From 1fb7294d24545467991eff1ca25375ecdf62e633 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 01:03:47 +0530
Subject: [PATCH 01/29] added logic to handle select argumnt using .shallow()

---
 R/fwrite.R | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index e94d00c3f3..d5897d0936 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -13,7 +13,8 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
            yaml = FALSE,
            bom = FALSE,
            verbose=getOption("datatable.verbose", FALSE),
-           encoding = "") {
+           encoding = "",
+           select) {
   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 +27,16 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   buffMB = as.integer(buffMB)
   nThread = as.integer(nThread)
   compressLevel = as.integer(compressLevel)
+
+  # Handle select argument using .shallow() 
+  if (!missing(select)) {
+    if (is.data.table(x)) {
+      cols = colnamesInt(x, select)
+      shallow_x = .shallow(x, cols)
+    } else {
+      shallow_x = x[select]
+    }
+  }
   # 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

From ee3e85f6b678a2b0de3b74096e89d85c668909af Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 01:19:29 +0530
Subject: [PATCH 02/29] add conditional passing

---
 R/fwrite.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index d5897d0936..94ec079e5f 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -131,7 +131,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
       x
     })
   }
-  .Call(CfwriteR, x, file, sep, sep2, eol, na, dec, quote, qmethod=="escape", append,
+  .Call(CfwriteR, if (missing(select)) x else shallow_x, file, sep, sep2, eol, na, dec, quote, qmethod=="escape", append,
         row.names, col.names, logical01, scipen, dateTimeAs, buffMB, nThread,
         showProgress, is_gzip, compressLevel, bom, yaml, verbose, encoding)
   invisible()

From 01d8bbeccbb48cd3e6e14d974e2b594ca71d71e0 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 01:37:26 +0530
Subject: [PATCH 03/29] assign x when selct is used

---
 R/fwrite.R | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 94ec079e5f..f5d38de753 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -32,9 +32,9 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   if (!missing(select)) {
     if (is.data.table(x)) {
       cols = colnamesInt(x, select)
-      shallow_x = .shallow(x, cols)
+      x = .shallow(x, cols)
     } else {
-      shallow_x = x[select]
+      x = x[select]
     }
   }
   # write.csv default is 'double' so fwrite follows suit. write.table's default is 'escape'
@@ -131,7 +131,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
       x
     })
   }
-  .Call(CfwriteR, if (missing(select)) x else shallow_x, file, sep, sep2, eol, na, dec, quote, qmethod=="escape", append,
+  .Call(CfwriteR, x, file, sep, sep2, eol, na, dec, quote, qmethod=="escape", append,
         row.names, col.names, logical01, scipen, dateTimeAs, buffMB, nThread,
         showProgress, is_gzip, compressLevel, bom, yaml, verbose, encoding)
   invisible()

From 97cb32bd8ba421320874fafca2d5a8b0a802bce9 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 02:14:53 +0530
Subject: [PATCH 04/29] added coverage tests

---
 R/fwrite.R            | 2 +-
 inst/tests/tests.Rraw | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index f5d38de753..77acb1c733 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -28,7 +28,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   nThread = as.integer(nThread)
   compressLevel = as.integer(compressLevel)
 
-  # Handle select argument using .shallow() 
+  # Handle select argument using .shallow()
   if (!missing(select)) {
     if (is.data.table(x)) {
       cols = colnamesInt(x, select)
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index a2549ccf6d..581fd6d0e0 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21549,3 +21549,10 @@ f = tempfile()
 writeLines(c('a', rep('0x1.ffffp0', 10000L), '0x1.ff\x9fp0', rep('0x1.ffffp0', 20000L)), f)
 test(2334, names(fread(f)), "a")
 unlink(f)
+
+# test for select parameter #4177
+DT = data.table(a=1:2, b=3:4)
+f = tempfile()
+fwrite(DT, f, select = "a") 
+test(2335.1, names(result = fread(f)), "a")
+unlink(f)

From 4f9928a4393ec79df2c222e0e264fdc9b88eadc5 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 02:22:05 +0530
Subject: [PATCH 05/29] typo

