##' Trace operator
##'
##' Calculates the trace of a square matrix.
##' @param x Square numeric matrix
##' @param \dots Additional arguments to lower level functions
##' @return \code{numeric}
##' @author Klaus K. Holst
##' @seealso \code{\link{crossprod}}, \code{\link{tcrossprod}}
##' @keywords math algebra
##' @examples
##'
##' tr(diag(1:5))
##' @export
"tr" <- function(x,...) UseMethod("tr")
##' @export
`tr.matrix` <-
function(x,na.rm=FALSE,...) {
if (length(x)==1)
return(x)
n <- nrow(x)
if (!n)
stop("0 x 0 matrix")
if (n != ncol(x))
stop("non-square matrix")
if (!na.rm && any(!is.finite(x)))
stop("infinite or missing values")
return(sum(diag(x),na.rm=na.rm))
}
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