improvements to handling and docs of dim<-

git-svn-id: https://svn.r-project.org/R/trunk@90454 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/base/man/dim.Rd

@@ -1,6 +1,6 @@
 % File src/library/base/man/dim.Rd
 % Part of the R package, https://www.R-project.org
-% Copyright 1995-2010 R Core Team
+% Copyright 1995-2026 R Core Team
 % Distributed under GPL 2 or later
 
 \name{dim}
@@ -18,7 +18,8 @@ dim(x) <- value
 \arguments{
   \item{x}{an \R object, for example a matrix, array or data frame.}
   \item{value}{for the default method, either \code{NULL} or
-    a numeric vector, which is coerced to integer (by truncation).}
+    a numeric vector, which is coerced to integer (by truncation).  After
+    coercion all the elements must be non-negative and not \code{NA}.}
 }
 \details{
   The functions \code{dim} and \code{dim<-} are \link{internal generic}
@@ -34,8 +35,8 @@ dim(x) <- value
   of mode \code{\link{integer}}.
 
   The replacement method changes the \code{"dim"} attribute (provided the
-  new value is compatible) and removes any \code{"dimnames"} \emph{and}
-  \code{"names"} attributes.
+  new value is compatible with the length) and removes any
+  \code{"dimnames"} \emph{and} \code{"names"} attributes.
 }
 \references{
   \bibshow{R:Becker+Chambers+Wilks:1988}

src/main/array.c

@@ -290,16 +290,19 @@ R_xlen_t dim2total(SEXP dim /* INTSXP */, bool *err)
         int d;
 
 	d = INTEGER(dim)[i];
-        /* Actually, NA_INTEGER is < 0, so we don't need to test for it
+	/* dn *= d can overflow, but the result is 0 if any dimension is
+	 * (unless NA, so we can't simply break here).
+	 */
+        if (d == 0)
+            dn = 0.0;
+       /* Actually, NA_INTEGER is < 0, so we don't need to test for it
            explicitly, but the value might change, so better be safe... 
            An optimizing compiler will likely strip the && part.
          */
-        // this can overflow, but the result is 0 if any dimenston is.
-        if (d == 0)
-            dn = 0.0;
-	else if (d >= 0 && d != NA_INTEGER)
+	if (d >= 0 && d != NA_INTEGER) {
             dn *= d;
-        else {
+	// Better to check for total here to avoid f/p overflow.
+        } else {
             if (d == NA_INTEGER)
 	        error(_("the dims contain missing values"));
             else
@@ -311,7 +314,7 @@ R_xlen_t dim2total(SEXP dim /* INTSXP */, bool *err)
 #else
     if (dn > INT_MAX)
 #endif
-	*err = true;
+	*err = true; // and callers should not use the return value.
     return (R_xlen_t) dn;
 }
 

tests/reg-tests-1e.R

@@ -3667,6 +3667,8 @@ for(nd in c(2:4, 33:37)) {
     stopifnot(identical(x, y), is.integer(d <- dim(x)), all.equal(d., d))
 }
 ## above creation of x & y failed for a couple of hours in R-devel
+## r90452 overflowed the computation of the product of dimeensions.
+## giving a negative answer on some platforms (x86_64, not macOS).
 
 
 ## PR#19116 -- <matrix>[[i, j]]: stochastic error with negative i
@@ -3684,7 +3686,7 @@ save("save", file=(tf <- tempfile("saveRdata")))
 sys.load.image(tf, FALSE); rm(tf, save)
 ## had called .Internal(RNGkind(..)) with wrong number of args
 
-## aperm and 
+## aperm and
 m <- matrix(1:10, 2, 5, dimnames = list(X = paste0("x", 1:2), Y = paste0("y", 1:5)))
 a <- array(m, dim = c(2, 5, 1), dimnames = list(X = paste0("x", 1:2), Y = paste0("y", 1:5), Z = "z1"))