x.abs()?

>> x=-1;
>> x.abs()
??? Attempt to reference field of non-structure array.

Why?

Notice that Matlab Help says "All MATLAB data types are implemented as object-oriented classes."

Answer 1

[Fangjun Jiang]

That is true, but it doesn't imply that x you declared will have abs as its method.

I think that claim you quoted means that MATLAB's data types are internally implemented as object-oriented classes. The double data type doesn't provide any method.

1. class(x) returns double
2. methodsview(x) causes an error.

[Jonathan]

I agree. The reason methodsview(x) causes an error is because isobject(x) returns false. From the help on isobject: "ISOBJECT(A) returns 1 if A is a MATLAB object and 0 otherwise." Juxtaposing this with a line from the Programming Fundamentals help section provides some clarity. "There are 15 fundamental classes in MATLAB." double is one of these fundamental MATLAB classes. However, it is not considered a MATLAB object.

That it is a class can be clearly seen from executing "class(x)", as above, or examining the metaclass for the double type: "mc = ?double". This reveals that there are native methods for the double class just as there are native methods for any MATLAB class. The particular methods for the double class can be inspected via the struct in mc or with the command "methods(x)", also "methods double". Also note that "hand = mc.Methods{1}.DefiningClass" returns a handle such that "isequal(hand, mc)" returns true.

[Daniel Shub]

@Jonathan, Why do you say that instances of the class double are not considered objects? In the "Subclassing MATLAB Built-In Classes" documentation it says "You can create an object of class double using an assignment statement, indexing expressions, or using converter functions." I read that as instances of class double are in fact objects and should obey all the rules of the MATLAB OO structure.

I look at the fact that isobject(double.empty) returns false as being an inconsistency in MATLAB arising from the fact that everything used to be a double.

[Jonathan]

@Daniel, This is a very good point. The reason I say that instances of the class double are not considered objects is because I use the result of isobject as part of my definition of object. In this sense, a 'MATLAB object' is anything that MATLAB treats as an object.

[Shunchao Wu]

I'd like to consider double as one fundmental data type, not the so called "class", and x as a normal variable, not an object. With regard to class(x), methods(x), methodsview(x) and isobject(x), I regard them as some games.

[Shunchao Wu]

Thank you all:)

Answer 2

[Daniel Shub]

I disagrees with both answers. Starting with

x = -1;

You can see that abs is a method with

methods(x)
metaclass(x)
methodsview(class(x))

Interesting, as Fangjun mentioned,

methodsview(x)

gives an error. Exploring the double class in general

?double
methodsview double

both list abs as a method.

Finally, creating a simply dummy class

classdef myClass < double
  methods
    function newClass = myClass(input)
      newClass = newClass@double(input);
    end
  end
end

makes it so that

y = myClass(-1);
y.abs()

works.

[Fangjun Jiang]

+1. Interesting finding! I guess TMW has some inconsistency problem needs to be sorted out.

Answer 3

[Drew Weymouth]

Although your variable x is technically a MATLAB "class" ( double ), it does not have any methods associated with it. While MATLAB does have some built-in classes with methods (such as Audioplayer for example), most data types are basically primitive values. To get the absolute value of any scalar value, use y = abs(x). If x is a matrix, then this will take the absolute value of each component. If x is complex, then it will give the magnitude.