Fix truncation of long column names in print.data.table()

NEWS.md

@@ -60,6 +60,8 @@
 
 13. `rbindlist()` (and therefore the `rbind()` method for `data.table`s) no longer raises an error upon encountering more than approximately 50000 columns in a list entry, [#7793](https://github.com/Rdatatable/data.table/issues/7793). The bug was introduced in `data.table` version 1.18.2.1. Thanks to @rickhelmus for the report and @aitap for the fix.
 
+14. `print.data.table()` now also truncates long column names and improves the "variables not shown" footer to stay within the console width, [#7797](https://github.com/Rdatatable/data.table/issues/7797). This ensures that visible headers, empty table summaries, and hidden column lists respect `options("width")`. Thanks @tdhock for the report and @venom1204 for the fix.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).

R/print.data.table.R

@@ -58,7 +58,12 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
       catf("Null %s (0 rows and 0 cols)\n", x_class)  # See FAQ 2.5 and NEWS item in v1.8.9
     } else {
       catf("Empty %s (%d rows and %d cols)", x_class, NROW(x), NCOL(x))
-      if (length(x)>0L) cat(": ",paste(head(names(x),6L),collapse=","),if(length(x)>6L)"...",sep="") # notranslate
+      if (length(x)>0L) {
+        # Minimal truncation for empty table summary line
+        e_trunc = getOption("datatable.prettyprint.char") %||% (getOption("width") - 5L)# 5L accounts for "Empty data.table..." prefix;
+        t_names = char.trunc(head(names(x), 6L), trunc.char = e_trunc)#6L is the max number of names shown in summary
+        cat(": ", paste(t_names, collapse=","), if(length(x)>6L)"...", sep="") # notranslate
+      }
       cat("\n") # notranslate
     }
     return(invisible(x))
@@ -74,7 +79,6 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
   }
   n_x = nrow(x)
   if ((topn*2L+1L)<n_x && (n_x>nrows || !topnmiss)) {
-    toprint = rbindlist(list(head(x, topn), tail(x, topn)), use.names=FALSE)  # no need to match names because head and tail of same x, and #3306
     rn = c(seq_len(topn), seq.int(to=n_x, length.out=topn))
     printdots = TRUE
     idx = c(seq_len(topn), seq(to=nrow(x), length.out=topn))
@@ -88,12 +92,14 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
   }
   require_bit64_if_needed(x)
   classes = classes1(toprint)
-  trunc.char = getOption("datatable.prettyprint.char")
-  if (is.null(trunc.char)) {
-    rn_w = if (isTRUE(row.names)) nchar(as.character(max(rn))) + 2L else 0L
-    trunc.char = max(0L, getOption("width") - rn_w - 3L)
-  }
+
+  rn_w = if (isTRUE(row.names)) nchar(as.character(max(rn))) + 2L else 0L# 2L accounts for the ": " added to row names (e.g., "10: "); 0L used when row.names=FALSE
+  width_limit = max(0L, getOption("width") - rn_w - 3L)# 3L accounts for separators and internal padding between columns
+  trunc.char = getOption("datatable.prettyprint.char") %||% width_limit
+  h_opt = getOption("datatable.prettyprint.char")
+  h_trunc = if (!is.null(h_opt) && is.infinite(h_opt)) Inf else width_limit
   toprint=format.data.table(toprint, na.encode=FALSE, timezone = timezone, trunc.char = trunc.char, ...)  # na.encode=FALSE so that NA in character cols print as <NA>
+  if (col.names != "none") colnames(toprint) = char.trunc(colnames(toprint), trunc.char = h_trunc)
 
