document select arg

Author: Toby Dylan Hocking

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.
 
-***
+***