How do I add a class to a class

I created this class to initialize properties of a specific rocket stage:

classdef stageClass < handle
    properties
       stageNum
       isp
       mtot
       mprop
       mstructure
       mpayload
    end
    methods 
        function obj = stageClass(stageNum)
            obj.stageNum = stageNum;
        end
    end
end

classdef rocket < handle
    %This class will create an object called rocket.
    %This rocket will have stages with properties
    
    properties
        stage = stageClass.empty;
    end
    
    methods(Static)
        function obj = rocket()
        end
        function obj = addStage(obj, stageNum)
            obj.stage(length(obj.stage)+1) = stageClass(stageNum);
        end
    end
end

I have another class called rocket from which I want to intialize a variable/ user input amount of stages. My goal here is to create a class that can initialize stages. For example I want the input to be: rocket.stages = 3. Then it would create 3 instances of the stages, each with unique properties which I could initialize individually.

How do I write a function that will add a stage to the rocket?

I referenced this question to write the rocket code:https://www.mathworks.com/matlabcentral/answers/17583-creating-multiple-instances-of-a-class-from-within-a-class

Answer 1

[Robert U]

Hi Alok Virkar,

There are several possibilities to fullfill the described functionality. But first there are issues with your class "rocket". Neither constructor method nor any method that changes properties may be "static". Static methods are well suited for value calculations or data operations, i.e. functions that do not manipulate the object nor depend on changing object properties.

My suggestion for adding rocket stages is to use a customized set-method for the property "stage" of class "rocket". I change the data type of "stage" to cell which makes it easy to support several stages.

classdef rocket < handle
    %This class will create an object called rocket.
    %This rocket will have stages with properties
    
    properties
        stage = {}; % could be a cell array Nx1 containing stageClass objects
    end
    
    methods
        function set.stage(obj,cStageClass)
            assert(all(cellfun(@(cIn) isa(cIn,'stageClass'),cStageClass)),'The input cell does not contain objects of class ''stageClass''.');
            if isempty(obj.stage)
                obj.stage(1) = cStageClass;
                fprintf(1,'<< Stage No. %d  added.\n',cStageClass{1}.stageNum);
            else
                dExistingStageNums = cellfun(@(cIn) cIn.stageNum,obj.stage);
                if any(dExistingStageNums == cStageClass{1}.stageNum)
                    warning('Stage No. %d exists already. It will not be added.',cStageClass{1}.stageNum);
                else
                    % append new stage
                    obj.stage(end+1) = cStageClass;
                    fprintf(1,'<< Stage No. %d  added.\n',cStageClass{1}.stageNum);
                    % sort according to stageNum
                    dExistingStageNums = cellfun(@(cIn) cIn.stageNum,obj.stage);
                    [~,indSort] = sort(dExistingStageNums);
                    obj.stage = obj.stage(indSort);
                end
            end
            
            
        end
    end
    
    methods
        function obj = rocket()
        end
        function addStage(obj,stageNum)
            validateattributes(stageNum,{'numeric'},{'vector','integer','positive'});
            for nStage = stageNum
                obj.stage = {stageClass(nStage)};
            end
        end
    end
end

The set-method controls whether a new stage is added or not to avoid duplicate numbers, and sorts the cells according to there stage number. Furthermore, I added exemplary input argument checks which will help you to create more robust code.

In difference to your description, each stage has to created by supplying its particular number. Call the following way:

myRocket = rocket; % create object of class "rocket"
myRocket.addStage(1:3); % create stages 1, 2, and 3
myRocket.addStage(5:-1:3); % create stages 4 and 5 (inverse order to check sorting, withdrawing 3 since it already exists)
myRocket.addStage([7,11,14]); % create stages 7,11,14 (arbitrary numbers)
myRocket.addStage(9);

Kind regards,

Robert

Answer 2

[Robert U]

Hi Alok Virkar,

another approch making use of object arrays which might be more convenient in use:

rocket.m

classdef rocket < handle
    %This class will create an object called rocket.
    %This rocket will have stages with properties
    
    properties
        stage = stageClass.empty; % object array 1xN of stageClass objects
    end
    
    methods
        function set.stage(obj,StageClass)
            assert(isa(StageClass,'stageClass'),'The input cell does not contain objects of class ''stageClass''.');
            for nStage = 1:numel(StageClass)
                dExistingStageNums = [obj.stage.stageNum];
                if any(dExistingStageNums == StageClass(nStage).stageNum)
                    warning('Stage No. %d exists already. It will not be added.',StageClass(nStage).stageNum);
                else
                    % append new stage
                    obj.stage(end+1) = StageClass(nStage);
                    fprintf(1,'<< Stage No. %d  added.\n',StageClass(nStage).stageNum);
                    % sort according to stageNum
                    dExistingStageNums = [obj.stage.stageNum];
                    [~,indSort] = sort(dExistingStageNums);
                    obj.stage = obj.stage(indSort);
                end
            end
            
            
        end
    end
    
    methods
        function obj = rocket()
        end
        function addStage(obj,stageNum)
            validateattributes(stageNum,{'numeric'},{'vector','integer','positive'});
            obj.stage = stageClass(stageNum);
        end
    end
end

stageClass.m

classdef stageClass < handle
    properties
       stageNum
       isp
       mtot
       mprop
       mstructure
       mpayload
    end
    methods 
        function obj = stageClass(dStageNum)
            if nargin ~= 0
                for indStage = 1:numel(dStageNum)
                    obj(indStage).stageNum = dStageNum(indStage);
                end
            end
        end
    end
end

Test sequence:

myRocket = rocket; % create object of class "rocket"
myRocket.addStage(1:3); % create stages 1, 2, and 3
myRocket.addStage(5:-1:3); % create stages 4 and 5 (inverse order to check sorting, withdrawing 3 since it already exists)
myRocket.addStage([7,11,14]); % create stages 7,11,14 (arbitrary numbers)
myRocket.addStage(9);

Kind regards,

Robert