Merge pull request #797 from stan-dev/empty-array-clarification

clarify empty array expression

Author: bob-carpenter

src/reference-manual/expressions.qmd

@@ -301,6 +301,19 @@ The empty expression `[ ]` is ambiguous and therefore is not
 allowed and similarly expressions such as `[ [ ] ]` or
 `[ [ ], [ ] ]` are not allowed.
 
+### Empty vectors and matrices {-}
+
+If needed, it is possible to create an empty vector with
+```stan
+rep_vector(e, 0)
+```
+where the first expression `e` needs to scalar of type `real`.
+
+If needed, it is possible to create an empty matrix with
+```stan
+rep_matrix(e, 0, 0)
+```
+where the first expression `e` needs to scalar of type `real`.
 
 ### Array expressions {-}
 
@@ -334,6 +347,21 @@ array[2, 3] int b = { { 1, 2, 3 },
                 { 4, 5, 6 } };
 ```
 
+### Empty arrays {-}
+
+The empty array expression (`{ }`) is not allowed. See more about restrictions on array expressions in subsection [Restrictions on values](#array-expressions).
+
+If needed, it is possible to create an empty array with
+```stan
+rep_array(e, 0)
+```
+where the first expression `e` determines the type of the array. For
+example, `rep_array(0.0, 0)` returns an empty real array of type
+`real[]`, whereas `rep_array({123}, 0)` returns an empty two
+dimensional integer array of type `int[ , ]`. Only the type of the
+first argument is used, so the integer arrays `{123}` and `{0}`
+produce equivalent values.
+
 
 ### Array expression types {-}