Can I use an enumeration as an index for a vector or MATLAB map?

Question

I am looking to use an enumeration to access elements of an array or dictionary, but no luck.

Enum:

classdef Enumeration1 < uint32 enumeration Left (1); Right (2); Neither (3); end end 

Using:

 directions(Enumeration1.Left) = 7; 

For me it should be the same as

 directions(1) = 7; 

but I get "Index indices must be either natural integers or booleans."

alternatively, when I use the containers.Map object, all the examples that I see have keys as strings. When I use the enumeration, I get "The type of the specified key does not match the type expected for this container." I can see from help containers.Map that uint32 is an acceptable key type.

How can I effectively index objects using a listed value?

Answer 1

If you look at the example provided at http://www.mathworks.com/help/matlab/matlab_oop/enumerations.html , you will see that the value of Enumeration1.Left not a value of 1 , but an object instead. You can confirm this by examining the returned object:

 a = Enumeration1.Left; whos a display(a) 

This shows that a is an object of class Enumeration1 with a size of 108 bytes and a value of Left . Converting Left to 1 is done using

 b = uint32(a); 

So the following should work:

 directions(uint32(Enumeration1.Left)) = 7; 

Interestingly, when I use Matlab 2012a, I can use the syntax you have above, and Matlab does not complain.

Answer 2

In general, to use objects as indexes , define a subsindex for your class.

Note that against everything else in MATLAB, subsindex should return indexes with a null value.

 classdef E < uint32 enumeration Left (1); Right (2); Neither (3); end methods function ind = subsindex(obj) ind = uint32(obj) - 1; end end end 

Example:

 >> x = 1:10; >> x(E.Right) ans = 2 

Note that even without defining a subsindex method subsindex classes that inherit from the built-in type should work as indexes as usual (at least it worked that way in my version of R2013a).

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If you want to work with container.Map, you must explicitly list the enum as uint32 . I think the containers.Map/subsref method does not use isa to check the type of indexing, instead use something like strcmp(class(obj),'..') , which explains the error message:

 Error using containers.Map/subsref Specified key type does not match the type expected for this container.