end indexing function as method input

I have a class definition that is used to map a binary data file. When the file is initially selected, the file is mapped, that is the file scanned and the bit location where the raw data begins. I then have a method called obj.data that calls the data from the binary file using the indeces that the user inputs. obj.data is disguised as a property but it's actually a method. So the user inputs the data they want into the method like they would index a matrix.

Ex: if data is 10 x 5

obj.data(1,2) tells the method to grab the 11th value in the file (because the data is stored 50 x 1 array).

I did this with a method:

function dataOut = data(obj, varargin)
            if nargin < 2 % obj.data returns the whole matrix
                i = 1:nRows;
                j = 1:nCols;
            else
                i = varargin{1};
                j = varargin{2};
            end
            ....
end

Anyway, my issue is how to deal with if the user puts in "end" to index the data. Ex: obj.data(end,1). When "end" is used as an input to this method, varagin is empty. I added a subsref method to catch ":" because ":" were actually causing an error but subsref fixed it, but subsref doesn't seem to identify "end". I tried adding an end method but it doesn't seem to get called when "end" is the input to a method. I did notice that obj.data('end',1) works but it's not intuitive to the user to do put end in quotes when indexing what they think is a property. I'd like to have it so the user doesn't have to know do anything special.

Don't know if it matters but I'm currently using Matlab 2018b.

Any ideas? Any advice would be appreciated.

[Rik]

I'm slightly surprised that would work, given the result below.

A=rand(4);
A('end')

Index exceeds the number of array elements (16).

On the other hand, maybe that isn't strange, as that would simply be a function input. How is Matlab supposed to know you are actually indexing? Maybe this also requires overloading size. I presume that is called implictly when you use end to index.

[Joseph Franco]

I think it works because the inputs are inputs to the method. Like you said, its a function input. So basically my issue is 'end' is recognized as a input but end is not. So I don't think Matlab does know I'm indexing but with my data method I'm able to trick it, basically. The two issues it can't be "tricked" are ":" and "end". I got around the ":" issue by overloading subsref but my issue is figuring out how to "trick" Matlab when the user puts in end. Interestingly, size(obj.data) seems work correctly. But, overloading it would probably be more efficient. Matlab is probably running a lot more code than necessary with the way I'm doing it.

Answer 1

[Rik]

If you create a dummy property (which is only used so Matlab treats your method as a property), you can use the overloaded subsref and end functions to call the actual code.

end is not a keyword in this context: it is a function. See this documentation page. You should be able to overload the end function with this signature:

function ind = end(A,k,n)

The arguments are described as follows:

• A is the object
• k is the index in the expression using the end syntax
• n is the total number of indices in the expression
• ind is the index value to use in the expression

You wrote an example yourself already:

classdef
    properties
        data = [];
    end
    methods
        function dataOut = subsref(obj,s)
            dataOut = obj.getData(s(2).subs{1},s(2).subs{2});
        end
        function ind = end(obj,k,n)
            switch k
                case 1
                    ind = nRows;
                case 2
                    ind = nCols;
            end
        end
        function dataOut = getData(obj,varagin)
            if nargin < 2 % obj.data returns the whole matrix
                i = 1:nRows;
                j = 1:nCols;
            else
                i = varargin{1};
                j = varargin{2};
            end
            obj.data = filemappingfunction(i,j);
        end
    end
end

[Joseph Franco]

Rik, thanks for your help. I tried overload end and I don't think this end method is called because obj.data is a method not a property. I'm not sure about this but I think this because I tried overloading the end method and I put a breakpoint in the end method and I wasn't able to get it to catch the code. I guess my real question is to confirm that the end method overload will only work for properties or will it work with methods?

[Joseph Franco]

It's also interesting when I try obj.data(end,1), the varargin array is empty.

[Rik]

Maybe you need a wrapper property. You can make it sparse to avoid it taking up any memory. Then you can use the overloaded subsref and end to redirect the calls to the method.

[Joseph Franco]

I think I know what you mean by wrapper property but I'm having a hard time visualizing it. Do you mean something like:

classdef
    properties
        data = [];
    end
    methods
        function dataOut = subsref(obj,s)
            dataOut = obj.getData(s(2).subs{1},s(2).subs{2});
        end
        function ind = end(obj,k,n)
            switch k
                case 1
                    ind = nRows;
                case 2
                    ind = nCols;
            end
        end
        function dataOut = getData(obj,varagin)
            if nargin < 2 % obj.data returns the whole matrix
                i = 1:nRows;
                j = 1:nCols;
            else
                i = varargin{1};
                j = varargin{2};
            end
            obj.data = filemappingfunction(i,j);
        end
    end
end

[Rik]

I hardly ever work with clases, but yes, that was my idea. (except that I didn't think long enough to realize you don't even need a sparse array the correct size and an empty array will work just fine as well)

[Joseph Franco]

Thanks Rik for all you help.

[Rik]

You're welcome. I will edit my answer tomorrow so it reflects the complete solution, after which I hope you will consider accepting it.