---
 inst/tests/tests.Rraw | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 581fd6d0e0..ed7ca2ccc7 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21554,5 +21554,5 @@ unlink(f)
 DT = data.table(a=1:2, b=3:4)
 f = tempfile()
 fwrite(DT, f, select = "a") 
-test(2335.1, names(result = fread(f)), "a")
+test(2335.1, names(fread(f)), "a")
 unlink(f)

From 8424176d38b77e7cacd9fd6912794273defa473a Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 02:27:46 +0530
Subject: [PATCH 06/29] right number

---
 inst/tests/tests.Rraw | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index dae734a4f8..89707b06f4 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21607,5 +21607,5 @@ test(2337.5, is.null(fwrite(data.table(a=numeric()), dec=",", sep=",")))
 DT = data.table(a=1:2, b=3:4)
 f = tempfile()
 fwrite(DT, f, select = "a") 
-test(2335.1, names(fread(f)), "a")
+test(2338.1, names(fread(f)), "a")
 unlink(f)

From d0956f3abfd091a287ff44941fac2a4acaa5329d Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 07:01:31 +0530
Subject: [PATCH 07/29] manual entry + news

---
 NEWS.md       | 2 ++
 R/fwrite.R    | 4 ++--
 man/fwrite.Rd | 1 +
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index fc69449438..1a5e167bd8 100644
--- a/NEWS.md
+++ b/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.
diff --git a/R/fwrite.R b/R/fwrite.R
index 8edd206bd5..6177e95ede 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -14,7 +14,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
            bom = FALSE,
            verbose=getOption("datatable.verbose", FALSE),
            encoding = "",
-           select) {
+           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'.")
@@ -29,7 +29,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   compressLevel = as.integer(compressLevel)
 
   # Handle select argument using .shallow()
-  if (!missing(select)) {
+  if (!null(select)) {
     if (is.data.table(x)) {
       cols = colnamesInt(x, select)
       x = .shallow(x, cols)
diff --git a/man/fwrite.Rd b/man/fwrite.Rd
index f16e4c9bd0..4e9efe0e96 100644
--- a/man/fwrite.Rd
+++ b/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.

From bb8320d111ac3526c2c713e9fd0e2c999a4d5b4c Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Tue, 5 Aug 2025 18:47:16 +0530
Subject: [PATCH 08/29] moved select logic to address matrix inputs also

---
 R/fwrite.R | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 6177e95ede..7010dd25e9 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -27,16 +27,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   buffMB = as.integer(buffMB)
   nThread = as.integer(nThread)
   compressLevel = as.integer(compressLevel)
-
-  # Handle select argument using .shallow()
-  if (!null(select)) {
-    if (is.data.table(x)) {
-      cols = colnamesInt(x, select)
-      x = .shallow(x, cols)
-    } else {
-      x = x[select]
-    }
-  }
+           
   # 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
@@ -49,6 +40,16 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
       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),

From 8e639919da262cbf45a92b5d1f7c6336bcb7a202 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 20:23:16 +0530
Subject: [PATCH 09/29] for matrix handling

---
 R/fwrite.R | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 6177e95ede..f8ac90aecc 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -28,15 +28,6 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   nThread = as.integer(nThread)
   compressLevel = as.integer(compressLevel)
 
-  # Handle select argument using .shallow()
-  if (!null(select)) {
-    if (is.data.table(x)) {
-      cols = colnamesInt(x, select)
-      x = .shallow(x, cols)
-    } else {
-      x = x[select]
-    }
-  }
   # 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
@@ -49,6 +40,15 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
       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),

From d237f5793234bf941b470203689b79751a9d4dcb Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Tue, 5 Aug 2025 20:37:58 +0530
Subject: [PATCH 10/29] add tests for other classes too

