Adds optional column count display to `print.data.table()` by venom1204 · Pull Request #7791 · Rdatatable/data.table

venom1204 · 2026-06-16T15:38:59Z

closes #6663.
in this pr I

Added datatable.show.ncols option (default FALSE).
Added show.ncols support .
Added header output for total column count.
Integrated column count with trunc.cols=TRUE output.

hi @tdhock @joshhwuu can you please have a look when you get time.
thanks

codecov · 2026-06-16T15:46:43Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (deb9acc) to head (e697d45).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7791   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          87       87           
  Lines       17140    17156   +16     
=======================================
+ Hits        16944    16960   +16     
  Misses        196      196

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github-actions · 2026-06-16T16:03:56Z

HEAD=issue6663 much slower for transform improved in #5493
HEAD=issue6663 slower P<0.001 for DT[by] max regression fixed in #7480

Generated via commit e697d45

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 44 seconds
Installing different package versions	23 seconds
Running and plotting the test cases	5 minutes and 25 seconds

tdhock

looks good, please add NEWS

tdhock · 2026-06-16T17:33:09Z

    cons_width = getOption("width")
    cols_to_print = widths < cons_width
    not_printed = colnames(toprint)[!cols_to_print]
+


please remove empty lines

ben-schwen · 2026-06-25T09:25:40Z

 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")


We have an options parameter in test, so please use it!

Please only use bracket blocks {} when it adds clarity, here it seems more complicated then using setup step.

Also you can set multiple options at once with options.

ben-schwen

There seems to be a problem when trunc.cols and nrow>20

options(width=40, datatable.show.ncols=TRUE)
DT = data.table(a=1:21,b=1:21,c=1:21,d=1:21,e=1:21,f=1:21,g=1:21,h=1:21)
print(DT, trunc.cols=TRUE, class=TRUE)
# Number of columns: 8, of which 3 are not shown: [f <int>, g <int>, h <int>]
# Error in rbind(toprint, matrix(if (quote) old else colnames(toprint),  : 
#   number of columns of matrices must match (see arg 3)

ben-schwen · 2026-06-25T11:11:02Z

    ))
  }
+  if (show.ncols && !isTRUE(trunc.cols) && !any(dim(x)==0L)) {
+    catf("Number of columns: %d\n", ncol(x))


Maybe use a helper here, similar to trunc_cols_message and dt_width

ben-schwen · 2026-06-25T11:12:04Z

    cols_to_print = widths < cons_width
    not_printed = colnames(toprint)[!cols_to_print]
+    if (show.ncols) {
+      n_not_printed = length(not_printed)


this seems like a near copy of trunc_cols_message, rework it to use it here

ben-schwen · 2026-06-25T11:13:58Z

+        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


trunc.cols is now overloaded with multiple tasks. maybe use a separate boolean for this.

ben-schwen · 2026-06-29T12:31:50Z

    cons_width = getOption("width")
    cols_to_print = widths < cons_width
    not_printed = colnames(toprint)[!cols_to_print]
+    if (show.ncols) {


The column count seems to be wrong when using show.indices=TRUE

DT = data.table(a=1:3, b=1:3, c=1:3, d=1:3, e=1:3) setindex(DT, a) options(width=20, datatable.show.ncols=TRUE) print(DT, trunc.cols=TRUE, show.indices=TRUE) # Index: <a> # Number of columns: 5, of which 4 are not shown: [c <int>, d <int>, e <int>, index:a <int>] # a b # <int> <int> # 1: 1 1 # 2: 2 2 # 3: 3 3

ben-schwen · 2026-06-29T15:12:34Z

    stopf("Valid options for col.names are 'auto', 'top', and 'none'")
  if (length(trunc.cols) != 1L || !is.logical(trunc.cols) || is.na(trunc.cols))
    stopf("Valid options for trunc.cols are TRUE and FALSE")
  stopifnot(isTRUEorFALSE(class))


Suggested change

stopifnot(isTRUEorFALSE(class))

stopifnot(isTRUEorFALSE(show.ncols))

see comment below about show.ncols=NA

ben-schwen · 2026-06-29T15:14:09Z

    cols_to_print = widths < cons_width
    not_printed = colnames(toprint)[!cols_to_print]
+    if (show.ncols) {
+      trunc_cols_message(not_printed, abbs, class, col.names, ncol=ncol(x))


It might be even as simple to exchange ncol(x) with ncol(toprint)

ben-schwen · 2026-06-29T15:18:08Z

+    if (show.ncols) {
+      trunc_cols_message(not_printed, abbs, class, col.names, ncol=ncol(x))
+      show_trunc_message = FALSE
+      }


Suggested change

}

}

wrong indentation

ben-schwen · 2026-06-29T15:19:15Z

      paste0("<", ixs, ">", collapse = ", ")
    ))
  }
+  if (show.ncols && !isTRUE(trunc.cols) && !any(dim(x)==0L)) {


can we move this below the any(dim(x)==0L) branch which comes next and drop the !any(dim(x)==0L) guard?

venom1204 added 4 commits June 15, 2026 21:24

added changes

e7e92b1

..

021d6e2

added test

864954e

added doc

1850b39

venom1204 requested a review from MichaelChirico as a code owner June 16, 2026 15:39

venom1204 added 2 commits June 16, 2026 15:49

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venom1204 requested review from joshhwuu and tdhock June 16, 2026 16:56

tdhock requested changes Jun 16, 2026

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venom1204 added 2 commits June 16, 2026 18:55

removed empty lines

3e58a32

added news

43dddf2

venom1204 requested a review from tdhock June 16, 2026 19:00

ben-schwen reviewed Jun 25, 2026

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venom1204 force-pushed the issue6663 branch from dcaad10 to 43dddf2 Compare June 25, 2026 10:00

venom1204 added 3 commits June 25, 2026 10:01

Merge remote-tracking branch 'origin/master' into issue6663

d638819

updated tests

aa54da6

.

d4d80f1

venom1204 requested a review from ben-schwen June 25, 2026 10:41

ben-schwen reviewed Jun 25, 2026

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venom1204 added 3 commits June 25, 2026 17:53

..

0c69365

added test

6757317

test

e697d45

venom1204 mentioned this pull request Jun 28, 2026

main discussion page for GSOC-2026 (VINIT) #1 venom1204/discussion-GSOC-2026#1

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venom1204 requested a review from ben-schwen June 28, 2026 09:59

ben-schwen reviewed Jun 29, 2026

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	stopifnot(isTRUEorFALSE(class))
	stopifnot(isTRUEorFALSE(show.ncols))

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