Adds optional column count display to print.data.table()

NEWS.md

@@ -32,6 +32,8 @@
 
 6. `yearqtr()` and `yearmon()` now gain an optional format specifier [#7694](https://github.com/Rdatatable/data.table/issues/7694). 'numeric' is the default, which preserves the original behavior, but 'character' formats `yearqtr()` as YYYYQ# (e.g. 2025Q2) and `yearmon()` as YYYYM## (e.g. 2025M02, 2025M10). Thanks to @jan-swissre for the report and @LunaticSage218 for the implementation.
 
+7. `print.data.table()` gains a `show.ncols` argument and `datatable.show.ncols` option to print the number of columns (labeled as `ncol:`) in the header, [#6663](https://github.com/Rdatatable/data.table/issues/6663). When `trunc.cols=TRUE`, this information is merged into the truncation message at the top, and the redundant footer message is suppressed. Thanks to @KyleHaynes for the suggestion and @venom1204 for the implementation.
+
 ### BUG FIXES
 
 1. `fread()` with `skip=0` and `(header=TRUE|FALSE)` no longer skips the first row when it has fewer fields than subsequent rows, [#7463](https://github.com/Rdatatable/data.table/issues/7463). Thanks @emayerhofer for the report and @ben-schwen for the fix.

R/onLoad.R

@@ -90,6 +90,7 @@
     datatable.print.keys=TRUE,          # for print.data.table
     datatable.print.trunc.cols=FALSE,   # for print.data.table
     datatable.show.indices=FALSE,       # for print.data.table
+    datatable.show.ncols=FALSE,         # for print.data.table
     datatable.allow.cartesian=FALSE,    # datatable.<argument name>
     datatable.join.many=TRUE,           # mergelist, [.data.table #4383 #914
     datatable.dfdispatchwarn=TRUE,      # not a function argument

R/print.data.table.R

@@ -8,6 +8,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
                print.keys=getOption("datatable.print.keys"),
                trunc.cols=getOption("datatable.print.trunc.cols"),
                show.indices=getOption("datatable.show.indices"),
+               show.ncols=getOption("datatable.show.ncols", FALSE),
                quote=FALSE,
                na.print=NULL,
                timezone=FALSE, ...) {
@@ -52,6 +53,9 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
       paste0("<", ixs, ">", collapse = ", ")
     ))
   }
+  if (show.ncols && !isTRUE(trunc.cols) && !any(dim(x)==0L)) {
+    catf("Number of columns: %d\n", ncol(x))
+  }
   if (any(dim(x)==0L)) {
     x_class = if (is.data.table(x)) "data.table" else "data.frame"  # a data.frame could be passed to print.data.table() directly, #3363
     if (all(dim(x)==0L)) {
@@ -116,14 +120,26 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     cons_width = getOption("width")
     cols_to_print = widths < cons_width
     not_printed = colnames(toprint)[!cols_to_print]
+    if (show.ncols) {
+      n_not_printed = length(not_printed)
+      if (n_not_printed > 0L) {
+        if (class && col.names != "none") classes = paste0(" ", tail(abbs, n_not_printed)) else classes = ""
+        catf("Number of columns: %d, of which %d %s not shown: %s\n",
+             ncol(x), n_not_printed, ngettext(n_not_printed, "is", "are"),
+             brackify(paste0(not_printed, classes)))
+        trunc.cols = FALSE
+      } else {
+        catf("Number of columns: %d\n", ncol(x))
+      }
+    }
     if (!any(cols_to_print)) {
-      trunc_cols_message(not_printed, abbs, class, col.names)
+      if (!show.ncols) trunc_cols_message(not_printed, abbs, class, col.names)
       return(invisible(x))
     }
     # When nrow(toprint) = 1, attributes get lost in the subset,
     #   function below adds those back when necessary
     toprint = toprint_subset(toprint, cols_to_print)
-    trunc.cols = length(not_printed) > 0L
+    trunc.cols = !show.ncols && length(not_printed) > 0L
   }
   print_default = function(x) {
     if (col.names != "none") cut_colnames = identity

inst/tests/tests.Rraw

@@ -21660,3 +21660,11 @@ test(2374.08, key(DT[, .(a, a)]), NULL)
 test(2374.09, key(subset(DT, select=c(a, a))), NULL)
 DT = data.table(a=1:2, a.1=3:4, val=10:11)
 test(2374.10, key(DT[, .(a.1, sum(val)), keyby=.(a, a)]), NULL)
+
+#6663 Option to print the number of columns
+test(2375.1, {options(datatable.show.ncols=TRUE); DT=as.data.table(iris); capture.output(print(DT))[1L]}, "Number of columns: 5")
+test(2375.2, {options(datatable.show.ncols=TRUE); DT=as.data.table(iris); setkey(DT, Sepal.Length); any(grepl("^Number of columns: 5$", capture.output(print(DT))))}, TRUE)
+test(2375.3, {options(width=20); options(datatable.show.ncols=TRUE); DT=data.table(a=1:3,b=1:3,c=1:3,d=1:3,e=1:3); any(grepl("^5 variables not shown", capture.output(print(DT, trunc.cols=TRUE))))}, FALSE)
+test(2375.4, {options(width=10); options(datatable.show.ncols=TRUE); DT=as.data.table(iris); capture.output(print(DT, trunc.cols=TRUE))[1L]}, "Number of columns: 5, of which 5 are not shown: [Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species]")
+test(2375.5, {options(datatable.show.ncols=FALSE); DT=as.data.table(iris); any(grepl("^Number of columns:", capture.output(print(DT))))}, FALSE)
+test(2375.6, {options(width=10); options(datatable.show.ncols=TRUE); DT=as.data.table(iris); capture.output(print(DT, trunc.cols=TRUE, class=TRUE))[1L]}, "Number of columns: 5, of which 5 are not shown: [Sepal.Length <num>, Sepal.Width <num>, Petal.Length <num>, Petal.Width <num>, Species <fctr>]")

man/data.table-options.Rd

@@ -33,6 +33,7 @@
    \item{\code{datatable.print.keys}}{A logical, default \code{FALSE}. If \code{TRUE}, the table's
      keys are printed above the data.}
    \item{\code{datatable.show.indices}}{A logical, default \code{TRUE}. A synonym for \code{datatable.print.keys} for historical reasons.}
+   \item{\code{datatable.show.ncols}}{A logical, default \code{FALSE}. If \code{TRUE}, the number of columns is printed in the header.}
    \item{\code{datatable.print.trunc.cols}}{A logical, default \code{FALSE}. If \code{TRUE} and a
      table has more columns than fit on the screen, it truncates the middle columns.}
    \item{\code{datatable.prettyprint.char}}{An integer, default \code{100L}. The maximum number of

man/print.data.table.Rd

@@ -25,6 +25,7 @@
     print.keys=getOption("datatable.print.keys"),       # default: TRUE
     trunc.cols=getOption("datatable.print.trunc.cols"), # default: FALSE
     show.indices=getOption("datatable.show.indices"),   # default: FALSE
+    show.ncols=getOption("datatable.show.ncols", FALSE), # default: FALSE
     quote=FALSE,
     na.print=NULL,
     timezone=FALSE, \dots)
@@ -43,6 +44,7 @@
   \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). 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{show.ncols}{ If \code{TRUE}, the number of columns is printed in the header. }
   \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}). }