Print column classes at the bottom if class is enabled (#6905)

* add column class at bottom if row > 20

* correct test number

* remove duplicate rbind() call

* correctly place bracket

* data.table style

* remove extra mat_abbs

* Restore and add issue number comments

Author: Mukulyadav2004

NEWS.md

@@ -8,6 +8,8 @@
 
 2. `melt()` now supports using `patterns()` with `id.vars`, [#6867](https://github.com/Rdatatable/data.table/issues/6867). Thanks to Toby Dylan Hocking for the suggestion and PR.
 
+3. `print.data.table()` now shows column classes at the bottom of large tables when `class=TRUE` and `col.names="auto"` (default) for tables with more than 20 rows, [#6902](https://github.com/Rdatatable/data.table/issues/6902). This follows the same behavior as column names at the bottom, making it easier to see column types for large tables without scrolling back to the top. Thanks to @TimTaylor for the suggestion and @Mukulyadav2004 for the PR.
+
 ## BUG FIXES
 
 1. Custom binary operators from the `lubridate` package now work with objects of class `IDate` as with a `Date` subclass, [#6839](https://github.com/Rdatatable/data.table/issues/6839). Thanks @emallickhossain for the report and @aitap for the fix.

R/print.data.table.R

@@ -142,7 +142,11 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
   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
-    toprint = rbind(toprint, matrix(if (quote) old else colnames(toprint), nrow=1L)) # fixes bug #97
+    toprint = rbind(
+      toprint,
+      matrix(if (quote) old else colnames(toprint), nrow=1L), # see #97
+      if (isTRUE(class)) matrix(abbs, nrow=1L) # #6902
+    )
   print_default(toprint)
   invisible(x)
 }

inst/tests/tests.Rraw

@@ -21134,3 +21134,10 @@ test(2312, fwrite(DF, nullfile(), encoding = "UTF-8", nThread = 2L), NULL)
 test(2313,
      melt(data.table(a=numeric(), b=numeric(), c=numeric()), id.vars=c('a', 'b')),
      data.table(a=numeric(), b=numeric(), variable=factor(levels='c'), value=numeric()))
+
+# Testing column footer display with col.names options in print.data.table #6902
+dt = data.table(id = 1:25)
+# Test with class=TRUE shows classes at bottom with default col.names="auto"
+test(2314.1, any(grepl("<int>", tail(capture.output(print(dt, class = TRUE)), 2))), TRUE)
+# Test that class=TRUE with col.names="top" doesn't show classes at bottom
+test(2314.2, !any(grepl("<int>", tail(capture.output(print(dt, class = TRUE, col.names = "top")), 2))), TRUE)

man/print.data.table.Rd

@@ -41,9 +41,9 @@
   \item{x}{ A \code{data.table}. }
   \item{topn}{ The number of rows to be printed from the beginning and end of tables with more than \code{nrows} rows. }
   \item{nrows}{ The number of rows which will be printed before truncation is enforced. }
-  \item{class}{ If \code{TRUE}, the resulting output will include above each column its storage class (or a self-evident abbreviation thereof). }
+  \item{class}{ If \code{TRUE}, the resulting output will include above each column its storage class (or a self-evident abbreviation thereof). When combined with \code{col.names="auto"} and tables >20 rows, classes will also appear at the bottom.}
   \item{row.names}{ If \code{TRUE}, row indices will be printed alongside \code{x}. }
-  \item{col.names}{ One of three flavours for controlling the display of column names in output. \code{"auto"} includes column names above the data, as well as below the table if \code{nrow(x) > 20}. \code{"top"} excludes this lower register when applicable, and \code{"none"} suppresses column names altogether (as well as column classes if \code{class = TRUE}. }
+  \item{col.names}{ One of three flavours for controlling the display of column names in output. \code{"auto"} includes column names above the data, as well as below the table if \code{nrow(x) > 20} (when \code{class=TRUE}, column classes will also appear at the bottom). \code{"top"} excludes this lower register when applicable, and \code{"none"} suppresses column names altogether (as well as column classes if \code{class = TRUE}. }
   \item{print.keys}{ If \code{TRUE}, any \code{\link{key}} and/or \code{\link[=indices]{index}} currently assigned to \code{x} will be printed prior to the preview of the data. }
   \item{trunc.cols}{ If \code{TRUE}, only the columns that can be printed in the console without wrapping the columns to new lines will be printed (similar to \code{tibbles}). }
   \item{show.indices}{ If \code{TRUE}, indices will be printed as columns alongside \code{x}. }