new function symmatrix

Author: baddstats

DESCRIPTION

@@ -1,5 +1,5 @@
 Package: spatstat.sparse
-Version: 3.1-0.006
+Version: 3.1-0.007
 Date: 2026-04-23
 Title: Sparse Three-Dimensional Arrays and Linear Algebra Utilities
 Authors@R: c(person("Adrian", "Baddeley", 

NAMESPACE

@@ -64,6 +64,7 @@ export("Summary.sparse3Darray")
 export("sumouter")
 export("sumsymouter")
 export("sumsymouterSparse")
+export("symmatrix")
 export("tensor1x1")
 export("tensorSparse")
 export("unionOfSparseIndices")

NEWS

@@ -1,16 +1,22 @@
 
-        CHANGES IN spatstat.sparse VERSION 3.1-0.006
+        CHANGES IN spatstat.sparse VERSION 3.1-0.007
 
 OVERVIEW
 
     o Simulate a Markov chain with sparse transition matrix.
 
+    o Recreate a symmetric matrix from its lower triangular entries.
+
 NEW FUNCTIONS
 
     o runSparseMarkovChain
     Generate simulated realisations of a Markov chain
     using a sparse matrix representation of the transition probability matrix.
 
+    o symmatrix
+    Recreate a symmetric matrix from a vector containing its lower triangular
+    (or upper triangular) entries.
+
         CHANGES IN spatstat.sparse VERSION 3.1-0
 
 OVERVIEW

R/matrixpower.R

@@ -112,3 +112,26 @@ matrixpower <- function(x, power, complexOK=TRUE) {
     dimnames(y) <- rev(dn)
   return(y)
 }
+
+symmatrix <- function(x, from=c("lower", "upper"), diag=TRUE) {
+  from <- match.arg(from)
+  x <- as.vector(x)
+  n <- length(x)
+  if(diag) {
+    m <- (sqrt(8*n+1)-1)/2
+    E <- m^2 + m - 2*n
+  } else {
+    m <- (sqrt(8*n+1)+1)/2
+    E <- m^2 - m - 2*n
+  }
+  if(E != 0)
+    stop("x has the wrong length for a triangular subset", call.=FALSE)
+  M <- matrix(, m, m)
+  A <- switch(from,
+              lower = lower.tri(M, diag=diag),
+              upper = upper.tri(M, diag=diag))
+  M[  A ] <- x
+  M[ !A ] <- t(M)[ !A ]
+  return(M)
+}
+

inst/doc/packagesizes.txt

@@ -19,4 +19,4 @@ date version nhelpfiles nobjects ndatasets Rlines srclines
 "2023-03-12" "3.0-1" 15 48 0 2092 740
 "2023-10-24" "3.0-3" 15 48 0 2092 740
 "2024-06-21" "3.1-0" 15 48 0 2092 740
-"2026-04-23" "3.1-0.006" 16 49 0 2194 943
+"2026-04-23" "3.1-0.007" 17 50 0 2217 943

inst/info/packagesizes.txt

@@ -19,4 +19,4 @@ date version nhelpfiles nobjects ndatasets Rlines srclines
 "2023-03-12" "3.0-1" 15 48 0 2092 740
 "2023-10-24" "3.0-3" 15 48 0 2092 740
 "2024-06-21" "3.1-0" 15 48 0 2092 740
-"2026-04-23" "3.1-0.006" 16 49 0 2194 943
+"2026-04-23" "3.1-0.007" 17 50 0 2217 943

man/symmatrix.Rd

@@ -0,0 +1,79 @@
+\name{symmatrix}
+\alias{symmatrix}
+\title{
+  Create a Symmetric Matrix from a Triangular Subset
+}
+\description{
+  Given a vector containing values for the upper or lower triangular
+  entries of a matrix, reconstruct the original matrix, assuming it
+  is symmetric.
+}
+\usage{
+symmatrix(x, from = c("lower", "upper"), diag = TRUE)
+}
+\arguments{
+  \item{x}{
+    A vector of atomic values.
+  }
+  \item{from}{
+    Character string (partially matched) specifying whether the
+    entries of \code{x} represent the upper or lower triangular
+    entries of the matrix.
+  }
+  \item{diag}{
+    Logical value specifying whether \code{x} includes diagonal entries
+    of the matrix.
+  }
+}
+\details{
+  This function reconstructs a symmetric matrix from its
+  lower triangular entries, or from its upper triangular entries.
+  \itemize{
+    \item
+    If \code{from="lower"} (the default), the vector \code{x} is assumed
+    to contain the lower-triangular entries of a matrix \code{M} (that is, the
+    entries \code{M[i,j]} in row \eqn{i} and column \eqn{j} where
+    \eqn{i \ge j}{i >= j})
+    listed in column-major order (that is, all of column 1 first, then all
+    the relevant entries in column 2, etc.). This vector could have been
+    obtained from the matrix by \code{x <- M[lower.tri(M, diag)]}.
+    \item
+    If \code{from="upper"}, the vector \code{x} is assumed
+    to contain the upper-triangular entries (those for which
+    \eqn{i \le j}{i <= j}) listed in column-major order.
+    This vector could have been
+    obtained from the matrix by \code{x <- M[upper.tri(M, diag)]}.
+  }
+  Assuming \code{M} was a
+  symmetric matrix, it will be reconstructed from these values.
+  The reconstructed matrix is returned.
+
+  If \code{diag} is FALSE, then the diagonal entries of the matrix
+  are assumed to have been omitted, and the
+  resulting matrix will contain \code{NA} entries in the diagonal.
+}
+\value{
+  A symmetric square matrix, containing values of the same atomic type
+  as \code{x}.
+}
+\author{
+  \adrian.
+}
+\seealso{
+  \code{\link[base]{lower.tri}}
+}
+\examples{
+  M <- matrix(c(10,  2, 3, 4,
+                 2, 20, 5, 6,
+                 3,  5, 30, 7,
+                 4,  6, 7, 40), 4, 4)
+  M
+
+  (x <- M[lower.tri(M, diag=TRUE)])
+  symmatrix(x)
+
+  (y <- M[upper.tri(M, diag=FALSE)])
+  symmatrix(y, from="upper", diag=FALSE)
+}
+\keyword{manip}
+\keyword{array}