Add select parameter to fwrite() to avoid temporary objects

NEWS.md

@@ -69,6 +69,8 @@
 
 15. New function `isoyear()` has been implemented as a complement to `isoweek()`, returning the ISO 8601 year corresponding to a given date, [#7154](https://github.com/Rdatatable/data.table/issues/7154). Thanks to @ben-schwen and @MichaelChirico for the suggestion and @venom1204 for the implementation.
 
+16. `fwrite()` gains `select` argument to write only specified columns, avoiding temporary object creation for memory efficiency, [#4177](https://github.com/Rdatatable/data.table/issues/4177). For `data.table` objects, this uses `.shallow()` to create shallow copies without data duplication. Thanks to @artidataio for feature request, @ColeMiller1 for suggesting implementation and @Mukulyadav2004 for the implementation.
+
 ### BUG FIXES
 
 1. `fread()` no longer warns on certain systems on R 4.5.0+ where the file owner can't be resolved, [#6918](https://github.com/Rdatatable/data.table/issues/6918). Thanks @ProfFancyPants for the report and PR.

R/fwrite.R

@@ -13,7 +13,8 @@
            yaml = FALSE,
            bom = FALSE,
            verbose=getOption("datatable.verbose", FALSE),
-           encoding = "") {
+           encoding = "",
+           select = NULL) {
   na = as.character(na[1L]) # fix for #1725
   if (length(encoding) != 1L || !encoding %chin% c("", "UTF-8", "native")) {
     stopf("Argument 'encoding' must be '', 'UTF-8' or 'native'.")
@@ -26,6 +27,7 @@
   buffMB = as.integer(buffMB)
   nThread = as.integer(nThread)
   compressLevel = as.integer(compressLevel)
+
   # write.csv default is 'double' so fwrite follows suit. write.table's default is 'escape'
   # validate arguments
   if (is.matrix(x)) { # coerce to data.table if input object is matrix
@@ -38,6 +40,16 @@
       x = as.data.table(x)
     }
   }
+  # Handle select argument using .shallow()
+  if (!null(select)) {
+    cols = colnamesInt(x, select)         
+    if (is.data.table(x)) {
+      x = .shallow(x, cols)
+    } else {
+      x = x[select]
+    }
+  }
+
   stopifnot(
     is.list(x),
     identical(quote,"auto") || isTRUEorFALSE(quote),

inst/tests/tests.Rraw

@@ -21602,3 +21602,10 @@ if (getRversion() >= "4.0.0") { # rely on stopifnot(named = ...) for correct mes
 test(2337.3, is.null(fwrite(data.table(c(0.1, 0.2)), dec=",", sep="\t")))
 test(2337.4, is.null(fwrite(data.table(a=numeric(), b=numeric()), dec=",", sep=",")))
 test(2337.5, is.null(fwrite(data.table(a=numeric()), dec=",", sep=",")))
+
+# test for select parameter #4177
+DT = data.table(a=1:2, b=3:4)
+f = tempfile()
+fwrite(DT, f, select = "a") 
+test(2338.1, names(fread(f)), "a")
+unlink(f)

man/fwrite.Rd

@@ -62,6 +62,7 @@ fwrite(x, file = "", append = FALSE, quote = "auto",
   \item{bom}{If \code{TRUE} a BOM (Byte Order Mark) sequence (EF BB BF) is added at the beginning of the file; format 'UTF-8 with BOM'.}
   \item{verbose}{Be chatty and report timings?}
   \item{encoding}{ The encoding of the strings written to the CSV file. Default is \code{""}, which means writing raw bytes without considering the encoding. Other possible options are \code{"UTF-8"} and \code{"native"}. }
+  \item{select}{Vector of column names or column numbers specifying which columns to include. When \code{NULL} (default), all columns are selected. This avoids creating temporary subsets for memory efficiency.}
 }
 \details{
 \code{fwrite} began as a community contribution with \href{https://github.com/Rdatatable/data.table/pull/1613}{pull request #1613} by Otto Seiskari. This gave Matt Dowle the impetus to specialize the numeric formatting and to parallelize: \url{https://h2o.ai/blog/2016/fast-csv-writing-for-r/}. Final items were tracked in \href{https://github.com/Rdatatable/data.table/issues/1664}{issue #1664} such as automatic quoting, \code{bit64::integer64} support, decimal/scientific formatting exactly matching \code{write.csv} between 2.225074e-308 and 1.797693e+308 to 15 significant figures, \code{row.names}, dates (between 0000-03-01 and 9999-12-31), times and \code{sep2} for \code{list} columns where each cell can itself be a vector.