---
 inst/tests/tests.Rraw | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 89707b06f4..9dbabc0419 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21608,4 +21608,13 @@ DT = data.table(a=1:2, b=3:4)
 f = tempfile()
 fwrite(DT, f, select = "a") 
 test(2338.1, names(fread(f)), "a")
+df = as.data.frame(DT)
+fwrite(df, f, select = "a")
+test(2338.2, names(fread(f)), "a")
+l = as.list(DT)
+fwrite(l, f, select = "a")
+test(2338.3, names(fread(f)), "a")
+m = as.matrix(DT)
+fwrite(m, f, select = "a")
+test(2338.4, names(fread(f)), "a")
 unlink(f)

From 3a718c08e22858a50492104799db529c221b92c4 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Tue, 5 Aug 2025 20:40:35 +0530
Subject: [PATCH 11/29] remove trail whit space

---
 R/fwrite.R | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 7010dd25e9..c537821d62 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -27,7 +27,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   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
@@ -42,14 +42,14 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!null(select)) {
-    cols = colnamesInt(x, 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),

From 23214d146b375ca066b024bd82c103ebb238a3d3 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Tue, 5 Aug 2025 22:17:47 +0530
Subject: [PATCH 12/29] is.null

---
 R/fwrite.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index c537821d62..6314069f37 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -41,7 +41,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
     }
   }
   # Handle select argument using .shallow()
-  if (!null(select)) {
+  if (!is.null(select)) {
     cols = colnamesInt(x, select) 
     if (is.data.table(x)) {
       x = .shallow(x, cols)

From 2744a89e364c5a2b2c4af6e1ba28078b58e3e814 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Tue, 5 Aug 2025 22:22:41 +0530
Subject: [PATCH 13/29] add tests

---
 R/fwrite.R            | 2 +-
 inst/tests/tests.Rraw | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index f8ac90aecc..6052fb0584 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -41,7 +41,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
     }
   }
   # Handle select argument using .shallow()
-  if (!null(select)) {
+  if (!is.null(select)) {
     cols = colnamesInt(x, select)
     if (is.data.table(x)) {
       x = .shallow(x, cols)
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 89707b06f4..9dbabc0419 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -21608,4 +21608,13 @@ DT = data.table(a=1:2, b=3:4)
 f = tempfile()
 fwrite(DT, f, select = "a") 
 test(2338.1, names(fread(f)), "a")
+df = as.data.frame(DT)
+fwrite(df, f, select = "a")
+test(2338.2, names(fread(f)), "a")
+l = as.list(DT)
+fwrite(l, f, select = "a")
+test(2338.3, names(fread(f)), "a")
+m = as.matrix(DT)
+fwrite(m, f, select = "a")
+test(2338.4, names(fread(f)), "a")
 unlink(f)

From 66d37442f2a42257438cf1f97651c25921c8ffce Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Tue, 5 Aug 2025 22:27:23 +0530
Subject: [PATCH 14/29] trailing space

---
 R/fwrite.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 6314069f37..3fe6abdf33 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -42,7 +42,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!is.null(select)) {
-    cols = colnamesInt(x, select) 
+    cols = colnamesInt(x, select)
     if (is.data.table(x)) {
       x = .shallow(x, cols)
     } else {

From d5c933bc64d94addb3b8206a879ba679bf734532 Mon Sep 17 00:00:00 2001
From: Mukul Kumar <mukulrao6480@gmail.com>
Date: Fri, 8 Aug 2025 21:48:56 +0530
Subject: [PATCH 15/29] added atime performance test

---
 .ci/atime/tests.R | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index 761ba3c2bb..d783db964f 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -286,5 +286,17 @@ test.list <- atime::atime_test_list(
     Slow = "548410d23dd74b625e8ea9aeb1a5d2e9dddd2927",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/548410d23dd74b625e8ea9aeb1a5d2e9dddd2927)
     Fast = "c0b32a60466bed0e63420ec105bc75c34590865e"),  # Commit in the PR (https://github.com/Rdatatable/data.table/pull/7144/commits) that uses a much faster implementation
 
+    # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
+    # Fixed in  https://github.com/Rdatatable/data.table/pull/7236
+    "fwrite select parameter improved in #4177" = atime::atime_test(
+    setup = {
+      set.seed(1)
+      DT = data.table(a = rnorm(N), b = rnorm(N), c = rnorm(N), d = rnorm(N), e = rnorm(N))
+      temp_file = tempfile()
+    },
+    expr = data.table::fwrite(DT, temp_file, select = c("a", "b", "c")),
+    Slow = "66cb6d2393cef30083b444346a7600a079207806",    # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+
     tests=extra.test.list)
 # nolint end: undesirable_operator_linter.

