Closes #936. Can re-assign/update factor cols by list type.

README.md

@@ -121,6 +121,8 @@
 
   32. `rbindlist` stack imbalance on all `NULL` list elements is now fixed. Closes [#980](https://github.com/Rdatatable/data.table/issues/980). Thanks to @ttuggle.
 
+  33. List columns can be assigned to columns of `factor` type by reference. Closes [#936](https://github.com/Rdatatable/data.table/issues/936). Thanks to @richierocks for the minimal example.
+
 #### NOTES
 
   1. Clearer explanation of what `duplicated()` does (borrowed from base). Thanks to @matthieugomez for pointing out. Closes [#872](https://github.com/Rdatatable/data.table/issues/872).

inst/tests/tests.Rraw

@@ -5908,6 +5908,10 @@ test(1478.2, sapply(ll, length), INT(1,2,0,0))
 test(1479, rbindlist(replicate(4,rbindlist(replicate(47, NULL), 
       use.names=TRUE, fill=TRUE)), use.names=TRUE, fill=TRUE), null.data.table())
 
+# #936, assigning list column to a factor column by reference
+DT <- data.table(x = factor(c("a", "b c", "d e f")))
+test(1480, DT[, x := strsplit(as.character(x), " ")], data.table(x=list("a", letters[2:3], letters[4:6])))
+
 ##########################
 
 

src/assign.c

@@ -377,7 +377,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
         if (isMatrix(thisvalue) && (j=INTEGER(getAttrib(thisvalue, R_DimSymbol))[1]) > 1)  // matrix passes above (considered atomic vector)
             warning("%d column matrix RHS of := will be treated as one vector", j);
         if ((coln+1)<=oldncol && isFactor(VECTOR_ELT(dt,coln)) &&
-            !isString(thisvalue) && TYPEOF(thisvalue)!=INTSXP && !isReal(thisvalue))  // !=INTSXP includes factor
+            !isString(thisvalue) && TYPEOF(thisvalue)!=INTSXP && !isReal(thisvalue) && !isNewList(thisvalue))  // !=INTSXP includes factor
             error("Can't assign to column '%s' (type 'factor') a value of type '%s' (not character, factor, integer or numeric)", CHAR(STRING_ELT(names,coln)),type2char(TYPEOF(thisvalue)));
         if (nrow>0) {
             if (vlen>targetlen)