Add list column to null data.table (#5739)

* add fix

* update tests

* add more tests

* switch back to cols

* Update NEWS.md

Co-authored-by: Michael Chirico <[email protected]>

* Update src/assign.c

Co-authored-by: Michael Chirico <[email protected]>

* standardize tests

---------

Co-authored-by: Michael Chirico <[email protected]>

NEWS.md

@@ -60,6 +60,8 @@
 
 7. `melt` returns an integer column for `variable` when `measure.vars` is a list of length=1, consistent with the documented behavior, [#5209](https://github.com/Rdatatable/data.table/issues/5209). Thanks to @tdhock for reporting and fixing. Any users who were relying on this behavior can change `measure.vars=list("col_name")` (output `variable` was column name, now is column index/integer) to `measure.vars="col_name"` (`variable` still is column name).
 
+8. Adding a list column to an empty `data.table` works consistently with other column types, [#5738](https://github.com/Rdatatable/data.table/issues/5738). Thanks to Benjamin Schwendinger for the report and the fix.
+
 ## NOTES
 
 1. `transform` method for data.table sped up substantially when creating new columns on large tables. Thanks to @OfekShilon for the report and PR. The implemented solution was proposed by @ColeMiller1.

inst/tests/tests.Rraw

@@ -18569,3 +18569,17 @@ test(2261.04, setNumericRounding(2L), 1L)
 # or not an object is an invisible copy or not, and prints it anyways.
 test(2261.05, capture.output(setNumericRounding(2L)), character(0))
 setNumericRounding(old)
+
+# Add list column to null data.table #5738
+x = list("a", 1)
+dt1 = data.table(a = x)
+dt2 = data.table(a = "a", b=1)
+dt3 = data.table(a=1:2, b=3:4)
+test(2262.1, null.data.table()[, a := x], dt1)
+test(2262.2, set(null.data.table(), j="a", value=x), dt1)
+test(2262.3, null.data.table()[, c("a","b") := x], dt2)
+test(2262.4, set(null.data.table(), j=c("a","b"), value=x), dt2)
+test(2262.5, null.data.table()[, c("a","b") := list(1:2, 3:4)], dt3)
+test(2262.6, set(null.data.table(), j=c("a","b"), value=list(1:2, 3:4)), dt3)
+test(2262.7, data.table(a=1, b=2)[, c("a", "b") := list(NULL, NULL)], null.data.table())
+test(2262.8, data.table(a=1, b=2)[, c("a", "b") := list(NULL)], null.data.table())

src/assign.c

@@ -360,8 +360,9 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
     // Not possible to test because R won't permit attributes be attached to NULL (which is good and we like); warning from R 3.4.0+ tested by 944.5
   }
   const int nrow = LENGTH(dt) ? length(VECTOR_ELT(dt,0)) :
-                                (isNewList(values) && length(values) ? length(VECTOR_ELT(values,0)) : length(values));
+                                (isNewList(values) && length(values) && (length(values)==length(cols)) ? length(VECTOR_ELT(values,0)) : length(values));
   //                            ^ when null data.table the new nrow becomes the fist column added
+  //                                                                    ^ distinguish between adding list column or multiple columns at once, #5738
   if (isNull(rows)) {
     numToDo = nrow;
     targetlen = nrow;