From b0e7f8263664f942fef9af32c3cc895f40e4a18d Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 8 Aug 2025 22:49:17 +0530
Subject: [PATCH 16/29] add condition to when to use select

---
 .ci/atime/tests.R | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index d783db964f..ff43c3e311 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -293,8 +293,10 @@ test.list <- atime::atime_test_list(
       set.seed(1)
       DT = data.table(a = rnorm(N), b = rnorm(N), c = rnorm(N), d = rnorm(N), e = rnorm(N))
       temp_file = tempfile()
+      has_select = "select" %chin% names(formals(data.table::fwrite))
     },
-    expr = data.table::fwrite(DT, temp_file, select = c("a", "b", "c")),
+    expr = if (has_select)
+      data.table::fwrite(DT, temp_file, select=c("a","b","c")),
     Slow = "66cb6d2393cef30083b444346a7600a079207806",    # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
     Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 

From 8fe700249bc1994195711047bc62c189c58c091b Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Thu, 14 Aug 2025 21:47:29 +0530
Subject: [PATCH 17/29] added select parameter to docs also

---
 man/fwrite.Rd | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/man/fwrite.Rd b/man/fwrite.Rd
index 99b7529d08..ef14f45400 100644
--- a/man/fwrite.Rd
+++ b/man/fwrite.Rd
@@ -22,7 +22,8 @@ fwrite(x, file = "", append = FALSE, quote = "auto",
   bom = FALSE,
   verbose = getOption("datatable.verbose", FALSE),
   encoding = "",
-  forceDecimal = FALSE)
+  forceDecimal = FALSE,
+  select = NULL)
 }
 \arguments{
   \item{x}{Any \code{list} of same length vectors; e.g. \code{data.frame} and \code{data.table}. If \code{matrix}, it gets internally coerced to \code{data.table} preserving col names but not row names}

From ec25903036141951e205e0aeef2ad12e6daa805f Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 22:01:52 +0530
Subject: [PATCH 18/29] setup and branching only in expr and added cases

---
 .ci/atime/tests.R | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index ff43c3e311..273321f3f5 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -289,16 +289,37 @@ test.list <- atime::atime_test_list(
     # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
     # Fixed in  https://github.com/Rdatatable/data.table/pull/7236
     "fwrite select parameter improved in #4177" = atime::atime_test(
+    # N row, 5 column
     setup = {
-      set.seed(1)
-      DT = data.table(a = rnorm(N), b = rnorm(N), c = rnorm(N), d = rnorm(N), e = rnorm(N))
+      set.seed(1L)
+      DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))
+      temp_file = tempfile()
+    },
+    expr = {
+      has_select = "select" %chin% names(formals(data.table::fwrite))
+      if (has_select) {
+        data.table::fwrite(DT, temp_file, select = c("a","b","c"))
+      } else {
+        data.table::fwrite(DT[, c("a","b","c"), with = FALSE], temp_file)
+      }
+    },
+    Slow = "66cb6d2393cef30083b444346a7600a079207806",      # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"       # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+    # 1 row, N columns
+    setup = {
+      DT = data.table(t(1:N))
       temp_file = tempfile()
+    },
+    expr = {
       has_select = "select" %chin% names(formals(data.table::fwrite))
+      if (has_select) {
+        data.table::fwrite(DT, temp_file, select = 1L)
+      } else {
+        data.table::fwrite(DT[, 1L, with = FALSE], temp_file)
+      }
     },
-    expr = if (has_select)
-      data.table::fwrite(DT, temp_file, select=c("a","b","c")),
-    Slow = "66cb6d2393cef30083b444346a7600a079207806",    # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
-    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+    Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"      # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 
     tests=extra.test.list)
 # nolint end: undesirable_operator_linter.

