print(col.names="none") more robust (#4271)

NEWS.md

@@ -121,6 +121,8 @@
 
 14. `by=...get()...` could fail with `object not found`, [#4873](https://github.com/Rdatatable/data.table/issues/4873) [#4981](https://github.com/Rdatatable/data.table/issues/4981). Thanks to @sindribaldur for reporting, and @OfekShilon for fixing.
 
+15. `print(x, col.names='none')` now removes the column names as intended for wide `data.table`s whose column names don't fit on a single line, [#4270](https://github.com/Rdatatable/data.table/issues/4270). Thanks to @tdhock for the report, and Michael Chirico for fixing.
+
 ## NOTES
 
 1. New feature 29 in v1.12.4 (Oct 2019) introduced zero-copy coercion. Our thinking is that requiring you to get the type right in the case of `0` (type double) vs `0L` (type integer) is too inconvenient for you the user. So such coercions happen in `data.table` automatically without warning. Thanks to zero-copy coercion there is no speed penalty, even when calling `set()` many times in a loop, so there's no speed penalty to warn you about either. However, we believe that assigning a character value such as `"2"` into an integer column is more likely to be a user mistake that you would like to be warned about. The type difference (character vs integer) may be the only clue that you have selected the wrong column, or typed the wrong variable to be assigned to that column. For this reason we view character to numeric-like coercion differently and will warn about it. If it is correct, then the warning is intended to nudge you to wrap the RHS with `as.<type>()` so that it is clear to readers of your code that a coercion from character to that type is intended. For example :

R/print.data.table.R

@@ -114,7 +114,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     toprint = rbind(head(toprint, topn + isTRUE(class)), "---"="", tail(toprint, topn))
     rownames(toprint) = format(rownames(toprint), justify="right")
     if (col.names == "none") {
-      cut_top(print(toprint, right=TRUE, quote=quote))
+      cut_colnames(print(toprint, right=TRUE, quote=quote))
     } else {
       print(toprint, right=TRUE, quote=quote)
     }
@@ -129,7 +129,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     #   option to shut this off per request of Oleg Bondar on SO, #1482
     toprint=rbind(toprint, matrix(if (quote) old else colnames(toprint), nrow=1L)) # fixes bug #97
   if (col.names == "none") {
-    cut_top(print(toprint, right=TRUE, quote=quote))
+    cut_colnames(print(toprint, right=TRUE, quote=quote))
   } else {
     print(toprint, right=TRUE, quote=quote)
   }
@@ -192,7 +192,8 @@ shouldPrint = function(x) {
 
 # for removing the head (column names) of matrix output entirely,
 #   as opposed to printing a blank line, for excluding col.names per PR #1483
-cut_top = function(x) writeLines(capture.output(x)[-1L])
+# be sure to remove colnames from any row where they exist, #4270
+cut_colnames = function(x) writeLines(grep("^\\s*(?:[0-9]+:|---)", capture.output(x), value=TRUE))
 
 # for printing the dims for list columns #3671; used by format.data.table()
 paste_dims = function(x) {

inst/tests/tests.Rraw

@@ -17596,3 +17596,7 @@ DT = data.table(a=1L)
 test(2186, DT[, if (TRUE) .(a=1L) else .(a=1L, b=2L)], DT,
      warning='j may not evaluate to the same number of columns for each group')
 
+# col.names='none' should apply when wrapping too, #4270
+DT = setDT(replicate(getOption('width'), 1, simplify = FALSE))
+test(2187, {print(DT, col.names='none'); TRUE}, notOutput="V")
+