   # FR #353 - add row.names = logical argument to print.data.table
   if (isTRUE(row.names)) rownames(toprint)=paste0(format(rn,right=TRUE,scientific=FALSE),":") else rownames(toprint)=rep.int("", nrow(toprint))
@@ -122,7 +128,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     cols_to_print = widths < cons_width
     not_printed = colnames(toprint)[!cols_to_print]
     if (!any(cols_to_print)) {
-      trunc_cols_message(not_printed, abbs, class, col.names)
+      trunc_cols_message(not_printed, abbs, class, col.names, trunc.char = h_trunc)
       return(invisible(x))
     }
     # When nrow(toprint) = 1, attributes get lost in the subset,
@@ -134,7 +140,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     if (col.names != "none") cut_colnames = identity
     cut_colnames(print(x, right=TRUE, quote=quote, na.print=na.print))
     # prints names of variables not shown in the print
-    if (trunc.cols) trunc_cols_message(not_printed, abbs, class, col.names)
+    if (trunc.cols) trunc_cols_message(not_printed, abbs, class, col.names, trunc.char = h_trunc)
   }
   if (printdots) {
     if (isFALSE(row.names)) {
@@ -253,7 +259,7 @@ format_list_item.data.frame = function(x, ...) {
 # Current implementation may have issues when dealing with strings that have combinations of full-width and half-width characters,
 # if this becomes a problem in the future, we could consider string traversal instead.
 char.trunc = function(x, trunc.char = getOption("datatable.prettyprint.char")) {
-  if (is.null(trunc.char)) return(x)
+  if (is.null(trunc.char) || is.infinite(trunc.char[1L])) return(x)
   trunc.char = max(0L, suppressWarnings(as.integer(trunc.char[1L])), na.rm=TRUE)
   if (!is.character(x) || trunc.char <= 0L) return(x)
   nchar_width = nchar(x, 'width', allowNA = TRUE)
@@ -291,12 +297,12 @@ toprint_subset = function(x, cols_to_print) {
   }
 }
 # message for when trunc.cols=TRUE and some columns are not printed
-trunc_cols_message = function(not_printed, abbs, class, col.names){
+trunc_cols_message = function(not_printed, abbs, class, col.names, trunc.char = getOption("datatable.prettyprint.char")){
   n = length(not_printed)
   if (class && col.names != "none") classes = paste0(" ", tail(abbs, n)) else classes = ""
   catf(
     ngettext(n, "%d variable not shown: %s\n", "%d variables not shown: %s\n"),
-    n, brackify(paste0(not_printed, classes)),
+    n, brackify(paste0(char.trunc(not_printed, trunc.char = trunc.char), classes)),
     domain=NA
   )
 }

inst/tests/tests.Rraw

@@ -21775,3 +21775,12 @@ test(2377.44, copy(dt)[!(a < 5 & b != "d"), .ROW := NULL], dt[1:3])
 dt = data.table(a=1:3)
 test(2377.91, truelength(dt$a), 0L)
 test(2377.92, {setallocrow(dt); truelength(dt$a)}, 3L)
+
+#7797 long column names go past width
+test(2378.1, print(data.table(abcdefghijklmnopqrstuvwxyz=1)), output="abcdefghijklmn...", options=list(width=20))
+test(2378.2, print(data.table(abcdefghijklmnopqrstuvwxyz=1)), output="1 variable not shown: \\[abcdefghijklmn... <num>\\]", options=list(width=20, datatable.print.trunc.cols=TRUE, datatable.print.class=TRUE))
+test(2378.3, print(data.table(abcdefghijklmnopqrst=1)), output="abcdefghijklmnopqrst", options=list(width=30))
+test(2378.4, print(data.table(abcdefghijklmnopqrstuvwxyz=1, zyxwvutsrqponmlkjihgfedcba=2)), output="abcdefghijklmnopqrstuvwxyz.*1 variable not shown: \\[zyxwvutsrqponmlkjihgfedcba <num>\\]", options=list(width=35, datatable.print.trunc.cols=TRUE, datatable.print.class=TRUE))
+test(2378.5, print(data.table(abcdefghijklmnopqrstuvwxyz=1)), output="1 variable not shown: \\[abcdefghijklmnopqrstuvwxyz <num>\\]", options=list(width=20, datatable.print.trunc.cols=TRUE, datatable.print.class=TRUE, datatable.prettyprint.char=Inf))
+test(2378.6, print(data.table(very_long_name_a=1, s_b=2, very_long_name_c=3)), output="very_long_name_a[ ]+s_b.*1 variable not shown: \\[very_long_name_c <num>\\]", options=list(width=35, datatable.print.trunc.cols=TRUE, datatable.print.class=TRUE))
+test(2378.7, print(data.table(extremely_long_a=1, s_b=2, extremely_long_c=3, s_d=4)), output="extremely_long_a[ ]+s_b.*2 variables not shown: \\[extremely_long_c <num>, s_d <num>\\]", options=list(width=30, datatable.print.trunc.cols=TRUE, datatable.print.class=TRUE))