From e54bd7135b85d5e5092ef0689e85c906595505be Mon Sep 17 00:00:00 2001
From: Toby Dylan Hocking <toby.dylan.hocking@usherbrooke.ca>
Date: Fri, 15 Aug 2025 12:36:24 -0400
Subject: [PATCH 19/29] document select arg

---
 vignettes/datatable-fread-and-fwrite.Rmd | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/vignettes/datatable-fread-and-fwrite.Rmd b/vignettes/datatable-fread-and-fwrite.Rmd
index 49cfa60db8..8a19d3e1ec 100644
--- a/vignettes/datatable-fread-and-fwrite.Rmd
+++ b/vignettes/datatable-fread-and-fwrite.Rmd
@@ -270,14 +270,23 @@ if (requireNamespace("bit64", quietly = TRUE)) {
 
 ### 2.4 Column Order and Subset Control
 
-To control the order and subset of columns written to file, subset the data.table before calling `fwrite()`. The `col.names` argument in `fwrite()` is a logical (TRUE/FALSE) that controls whether the header row is written, not which columns are written.
+To control the order and subset of columns written to file, you can use `[.data.table` to make a new table before calling `fwrite()`, but it is more efficient to use the `select` argument, which avoids making a copy.
 
 ```{r}
 dt = data.table(A = 1:3, B = 4:6, C = 7:9)
-
 # Write only columns C and A, in that order
 fwrite(dt[, .(C, A)], "out.csv")
 cat(readLines("out.csv"), sep = "\n")
+fwrite(dt, "out.csv", select=c("C","A"))
+cat(readLines("out.csv"), sep = "\n")
+file.remove("out.csv")
+```
+
+The `col.names` argument in `fwrite()` is a logical (TRUE/FALSE) that controls whether the header row is written, not which columns are written.
+
+```{r}
+fwrite(dt, "out.csv", col.names=FALSE)
+cat(readLines("out.csv"), sep = "\n")
 file.remove("out.csv")
 ```
 
@@ -292,4 +301,4 @@ For users interested in detailed, up-to-date performance comparisons, we recomme
 
 These benchmarks consistently show that `fread` and `fwrite` are highly competitive and often state-of-the-art for performance in the R ecosystem.
 
-***
\ No newline at end of file
+***

From c2c5d89a7a3d75c5cf2da5beda9f643146309c15 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 22:15:42 +0530
Subject: [PATCH 20/29] bypass name res for num select, avoid no-op shallow, 
 and cols in place of select(keep x as list/dt)

---
 R/fwrite.R | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index e28f6e9567..3e72f56fff 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -43,11 +43,17 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!is.null(select)) {
-    cols = colnamesInt(x, select)
+    if (is.integer(select) || is.numeric(select)) {  # numeric/integer avoids O(#cols) name-match overhead
+        cols = as.integer(select)
+    } else {
+        cols = colnamesInt(x, select)
+    }
     if (is.data.table(x)) {
-      x = .shallow(x, cols)
+        if (length(cols) < NCOL(x) || !identical(cols, seq_len(NCOL(x)))) { # only build a shallow view when columns are reduced or reordered
+           x = .shallow(x, cols)
+        }
     } else {
-      x = x[select]
+      x = x[cols]
     }
   }
 

From 926bb0e6e004058815b9a6bbcc4d15e57bf04f44 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 22:19:53 +0530
Subject: [PATCH 21/29] better writing style

Co-authored-by: Toby Dylan Hocking <tdhock5@gmail.com>
---
 R/fwrite.R | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index 3e72f56fff..cb3e724ab4 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -43,10 +43,10 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!is.null(select)) {
-    if (is.integer(select) || is.numeric(select)) {  # numeric/integer avoids O(#cols) name-match overhead
-        cols = as.integer(select)
+    cols = if (is.integer(select) || is.numeric(select)) {  # numeric/integer avoids O(#cols) name-match overhead
+        as.integer(select)
     } else {
-        cols = colnamesInt(x, select)
+        colnamesInt(x, select)
     }
     if (is.data.table(x)) {
         if (length(cols) < NCOL(x) || !identical(cols, seq_len(NCOL(x)))) { # only build a shallow view when columns are reduced or reordered

