Fix: Ensure column widths are data-driven when col.names = "none" in print.data.table (#7130)

* added condition

* indentation

* news.md

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

Author: venom1204

NEWS.md

@@ -84,6 +84,8 @@
  
 20. `droplevels()` works on 0-row data.tables, [#7043](https://github.com/Rdatatable/data.table/issues/7043). The result will have factor columns `factor(character())`, consistent with the data.frame method. Thanks @advieser for the report and @MichaelChirico for the fix.
 
+21. `print(..., col.names = 'none')` now correctly adapts column widths to the data content, ignoring the original column names and producing a more compact output, [#6882](https://github.com/Rdatatable/data.table/issues/6882). Thanks to @brooksambrose for the report and @venom1204 for the PR.
+
 ### NOTES
 
 1. Continued work to remove non-API C functions, [#6180](https://github.com/Rdatatable/data.table/issues/6180). Thanks Ivan Krylov for the PRs and for writing a clear and concise guide about the R API: https://aitap.codeberg.page/R-api/.

R/print.data.table.R

@@ -141,6 +141,8 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     print_default(toprint)
     return(invisible(x))
   }
+  if (col.names == "none")
+    colnames(toprint) = rep.int("", ncol(toprint))
   if (nrow(toprint)>20L && col.names == "auto")
     # repeat colnames at the bottom if over 20 rows so you don't have to scroll up to see them
     #   option to shut this off per request of Oleg Bondar on SO, #1482

inst/tests/tests.Rraw

@@ -21365,3 +21365,11 @@ test(2328.1, levels(droplevels(DT)$f), character())
 DT[, i := integer()]
 DT[, f2 := factor()]
 test(2328.2, droplevels(DT), data.table(f=factor(), i=integer(), f2=factor()))
+
+#6882 print() output with col.names="none"
+dt = data.table(short = 1:3, verylongcolumnname = 4:6)
+test(2329.1, print(dt, col.names = "none"), output = "1: 1 4\n2: 2 5\n3: 3 6\n")
+dt = data.table(x = 123456, y = "wide_string")
+test(2329.2, print(dt, col.names = "none"), output = "1: 123456 wide_string\n")
+dt = data.table(a = NA_integer_, b = NaN)
+test(2329.3, print(dt, col.names = "none"), output = "1: NA NaN\n")