print.data.table now warns about int64 cols if bit64 isn't loaded.

R/data.table.R

@@ -135,6 +135,10 @@ print.data.table <- function(x,
         printdots = FALSE
     }
     toprint=format.data.table(toprint, ...)
+    # fix for #975.
+    if (any(sapply(x, function(col) "integer64" %in% class(col))) && !"package:bit64" %in% search()) {
+        warning("Some columns have been read as type 'integer64' but package bit64 isn't loaded. Those columns will display as strange looking floating point data. There is no need to reload the data. Just require(bit64) to obtain the integer64 print method and print the data again.")
+    }
     # FR #5020 - add row.names = logical argument to print.data.table
     if (isTRUE(row.names)) rownames(toprint)=paste(format(rn,right=TRUE,scientific=FALSE),":",sep="") else rownames(toprint)=rep.int("", nrow(toprint))
     if (is.null(names(x))) colnames(toprint)=rep("NA", ncol(toprint)) # fixes bug #4934

README.md

@@ -31,6 +31,8 @@
 
   8. `fread()` gains `blank.lines.skip` argument that continues reading by skipping empty lines. Default is `FALSE` for backwards compatibility, [#530](https://github.com/Rdatatable/data.table/issues/530). Thanks to @DirkJonker.
 
+  9. `print.data.table` now warns when `bit64` package isn't loaded but the `data.table` contains `integer64` columns, [#975](https://github.com/Rdatatable/data.table/issues/975). Thanks to @StephenMcInerney.
+
 #### BUG FIXES
 
   1. Now compiles and runs on IBM AIX gcc. Thanks to Vinh Nguyen for investigation and testing, [#1351](https://github.com/Rdatatable/data.table/issues/1351).