From cbffb5bc93b60641d0057dc1eac8e6731b3cddb4 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 22:50:56 +0530
Subject: [PATCH 22/29] seperate both cases

---
 .ci/atime/tests.R | 70 ++++++++++++++++++++++++-----------------------
 1 file changed, 36 insertions(+), 34 deletions(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index 273321f3f5..f96619fb18 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -285,41 +285,43 @@ test.list <- atime::atime_test_list(
     expr = data.table::isoweek(x),
     Slow = "548410d23dd74b625e8ea9aeb1a5d2e9dddd2927",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/548410d23dd74b625e8ea9aeb1a5d2e9dddd2927)
     Fast = "c0b32a60466bed0e63420ec105bc75c34590865e"),  # Commit in the PR (https://github.com/Rdatatable/data.table/pull/7144/commits) that uses a much faster implementation
-
+  
     # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
-    # Fixed in  https://github.com/Rdatatable/data.table/pull/7236
-    "fwrite select parameter improved in #4177" = atime::atime_test(
-    # N row, 5 column
-    setup = {
-      set.seed(1L)
-      DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))
-      temp_file = tempfile()
-    },
-    expr = {
-      has_select = "select" %chin% names(formals(data.table::fwrite))
-      if (has_select) {
-        data.table::fwrite(DT, temp_file, select = c("a","b","c"))
-      } else {
-        data.table::fwrite(DT[, c("a","b","c"), with = FALSE], temp_file)
-      }
-    },
-    Slow = "66cb6d2393cef30083b444346a7600a079207806",      # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
-    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"       # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
-    # 1 row, N columns
-    setup = {
-      DT = data.table(t(1:N))
-      temp_file = tempfile()
-    },
-    expr = {
-      has_select = "select" %chin% names(formals(data.table::fwrite))
-      if (has_select) {
-        data.table::fwrite(DT, temp_file, select = 1L)
-      } else {
-        data.table::fwrite(DT[, 1L, with = FALSE], temp_file)
-      }
-    },
-    Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
-    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"      # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+    # Fixed in https://github.com/Rdatatable/data.table/pull/7236
+   "fwrite select parameter improved in #4177 (N rows and 5 columns, select a,b,c)" = atime::atime_test(
+      setup = {
+        set.seed(1L)
+        DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))
+        temp_file = tempfile()
+      },
+      expr = {
+        has_select = "select" %chin% names(formals(data.table::fwrite))
+        if (has_select) {
+          data.table::fwrite(DT, temp_file, select = c("a","b","c"))
+        } else {
+          data.table::fwrite(DT[, c("a","b","c"), with = FALSE], temp_file)
+        }
+      },
+      Slow = "66cb6d2393cef30083b444346a7600a079207806",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+      Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85",   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+
+    # Wide case: 1 row, N columns; select a single column
+    "fwrite select parameter improved in #4177 (N columns and 1 row, select 1)" = atime::atime_test(
+      setup = {
+        DT = data.table(t(1:N))
+        temp_file = tempfile()
+        select_idx = 1L
+      },
+      expr = {
+        has_select = "select" %chin% names(formals(data.table::fwrite))
+        if (has_select) {
+          data.table::fwrite(DT, temp_file, select = select_idx)
+        } else {
+          data.table::fwrite(DT[, select_idx, with = FALSE], temp_file)
+        }
+      },
+      Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+      Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85",     # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 
     tests=extra.test.list)
 # nolint end: undesirable_operator_linter.

