. A sparse grid is more efficient than the full grid in the GaussHermiteCubeRule function. This has been adapted from the SparseGrid package.

The TransitionMatrix function has several uses. A user may supply a vector of marginal posterior samples of a discrete Markov chain as theta.y, and an observed posterior transition matrix is returned. Otherwise, a user may supply data (y.theta) and/or a prior (p.theta), in which case a posterior transition matrix is returned. A common row-wise prior is the dirichlet distribution. Transition probabilities are from row element to column element.

The tr function returns the trace of a matrix. The trace of a matrix is the sum of the elements in the main diagonal of a square matrix. For example, the trace of a k \times k matrix \textbf{X}, is \sum_{k=1} \textbf{X}_{k,k}.

The upper.triangle function returns a vector of the lower triangular elements of a matrix, and the diagonal is included when diag=TRUE.

Author(s)

Statisticat, LLC. software@bayesian-inference.com

References

Higham, N.J. (2002). "Computing the Nearest Correlation Matrix - a Problem from Finance". IMA Journal of Numerical Analysis, 22, p. 329–343.