tweak write.table() performance (PR#18500)

git-svn-id: https://svn.r-project.org/R/trunk@84133 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/NEWS.Rd

@@ -271,6 +271,11 @@
       an F test depending on whether the dispersion is fixed or
       free.  Test statistics can be suppressed by giving argument
       \code{test} a false logical value.
+
+      \item The performance of \code{write.table()} has been improved
+      for data frames with a large number of unclassed columns.
+      (Thanks to Gabriel Becker's \PR{18500}, prompted by a report from
+      Toby Dylan Hocking on the R-devel mailing list.)
     }
   }
 

src/library/utils/R/write.table.R

@@ -1,7 +1,7 @@
 #  File src/library/utils/R/write.table.R
 #  Part of the R package, https://www.R-project.org
 #
-#  Copyright (C) 1995-2019 The R Core Team
+#  Copyright (C) 1995-2023 The R Core Team
 #
 #  This program is free software; you can redistribute it and/or modify
 #  it under the terms of the GNU General Public License as published by
@@ -135,9 +135,8 @@ function (x, file = "", append = FALSE, quote = TRUE, sep = " ",
     if(is.matrix(x) && !is.atomic(x)) mode(x) <- "character"
     if(is.data.frame(x)) {
         ## convert columns we can't handle in C code
-        x[] <- lapply(x, function(z) {
-            if(is.object(z) && !is.factor(z)) as.character(z) else z
-        })
+        needconv <- vapply(x, function(z) is.object(z) && !is.factor(z), TRUE)
+        x[needconv] <- lapply(x[needconv], as.character)
     }
 
     invisible(.External2(C_writetable, x, file, nrow(x), p, rnames, sep, eol,