From ea91282210ba9dc09b71a7c494f32626fb216ae3 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 22:56:18 +0530
Subject: [PATCH 23/29] use only is.numeric(x)

---
 R/fwrite.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index cb3e724ab4..ee04dfe7ca 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -43,7 +43,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!is.null(select)) {
-    cols = if (is.integer(select) || is.numeric(select)) {  # numeric/integer avoids O(#cols) name-match overhead
+    cols = if (is.numeric(x)) {  # numeric/integer avoids O(#cols) name-match overhead
         as.integer(select)
     } else {
         colnamesInt(x, select)

From 28705de3b7d1f2fa1f611ce8d5fe8a6c612ecf87 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Fri, 15 Aug 2025 23:00:45 +0530
Subject: [PATCH 24/29] better manual docs


From 94a9b596c038d411ecf021805dc4827a3997c0fb Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Wed, 20 Aug 2025 18:01:26 +0530
Subject: [PATCH 25/29] closed parenthisis

---
 .ci/atime/tests.R | 70 +++++++++++++++++++++++------------------------
 1 file changed, 35 insertions(+), 35 deletions(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index f96619fb18..322d8ff68f 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -286,42 +286,42 @@ test.list <- atime::atime_test_list(
     Slow = "548410d23dd74b625e8ea9aeb1a5d2e9dddd2927",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/548410d23dd74b625e8ea9aeb1a5d2e9dddd2927)
     Fast = "c0b32a60466bed0e63420ec105bc75c34590865e"),  # Commit in the PR (https://github.com/Rdatatable/data.table/pull/7144/commits) that uses a much faster implementation
   
-    # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
-    # Fixed in https://github.com/Rdatatable/data.table/pull/7236
-   "fwrite select parameter improved in #4177 (N rows and 5 columns, select a,b,c)" = atime::atime_test(
-      setup = {
-        set.seed(1L)
-        DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))
-        temp_file = tempfile()
-      },
-      expr = {
-        has_select = "select" %chin% names(formals(data.table::fwrite))
-        if (has_select) {
-          data.table::fwrite(DT, temp_file, select = c("a","b","c"))
-        } else {
-          data.table::fwrite(DT[, c("a","b","c"), with = FALSE], temp_file)
-        }
-      },
-      Slow = "66cb6d2393cef30083b444346a7600a079207806",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
-      Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85",   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+  # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
+  # Fixed in https://github.com/Rdatatable/data.table/pull/7236
+  "fwrite select parameter improved in #4177 (N rows and 5 columns, select a,b,c)" = atime::atime_test(
+    setup = {
+      set.seed(1L)
+      DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))
+      temp_file = tempfile()
+    },
+    expr = {
+      has_select = "select" %chin% names(formals(data.table::fwrite))
+      if (has_select) {
+        data.table::fwrite(DT, temp_file, select = c("a","b","c"))
+      } else {
+        data.table::fwrite(DT[, c("a","b","c"), with = FALSE], temp_file)
+      }
+    },
+    Slow = "66cb6d2393cef30083b444346a7600a079207806",   # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 
-    # Wide case: 1 row, N columns; select a single column
-    "fwrite select parameter improved in #4177 (N columns and 1 row, select 1)" = atime::atime_test(
-      setup = {
-        DT = data.table(t(1:N))
-        temp_file = tempfile()
-        select_idx = 1L
-      },
-      expr = {
-        has_select = "select" %chin% names(formals(data.table::fwrite))
-        if (has_select) {
-          data.table::fwrite(DT, temp_file, select = select_idx)
-        } else {
-          data.table::fwrite(DT[, select_idx, with = FALSE], temp_file)
-        }
-      },
-      Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
-      Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85",     # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
+  # Wide case: 1 row, N columns; select a single column
+  "fwrite select parameter improved in #4177 (N columns and 1 row, select 1)" = atime::atime_test(
+    setup = {
+      DT = data.table(t(1:N))
+      temp_file = tempfile()
+      select_idx = 1L
+    },
+    expr = {
+      has_select = "select" %chin% names(formals(data.table::fwrite))
+      if (has_select) {
+        data.table::fwrite(DT, temp_file, select = select_idx)
+      } else {
+        data.table::fwrite(DT[, select_idx, with = FALSE], temp_file)
+      }
+    },
+    Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)
+    Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),     # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 
     tests=extra.test.list)
 # nolint end: undesirable_operator_linter.

