rowsum <- function(x, group, reorder=TRUE, ...) UseMethod("rowsum")

rowsum.default <- function(x, group, reorder=TRUE, na.rm = FALSE, ...)
{
    if (!is.numeric(x))
        stop("'x' must be numeric")
    if (length(group) != NROW(x))
        stop("incorrect length for 'group'")
    if (any(is.na(group)))
        warning("missing values for 'group'")
    ugroup <- unique(group)
    if (reorder) ugroup <- sort(ugroup, na.last=TRUE, method="quick")

    rval <- .Call("Rrowsum_matrix", x, NCOL(x), group, ugroup, na.rm,
                  PACKAGE="base")

    dimnames(rval) <- list(as.character(ugroup),dimnames(x)[[2]])
    rval
}

rowsum.data.frame <- function(x, group, reorder=TRUE, na.rm = FALSE, ...)
{
    if (!is.data.frame(x)) stop("not a data frame") ## make MM happy
    if (length(group) != NROW(x))
        stop("incorrect length for 'group'")
    if (any(is.na(group)))
        warning("missing values for 'group'")
    ugroup <- unique(group)
    if (reorder) ugroup <- sort(ugroup, na.last=TRUE, method="quick")

    rval <- .Call("Rrowsum_df", x, NCOL(x), group, ugroup, na.rm,
                  PACKAGE="base")

    as.data.frame(rval, row.names=as.character(ugroup))
}