Updated warning for referencing a non-existent value during creation of new column (#6016)

* Updated warning message in assign.c, as well as updated tests 316, 944.1 and 944.3

* added spaces for consistency

* Update src/assign.c warning

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

* updated tests

---------

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

Author: joshhwuu

inst/tests/tests.Rraw

@@ -1012,7 +1012,7 @@ test(313, DT[,a:=1:3], data.table(a=1:3))    # test changed in 1.12.2; can now a
 DT = data.table(a=20:22)
 test(314, {DT[,b:=23:25];DT[,c:=26:28]}, data.table(a=20:22,b=23:25,c=26:28))   # add in series
 test(315, DT[,c:=NULL], data.table(a=20:22,b=23:25))   # delete last
-test(316, DT[,c:=NULL], data.table(a=20:22,b=23:25), warning="Column 'c' does not exist to remove")
+test(316, DT[,c:=NULL], data.table(a=20:22,b=23:25), warning="Tried to assign NULL to column 'c', but this column does not exist to remove")
 
 # Test adding, removing and updating columns via [<- in one step
 DT = data.table(a=1:6,b=1:6,c=1:6)
@@ -2809,9 +2809,9 @@ test(943, merge(X,Y,all.y=TRUE,by="a"), data.table(a=2:4,b=INT(5:6,NA),"d 1"=5:7
 
 # Test error message about NULL type
 DT = data.table(NULL)
-test(944.1, DT[, foo:=NULL], DT, warning="Column 'foo' does not exist to remove")
+test(944.1, DT[, foo:=NULL], DT, warning="Tried to assign NULL to column 'foo', but this column does not exist to remove")
 test(944.2, DT[,a:=1L], data.table(a=1L))  # can now add columns to an empty data.table from v1.12.2
-test(944.3, DT[,aa:=NULL], data.table(a=1L), warning="Column 'aa' does not exist to remove")
+test(944.3, DT[,aa:=NULL], data.table(a=1L), warning="Tried to assign NULL to column 'aa', but this column does not exist to remove")
 test(944.4, DT[,a:=NULL], data.table(NULL))
 if (base::getRversion() >= "3.4.0") {
   test(944.5, typeof(structure(NULL, class=c("data.table","data.frame"))), 'list', warning="deprecated, as NULL cannot have attributes")  # R warns which is good and we like

src/assign.c

@@ -430,7 +430,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
       if (newcolnum<0 || newcolnum>=length(newcolnames))
         error(_("Internal error in assign.c: length(newcolnames)=%d, length(names)=%d, coln=%d"), length(newcolnames), length(names), coln); // # nocov
       if (isNull(thisvalue)) {
-        warning(_("Column '%s' does not exist to remove"),CHAR(STRING_ELT(newcolnames,newcolnum)));
+        warning(_("Tried to assign NULL to column '%s', but this column does not exist to remove"), CHAR(STRING_ELT(newcolnames,newcolnum)));
         continue;
       }
       // RHS of assignment to new column is zero length but we'll use its type to create all-NA column of that type