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Objects in Index Expressions

Objects as Indexes

MATLAB® can use objects as indices in indexed expressions. The rules of array indexing apply — indices must be positive integers. Therefore, MATLAB must be able to derive a value from the object that is a positive integer for use in the indexed expression.

Indexed expressions like X(A), where A is an object, cause MATLAB to call the subsindex function. However, if an indexing expression results in a call to an overloaded method from matlab.mixin.indexing.RedefinesParen, matlab.mixin.indexing.RedefinesDot, or matlab.mixin.indexing.RedefinesBrace defined by class X, then MATLAB does not call subsindex.

Ways to Implement Objects as Indices

There are several ways to implement indexing of one object by another object, X(A):

  • Define a subsindex method in the class of A that converts A to an integer. MATLAB calls A's subsindex method to perform indexing operations when the class of X does not overload methods from matlab.mixin.indexing.RedefinesParen, matlab.mixin.indexing.RedefinesDot, or matlab.mixin.indexing.RedefinesBrace.

  • If the class of X overloads methods from RedefinesParen, RedefinesDot, or RedefinesBrace these methods can call the subsindex method of A explicitly. The class of A must implement a subsindex method that returns an appropriate value.

subsindex Implementation

subsindex must return the value of the object as a zero-based integer index value in the range 0 to prod(size(X))-1.

Suppose that you want to use object A to index into object B. B can be a single object or an array, depending on the class designs.

C = B(A);

Here are two examples of subsindex methods. The first assumes you can convert class A to a uint8. The second assumes class A stores an index value in a property.

  • The subsindex method implemented by class A can convert the object to numeric format to be used as an index:

    function ind = subsindex(obj)
       ind = uint8(obj);

    The class of obj implements a uint8 method to provide the conversion from the object to an integer value.

  • Class A implements subsindex to return a numeric value that is stored in a property:

    function ind = subsindex(obj)
       ind = obj.ElementIndex;


subsindex values are 0-based, not 1-based.

See Also

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