Object properties that automatically update but aren't Depenent

I'm new to using object oriented programming, so I'm honestly just experimenting here, and maybe there's a better way to do this. For context I'm trying to create an object for an ideal gas.

classdef idealGas
    properties
        P
        V
        n
        R = 8.314; %J/mol/K
        T
    end
end

Ideal gasses are defined by P*V = n*R*T, so I would like to be able to set any 3 (R is constant) and have the fourth calculated automatically. If only one of them were dependent, I could do

classdef idealGas
    properties
        P
        V
        n
        R = 8.314;
    end
    properties (Dependent)
        T
    end
    methods
        function T = get.T(obj)
            T = obj.P*obj.V/obj.n/obj.R;
        end
    end
end

gas1 = idealGas;
gas1.P = 100;
gas1.V = 2;
gas1.n = 5;

T = gas1.T;

I have two problems with this. First is that it only lets T be dependent; I would like to be able to specify any three and have it calculate the fourth. Second is that is would have to recalculate T every time I call it. This might not be a problem for this example since its a very simple operation, but for different objects it might not be. I would like it to be able to store T without recalculating every time I call it.

I tried having assigning other properties in a set method for P like follows, but it didn't like me accessing other properties in the set method for P.

function obj = set.P(obj,pressure)
    if ~isempty(obj.P)
        error('Pressure is already set, please change pressure via specific process')
    end
    obj.P = pressure;
    obj.V = obj.n*obj.Rbar*obj.T/pressure;
    obj.n = pressure*obj.V/obj.Rbar/obj.T;
    obj.T = pressure*obj.V/obj.n/obj.Rbar;
end

Also even if that worked, I'd still have to do it for every property, which feels clunky.

Any ideas would be appreciated

[Les Beckham]

This is a fascinating question. I too am not very comfortable with using OOP in Matlab. But I'm trying to learn.

I experimented a bit with this and did not come up with any good solution. I suspect that you could create different objects where one and only one property was dependent on the others (idealGasPVT, idealGasnPT, idealGasnVT, etc.) but that really seems like an unsatisfying approach. I feel that this should be possible. I'm hoping that someone smarter than me will provide a real answer.

Here is my (not working) code that I used to experiment with this.

This code generates the same CodeAnalyzer errors/warnings you refer to, which refers to loading and saving of objects, not creating or updating them (which is less than helpful in this case).

% idealGas.m
% https://www.mathworks.com/matlabcentral/answers/721574-object-properties-that-automatically-update-but-aren-t-depenent?s_tid=srchtitle

classdef idealGas
    properties
        P % N/m^2
        V % m^2
        n % moles
        T % Kelvin
    end % end properties
    properties(Constant)
        R = 8.314; % J/mol/K
    end % end Constant properties
    methods
        % Constructors
        function ig = idealGas(P, V, n, T)
            if (isempty(P))
                ig.V = V;
                ig.T = T;
                ig.n = n;
                ig.P = get(P);
            elseif (isempty(V))
                ig.P = P;
                ig.n = n;
                ig.T = T;
                ig.V = get(V);
            elseif (isempty(n))
                ig.P = P;
                ig.V = V;
                ig.T = T;
                ig.n = get(n);
            elseif (isempty(T))
                ig.P = P;
                ig.V = V;
                ig.n = n;
                ig.n = get(T);
            end
        end

        % Get methods
        function P = get.P(obj)
            P = obj.n * obj.R * obj.T / obj.V;
        end
        function V = get.V(obj)
            V = obj.n * obj.R * obj.T / obj.P;
        end
        function T = get.T(obj)
            T = obj.P * obj.V / obj.n / obj.R;
        end
        function n = get.n(obj)
            n = obj.P * obj.V / obj.R / obj.T;
        end

        % Set methods
        function obj = set.P(obj, pressure)
            obj.P = pressure;
            obj.V = obj.n * obj.R * obj.T / obj.P;
            obj.n = obj.P * obj.V / obj.R / obj.T;
            obj.T = obj.P * obj.V / obj.n / obj.R;
        end % end set.P

        function obj = set.V(obj, volume)
            obj.V = volume;
            obj.P = obj.n * obj.R * obj.T / obj.V;
            obj.n = obj.P * obj.V / obj.R / obj.T;
            obj.T = obj.P * obj.V / obj.n / obj.R;
        end % end set.V

        function obj = set.T(obj, temp)
            obj.T = temp;
            obj.V = obj.n * obj.R * obj.T / obj.P;
            obj.n = obj.P * obj.V / obj.R / obj.T;
            obj.T = obj.P * obj.V / obj.n / obj.R;
        end % end set.V
    end % end methods
end % end classdef

Answer 1

[Steven Lord]

function obj = set.P(obj,pressure)
    if ~isempty(obj.P)
        error('Pressure is already set, please change pressure via specific process')
    end
    obj.P = pressure;
    obj.V = obj.n*obj.Rbar*obj.T/pressure;
    obj.n = pressure*obj.V/obj.Rbar/obj.T;
    obj.T = pressure*obj.V/obj.n/obj.Rbar;
end

So obj.P will have the new pressure value. obj.V will be calculated using the old n, Rbar, and T and the new P. Then obj.n will be calculated using the new pressure and V and the old Rbar and T. Do you then need to go back and recalculate obj.V using the new pressure and n and the old Rbar and T?

Giving a new value for any of the parameters will have a ripple effect on the others. I probably wouldn't try to use property set methods but I might use property get methods. If you're concerned about recomputing one of the parameters using a lengthy calculation consider using a memoize object.

[Laird Mendelson]

Yep! Yeah, I realized that was the problem with my method. Memoized objects seem perfect for what I'm trying to do. Thanks for the help!