Adding a Null element to a List<>

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Permalink Reply by peter fotiadis on November 29, 2014 at 12:13am

Use nullable type(s) (see the ? used in List<Vector3d?> )

Attachments:

• nullable_vector_list.gh, 16 KB

Permalink Reply by David Rutten on November 29, 2014 at 2:13am

Or you can use Vector3d.Unset as a placeholder indicating an invalid value.

Or you can directly output a list of GH_Vector, which is a class and thus supports null values.

Permalink Reply by Garcia del Castillo on December 3, 2014 at 11:38am

Awesome, thanks to both.

Permalink Reply by peter fotiadis on December 3, 2014 at 1:01pm

And here's the trad update (more complexity, more bugs, one division by zero == progress)

have nullable fun

Attachments:

• nullable_vector_list_tree.gh, 303 KB

Permalink Reply by Garcia del Castillo on December 3, 2014 at 3:17pm

Wow Peter, thank you for such a comprehensive response.

Permalink Reply by Garcia del Castillo on December 3, 2014 at 7:12pm

For the sake of completeness and indexability, I would like to post here some valuable comments in Peter's definition:

VB.Net’s Nothing keyword is is not the same as C#’s null. MSDN states, “Assigning Nothing to a variable sets it to the default value for its declared type. If that type contains variable members, they are all set to their default value”.

In other words, the Nothing keyword is actually equivalent to C#’s default(T) keyword, where T is the type that the expression is used as.

This can lead to nasty surprises with nullable types in conditional operators.

In C#, the expression (...) ? null : 1 will not compile, since “there is no implicit conversion between '<null>' and 'int'”.  Since  null is an untyped expression, the type of the conditional is inferred to be int, resulting in an error because null cannot be converted to int.

In VB.Net, by contrast, the equivalent expression, If((...), Nothing, 1), will compile, but will have unexpected results.Here too,  Nothing is an untyped expression, so the type of the conditional is inferred to be Integer.  However, unlike null, Nothing can be converted to Integer, so this is compiled as If((...), 0, 1),  which is probably not what the programmer intended.

In both languages, the solution is to use an expression which is actually typed as int?, by writing new int?(), (in C#) or New Integer?() (in VB.Net).

Thanks a lot again Peter, your example was super helpful.