Using Property Blocks : Set/get on properties within a single property block

Hello all,

Yet another classdef question from me. Let us say I have a bunch of properties for some custom class, and that some are data properties that are numerical arrays, and some are metadata properties that are of mixed classes. It is useful to use the following:

classdef myClass
    properties % Metadata 
        name     char
        author   char
        date     datetime
    end
    
    properties % Data
        a (:,1) double
        b (:,1) double
        c (:,1) double
        d (:,1) double
    end
end

Now, I may want to index into my data properties or, more likely, truncate a single object or concatenate two of them. Now, I clearly want to only trunc/cat the data properties, but for now I am stuck adding a method like the following:

function obj = trunc(obj,s,e)
    obj.a = obj.a(s:e);
    obj.b = obj.b(s:e);
    obj.c = obj.c(s:e);
    obj.d = obj.d(s:e);
end

function obj = cat(obj,obj2)
    obj.a = cat(1,obj.a,obj2.a)
    obj.b = cat(1,obj.b,obj2.b)
    obj.c = cat(1,obj.a,obj2.c)
    obj.d = cat(1,obj.b,obj2.d)
end

Nominally I would have something like this pseudocode

function obj = trunc(obj,s,e)
    datProps = properties(obj,'block','Data')
    for ii = 1:length(datProps)
        obj.(datProps{ii}) = obj.(datProps{ii})(s:e)
    end
end

and similarly for cat. Is there any way of doing this? It would be extremely useful for classes with a large number of properties.

Side note; I have a parallel concern with the results of doc myClass as it currently returns all of the properties but in an alphabetical/case sorted list. I can think of a number of ways to leverage both properties and methods blocks if both the comments, and a block label, could be attached, and/or if functions could apply only to properties within a set block.

After writing, this may be better for suggestions, but maybe someone has an idea for how to do this in 2019?

-DP

Answer 1

[per isakson]

The problem is obviously to implement 'block','Data'.

Try this

>> myc = myClass
myc = 
  myClass with properties:
      name: ''
    author: ''
      date: [0×0 datetime]
         a: [100×1 double]
         b: [100×1 double]
         c: [100×1 double]
         d: [100×1 double]
         
>> myc = myc.trunc(10,90)
myc = 
  myClass with properties:
      name: ''
    author: ''
      date: [0×0 datetime]
         a: [81×1 double]
         b: [81×1 double]
         c: [81×1 double]
         d: [81×1 double]
>> 

where

classdef    myClass < myData
    properties % Metadata 
        name     char
        author   char
        date     datetime
    end
    methods
        function this = trunc( this, ixb, ixe )
            
            mc  = metaclass( this );
            len = length( mc.PropertyList );
            
            for jj = 1 : len
                if strcmp( mc.PropertyList(jj).DefiningClass.Name, 'myData' )
                    this.( mc.PropertyList(jj).Name ) ...  
                    =   this.( mc.PropertyList(jj).Name )(ixb:ixe); 
                end
            end
        end        
    end
end

and

classdef    myData
    properties % Data
        a (:,1) double  = randn( 100,1);
        b (:,1) double  = randn( 100,1);
        c (:,1) double  = randn( 100,1);
        d (:,1) double  = randn( 100,1);
    end
end

I don't know, but if nothing else this is an exercise with meta.class.

Maybe, a better alternative would be to put a,b,c,d as variables in a table.

[D. Plotnick]

Thanks, there are a lot of interesting pieces in there that I hadn't seen before (I have been getting properties using propList = properties(this)) and had totally forgottten about metaclass.

I have opted for a variation on your final suggestion, which is in my comment to Steven Lord below.

For now, it sounds like this is not a native capability of Matlab and has to be shoehorned in. I think I will submit it as a suggestion.

Answer 2

[Steven Lord]

I second some of Jan's suggestions about perhaps packing the properties into a table array (or maybe a struct, vector, or even an object.) I wanted to comment on a different part of your question, your side note about object display. This might even help with your truncation question now that I think about it.

You can use the matlab.mixin.CustomDisplay base class to customize how your class gets displayed. See the documentation for this capability for some examples. You could write a getPropertyGroups method to divide the properties of your object into related collections of properties that are displayed as separate groups.

You could even potentially abuse that system a little bit to help with your truncation question. MATLAB will only call getPropertyGroups with one input to customize the display. You could have your version of that method additionally allow a two input syntax where the inputs are the object and a 'type' of property ('Data' or 'Metadata' in the example class you showed.) The two input syntax would return a list of properties of the type you requested. You could then use dynamic property referencing to refer to those properties like the below example, which accesses the table property named Properties using the char vector 'Properties' to create P2.

T = array2table(magic(4));
P1 = T.Properties;
P2 = T.('Properties');
isequal(P1, P2)

[D. Plotnick]

Thanks, it is a shame I can only accept one answer, since you and per isakson each answered seperate parts of my question. The customDisplay looks potentially very useful, and I will be digging into there.

For now, I basically did what each of you suggested in making a table of the different types of properties. Instead, I created a cell array of the names, and simply used something like

for ii = 1:length(propList)
if ismember(propList{ii},dataProperties)
    obj.(propList{ii}) = obj.(propList{ii})(ei:ee);
end
end

[Steven Lord]

If you feel both answers were useful, you could accept one and vote for the other (or accept one and vote for both.)

[D. Plotnick]

Done! Also, submitted to feature requests.