allow length(<expr>) <- n

git-svn-id: https://svn.r-project.org/R/trunk@88410 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: maechler

doc/NEWS.Rd

@@ -176,6 +176,11 @@
       \item \code{tools::checkReplaceFuns()} now deals better with
       replacement methods \emph{not} available as regular functions in the
       namespace.
+
+      \item \code{length(xpr) <- n} now also works for \code{expression}
+      vectors, the same as for \code{list}s, i.e., padding with \code{NULL}
+      entries if the length is increased.  Thanks to \I{Mikael Jagan}'s
+      \PR{18917}.
     }
   }
 }

src/library/base/man/length.Rd

@@ -1,6 +1,6 @@
 % File src/library/base/man/length.Rd
 % Part of the R package, https://www.R-project.org
-% Copyright 1995-2017 R Core Team
+% Copyright 1995-2025 R Core Team
 % Distributed under GPL 2 or later
 
 \name{length}
@@ -27,8 +27,11 @@ length(x) <- value
 
   The replacement form can be used to reset the length of a vector.  If
   a vector is shortened, extra values are discarded and when a vector is
-  lengthened, it is padded out to its new length with \code{\link{NA}}s
-  (\code{nul} for raw vectors).
+  lengthened, it is padded out to its new length with \code{\link{NA}}s for
+  atomic vectors but by \code{nul}, i.e., \code{as.raw(0)} for
+  \code{\link{raw}} vectors, whereas \code{\link{list}} and
+  \code{\link{expression}} vectors use \code{\link{NULL}}, the latter three
+  cases in line with vector creation by \code{\link{vector}("<mode>", length)}.
 
   Both are \link{primitive} functions.
 }
@@ -38,7 +41,8 @@ length(x) <- value
   \code{\link{integer}} of length 1, except for vectors of more than
   \eqn{2^{31}-1}{2^31 - 1} elements, when it returns a double.
 
-  For vectors (including lists) and factors the length is the number of
+  For vectors (including lists and expressions) and factors the length is
+  the number of
   elements.  For an environment it is the number of objects in the
   environment, and \code{NULL} has length 0.  For expressions and
   pairlists (including \link{language objects} and dot-dot-dot lists) it is the

src/main/builtin.c

@@ -904,6 +904,7 @@ SEXP xlengthgets(SEXP x, R_xlen_t len)
 	}
 	break;
     case VECSXP:
+    case EXPRSXP:
 	for (i = 0; i < len; i++)
 	    if (i < lenx) {
 		SET_VECTOR_ELT(rval, i, VECTOR_ELT(x, i));
@@ -940,12 +941,10 @@ SEXP lengthgets(SEXP x, R_len_t len)
 
 attribute_hidden SEXP do_lengthgets(SEXP call, SEXP op, SEXP args, SEXP rho)
 {
-    SEXP x, ans;
-
     checkArity(op, args);
     check1arg(args, call, "x");
 
-    x = CAR(args);
+    SEXP ans, x = CAR(args);
 
     /* DispatchOrEval internal generic: length<- */
     if(isObject(x) && DispatchOrEval(call, op, "length<-", args,

tests/reg-tests-1e.R

@@ -2021,6 +2021,13 @@ stopifnot(ch %in% D, D %in% ch, !(c2 %in% D), !(D %in% c2))
 ## had failed in R-devel around 2025-06-26 (and before R 4.3.0)
 
 
+## length(<expression>) <- value
+x <- expression(.)
+length(x) <- 0L
+stopifnot(identical(x, expression()),
+          identical(`length<-`(x, 2L), expression(NULL, NULL)))
+
+
 
 ## keep at end
 rbind(last =  proc.time() - .pt,