From 377620ee8ab80699786a8bf36aa4367fe0d3ac9b Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Wed, 20 Aug 2025 21:51:12 +0530
Subject: [PATCH 26/29] improved test stmt

Co-authored-by: Toby Dylan Hocking <tdhock5@gmail.com>
---
 .ci/atime/tests.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index 322d8ff68f..bf34f7a1b7 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -288,7 +288,7 @@ test.list <- atime::atime_test_list(
   
   # Test case adapted from https://github.com/Rdatatable/data.table/issues/4177 which is where the issue was reported.
   # Fixed in https://github.com/Rdatatable/data.table/pull/7236
-  "fwrite select parameter improved in #4177 (N rows and 5 columns, select a,b,c)" = atime::atime_test(
+  "fwrite(select) #4177 Nx5" = atime::atime_test(
     setup = {
       set.seed(1L)
       DT = data.table(a=rnorm(N), b=rnorm(N), c=rnorm(N), d=rnorm(N), e=rnorm(N))

From d8348118dcd425c844b017f22c75e4480dd4bef9 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Wed, 20 Aug 2025 21:51:31 +0530
Subject: [PATCH 27/29] imprvd tst stmt

Co-authored-by: Toby Dylan Hocking <tdhock5@gmail.com>
---
 .ci/atime/tests.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index bf34f7a1b7..81f52d12a7 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -306,7 +306,7 @@ test.list <- atime::atime_test_list(
     Fast = "1887699fe965b5aa1fb8cb16b5507b7a5cbf5c85"),   # Commit in the PR (https://github.com/Rdatatable/data.table/pull/4177/commits) that adds select parameter
 
   # Wide case: 1 row, N columns; select a single column
-  "fwrite select parameter improved in #4177 (N columns and 1 row, select 1)" = atime::atime_test(
+  "fwrite(select) #4177 1xN" = atime::atime_test(
     setup = {
       DT = data.table(t(1:N))
       temp_file = tempfile()

From 08f6fee012ce03322497e4292e63a7ec0354cfb6 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Sat, 23 Aug 2025 03:02:14 +0530
Subject: [PATCH 28/29] use select parm instead of x in is.numeric

---
 R/fwrite.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/R/fwrite.R b/R/fwrite.R
index ee04dfe7ca..6c25d74b1f 100644
--- a/R/fwrite.R
+++ b/R/fwrite.R
@@ -43,7 +43,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
   }
   # Handle select argument using .shallow()
   if (!is.null(select)) {
-    cols = if (is.numeric(x)) {  # numeric/integer avoids O(#cols) name-match overhead
+    cols = if (is.numeric(select)) {  # numeric/integer avoids O(#cols) name-match overhead
         as.integer(select)
     } else {
         colnamesInt(x, select)

From b072be59d73179b4ec8c4052fefa8f0e44a51709 Mon Sep 17 00:00:00 2001
From: Mukul <145585624+Mukulyadav2004@users.noreply.github.com>
Date: Sat, 23 Aug 2025 21:45:48 +0530
Subject: [PATCH 29/29]  fwrite call to use data.table subsetting

---
 .ci/atime/tests.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.ci/atime/tests.R b/.ci/atime/tests.R
index 81f52d12a7..e52369b665 100644
--- a/.ci/atime/tests.R
+++ b/.ci/atime/tests.R
@@ -317,7 +317,7 @@ test.list <- atime::atime_test_list(
       if (has_select) {
         data.table::fwrite(DT, temp_file, select = select_idx)
       } else {
-        data.table::fwrite(DT[, select_idx, with = FALSE], temp_file)
+        data.table::fwrite(data.table:::`[.data.table`(DT, , select_idx, with = FALSE), temp_file)
       }
     },
     Slow = "66cb6d2393cef30083b444346a7600a079207806",     # Parent of the first commit in the PR (https://github.com/Rdatatable/data.table/commit/66cb6d2393cef30083b444346a7600a079207806)