Closes #2930 -- bugfix to as.matrix.data.table()

NEWS.md

@@ -7,6 +7,8 @@
 
 #### BUG FIXES
 
+1. Passing a column to use as the rownames to `as.matrix.data.table()` now works when the input `data.table` has a single row, [#2930](https://github.com/Rdatatable/data.table/issues/2930). Thanks to @malcook for reporting, @sritchie73 for fixing, and @HughParsonage for feedback  
+
 #### NOTES
 
 

R/data.table.R

@@ -1887,14 +1887,19 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
 #    x
 #}
 
-as.matrix.data.table <- function(x, rownames, ...) {
+as.matrix.data.table <- function(x, rownames, rownames.literal=FALSE, ...) {
   rn <- NULL
   rnc <- NULL
   if (!missing(rownames)) { # Convert rownames to a column index if possible
-    if (length(rownames) == nrow(x)) {
+    if (length(rownames) == nrow(x) && nrow(x) > 1) {
       # rownames argument is a vector of row names, no column in x to drop.
       rn <- rownames
       rnc <- NULL
+    } else if (nrow(x) == 1 && isTRUE(rownames.literal)) {
+      # When x has a single row, the user may still want to supply vector of rownames, 
+      # but we need to distinguish from the case when the rownames is a column of x.
+      rn <- rownames
+      rnc <- NULL
     } else if (!is.null(rownames) && length(rownames) != 1L) { # vector(0) will throw an error, but NULL will pass through
       stop(sprintf("rownames must be a single column in x or a vector of row names of length nrow(x)=%d", nrow(x)))
     } else if (!(is.null(rownames) || is.logical(rownames) || is.character(rownames) || is.numeric(rownames))) {

inst/tests/tests.Rraw

@@ -11702,6 +11702,12 @@ test(1899.12, as.matrix(DT, FALSE), mat2)
 setkey(DT, id, X)
 test(1899.13, as.matrix(DT, TRUE), mat, warning="rownames is TRUE but multiple keys")
 
+# Check handling of cases where the data.table only has 1 row, raised by Issue #2930:
+mat4 <- matrix(c("a", 1, 5), nrow=1, dimnames=list(c("x"), c("id", "X", "Y")))
+test(1899.14, as.matrix(DT[1,], 1), mat[1,,drop=FALSE])
+test(1899.15, as.matrix(DT[1,], "id"), mat[1,,drop=FALSE])
+test(1899.16, as.matrix(DT[1,], "x", rownames.literal=TRUE), mat4) # "x" not a column in DT, so use "x" as the rownames
+
 # index argument for fread, #2633
 DT_str = c('a,b\n3,1\n2,2\n1,1\n2,1\n3,2')
 test(1900.1, attributes(attr(fread(DT_str, index = 'a'), 'index')),

man/as.matrix.Rd

@@ -7,7 +7,7 @@ Converts a \code{data.table} into a \code{matrix}, optionally using one
 of the columns in the \code{data.table} as the \code{matrix} \code{rownames}.
 }
 \usage{
-\method{as.matrix}{data.table}(x, rownames, ...)}
+\method{as.matrix}{data.table}(x, rownames, rownames.literal, ...)}
 
 \arguments{
 \item{x}{a \code{data.table}}
@@ -17,6 +17,11 @@ the \code{rownames} in the returned \code{matrix}. If \code{TRUE} the
 single column, otherwise the first column in the \code{data.table} will
 be used. Alternative a vector of length \code{nrow(x)} to assign as the
 row names of the returned \code{matrix}.}
+\item{rownames.literal}{optional, when x is a \code{data.table} that has
+only a single row, set \code{rownames.literal} to \code{TRUE} if you want
+to use the vector supplied to the \code{rownames} argument as the row 
+names of the returned \code{matrix} instead of looking for \code{rownames}
+as a column of \code{x}.}
 \item{\dots}{additional arguments to be passed to or from methods.}
 }