Add "na.print" as a new argument to "print.data.table"

NEWS.md

@@ -74,6 +74,8 @@
 
 11. Using `print.data.table` when truncation is needed with `row.names = FALSE` prints the indicator `---` in every value column instead of adding a blank column where the `rownames` would have been just to include `---`, [#4083](https://github.com/Rdatatable/data.table/issues/4083). Thanks @MichaelChirico for the report and @joshhwuu for the fix.
 
+12. `print.data.table` now honors `na.print`, as seen in `print.default`, allowing for string replacement of `NA` values when printing. Thanks @HughParsonage for the report and @joshhwuu for the fix.
+
 # data.table [v1.15.0](https://github.com/Rdatatable/data.table/milestone/29)  (30 Jan 2024)
 
 ## BREAKING CHANGE

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"),
                quote=FALSE,
+               na.print=NULL,
                timezone=FALSE, ...) {
   # topn  - print the top topn and bottom topn rows with '---' inbetween (5)
   # nrows - under this the whole (small) table is printed, unless topn is provided (100)
@@ -118,9 +119,9 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     }
     rownames(toprint) = format(rownames(toprint), justify="right")
     if (col.names == "none") {
-      cut_colnames(print(toprint, right=TRUE, quote=quote))
+      cut_colnames(print(toprint, right=TRUE, quote=quote, na.print=na.print))
     } else {
-      print(toprint, right=TRUE, quote=quote)
+      print(toprint, right=TRUE, quote=quote, na.print=na.print)
     }
     if (trunc.cols && length(not_printed) > 0L)
       # prints names of variables not shown in the print
@@ -133,9 +134,9 @@ 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_colnames(print(toprint, right=TRUE, quote=quote))
+    cut_colnames(print(toprint, right=TRUE, quote=quote, na.print=na.print))
   } else {
-    print(toprint, right=TRUE, quote=quote)
+    print(toprint, right=TRUE, quote=quote, na.print=na.print)
   }
   if (trunc.cols && length(not_printed) > 0L)
     # prints names of variables not shown in the print

inst/tests/tests.Rraw

@@ -18454,3 +18454,20 @@ test(2256.6, fread('a;b\n1,14;5', verbose=TRUE), data.table(a=1.14, b=5L), outpu
 # helpful error about deleting during grouping, #1873
 DT = data.table(id = c(1, 1, 2, 2), a = 1:4, b = 5:8)
 test(2257, DT[ , c("c", "a") := .(a + 1, NULL), by=id], error="it's not possible to delete parts of a column")
+
+# testing printing data.tables with na.print, #3152
+DT = data.table(x=c(NA, "a", "b"))
+test(2258.1, capture.output(print(DT, na.print=".")), c("   x", "1: .", "2: a", "3: b"))
+test(2258.2, capture.output(print(DT, na.print="_")), c("   x", "1: _", "2: a", "3: b"))
+test(2258.3, capture.output(print(DT, na.print="NA")), c("    x", "1: NA", "2:  a", "3:  b"))
+test(2258.4, capture.output(print(DT, na.print=TRUE)), error="invalid 'na.print' specification")
+test(2258.5, capture.output(print(DT, na.print=".", quote=TRUE)), c('   "x"', "1:   .", '2: "a"', '3: "b"'))
+test(2258.6, capture.output(print(DT, na.print=".", right=TRUE)), c("   x", "1: .", "2: a", "3: b"))
+# tests for other call sites
+# col.names="none"
+test(2258.7, capture.output(print(DT, na.print=".", col.names="none")), c("1: .", "2: a", "3: b"))
+# printdots=TRUE, col.names="none"
+DT = data.table(x = c(NA, "e", "b", "j", "w", NA))
+test(2258.8, capture.output(print(DT, na.print=".", topn=2, col.names="none")), c(" 1: .", " 2: e", "---  ", " 5: w", " 6: ."))
+# printdots=TRUE, col.names!="none"
+test(2258.9, capture.output(print(DT, na.print=".", topn=2)), c("    x", " 1: .", " 2: e", "---  ", " 5: w", " 6: ."))

man/print.data.table.Rd

@@ -26,6 +26,7 @@
     print.keys=getOption("datatable.print.keys"),       # default: TRUE
     trunc.cols=getOption("datatable.print.trunc.cols"), # default: FALSE
     quote=FALSE,
+    na.print=NULL,
     timezone=FALSE, \dots)
 
   format_col(x, \dots)
@@ -47,6 +48,7 @@
   \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{quote}{ If \code{TRUE}, all output will appear in quotes, as in \code{print.default}. }
   \item{timezone}{ If \code{TRUE}, time columns of class POSIXct or POSIXlt will be printed with their timezones (if attribute is available). }
+  \item{na.print}{ The string to be printed in place of \code{NA} values, as in \code{print.default}. }
   \item{\dots}{ Other arguments ultimately passed to \code{format}. }
 }
 \value{