nnzero.Rd
\name{nnzero}
\title{The Number of Non-Zero Values of a Matrix}
\alias{nnzero}
\alias{nnzero,ANY-method}
\alias{nnzero,denseMatrix-method}
\alias{nnzero,diagonalMatrix-method}
\alias{nnzero,indMatrix-method}
\alias{nnzero,sparseMatrix-method}
\alias{nnzero,CHMfactor-method}
\description{
Returns the number of non-zero values of a numeric-like \R object, and
in particular an object \code{x} inheriting from class
\code{\linkS4class{Matrix}}.
}
\usage{
nnzero(x, na.counted = NA)
}
\arguments{
\item{x}{an \R object, typically inheriting from class
\code{\linkS4class{Matrix}} or \code{\link{numeric}}.}
\item{na.counted}{a \code{\link{logical}} describing how
\code{\link{NA}}s should be counted. There are three possible
settings for \code{na.counted}:
\describe{
\item{TRUE}{\code{NA}s \emph{are} counted as non-zero (since
\dQuote{they are not zero}).}
\item{NA}{(default)the result will be \code{NA} if there are \code{NA}'s in
\code{x} (since \dQuote{NA's are not known, i.e., \emph{may be} zero}).}
\item{FALSE}{\code{NA}s are \emph{omitted} from \code{x} before
the non-zero entries are counted.}
}
For sparse matrices, you may often want to use \code{na.counted = TRUE}.
}
}
% \details{
% }
\section{Methods}{
\describe{
\item{\code{signature(x = "ANY")}}{the default method for
non-\code{\linkS4class{Matrix}} class objects, simply counts the
number \code{0}s in \code{x}, counting \code{NA}'s depending on
the \code{na.counted} argument, see above.}
\item{\code{signature(x = "denseMatrix")}}{conceptually the same as
for traditional \code{\link{matrix}} objects, care has to be taken
for \code{"\linkS4class{symmetricMatrix}"} objects.}
\item{\code{signature(x = "diagonalMatrix")}, and
\code{signature(x = "indMatrix")}}{fast simple methods for these
special \code{"sparseMatrix"} classes.}
\item{\code{signature(x = "sparseMatrix")}}{typically, the most
interesting method, also carefully taking
\code{"\linkS4class{symmetricMatrix}"} objects into account.}
}
}
\value{
the number of non zero entries in \code{x} (typically
\code{\link{integer}}).
Note that for a \emph{symmetric} sparse matrix \code{S} (i.e., inheriting from
class \code{\linkS4class{symmetricMatrix}}), \code{nnzero(S)} is
typically \emph{twice} the \code{length(S@x)}.
}
%\author{Martin}
\seealso{The \code{\linkS4class{Matrix}} class also has a
\code{\link{length}} method; typically, \code{length(M)} is much
larger than \code{nnzero(M)} for a sparse matrix M, and the latter is
a better indication of the \emph{size} of \code{M}.
\code{\link{drop0}}, \code{\link{zapsmall}}.
}
\examples{
m <- Matrix(0+1:28, nrow = 4)
m[-3,c(2,4:5,7)] <- m[ 3, 1:4] <- m[1:3, 6] <- 0
(mT <- as(m, "dgTMatrix"))
nnzero(mT)
(S <- crossprod(mT))
nnzero(S)
str(S) # slots are smaller than nnzero()
stopifnot(nnzero(S) == sum(as.matrix(S) != 0))# failed earlier
data(KNex)
M <- KNex$mm
class(M)
dim(M)
length(M); stopifnot(length(M) == prod(dim(M)))
nnzero(M) # more relevant than length
## the above are also visible from
str(M)
}
\keyword{attribute}