Set the initial type of a vector in Matlab

Question

I'd like to declare an empty vector that accepts insertion of user-defined types. In the following examples node is a type I've defined with classdef node ...

The following code is rejected by the Matlab interpreter because the empty vector is automatically initialized as type double, so it can't have a node inserted into it.

>> a = [];
>> a(1) = node(1,1,1);
The following error occurred converting from node to double:
Conversion to double from node is not possible.

The code below is accepted because the vector is initialized with a node in it, so it can later have nodes inserted.

>> a = [node(1,1,1)];
>> a(1) = node(1,2,1);

However, I want to create an empty vector that can have nodes inserted into it. I can do it awkwardly like this:

>> a = [node(1,1,1)];
>> a(1) = [];

What's a better way? I'm looking for something that declares the initial type of the empty vector to be node. If I could make up the syntax, it would look like:

>> a = node[];

But that's not valid Matlab syntax. Is there a good way to do this?

Answer 1

You don't specify what your class contains, but yes, generally speaking it is possible to use array creation functions such as zeros, ones, and others for user-defined classes as well.

For in-built classes, you might have a call like

A = zeros(2,3,'uint8');

to create a 2-by-3 matrix of zeros of datatype uint8. The similar syntax can also be applied for appropriate types of user-defined classes, for instance:

A = zeros(2,3,'MyClass');

where 'MyClass' is the name of your class, or by giving an example:

p = MyClass(...);
A = zeros(2,3,'like',p);

The source for this information, along with a specification of how to implement support for array creation funtions in user-defined classes may be found here.

A call such as zeros(0,0,'MyClass') would then produce an empty vector of type MyClass.

Answer 2

Empty object can be created by

A = MyClass.empty;

It works with your own class, but also with Matlab's class such as

A = int16.empty;

This method is able to create multi-dimensional empty objects with this syntax

A = MyClass.empty(n,m,0,p,q);

as long as one dimension is set to zero.

See the doc.