Function in class not working which normally works

Hi,

I get the message:

Undefined function 'get_Var' for input arguments of type 'char'.

Error in ClmRun/getVar_std_tot (line 35) data = get_Var(file, name);

Error in ClmRun (line 14) data = getVar_std_tot(run, 'area');

when trying to create a variable of the class ClmRun. Strangely, the same kind of function works perfect when I use it in a non-class environment (just a standard function).

Here is the code of the class I try to create:

classdef ClmRun

    properties
        CASE    % The Case name which should be the beginning of all the final files
        nlon    % Number of grid cells in longitude direction
        nlat    % Number of grid cells in latitude direction

    end

    methods
        function run = ClmRun(name)     % Constructor
            if(ischar(name))
                run.CASE = name;
                data = getVar_std_tot(run, 'area');

            else
                error('The case of the run should be a string')
            end
        end

        function output(run)            % Disp the CASE
            disp(run.CASE)
        end

        function data = getVar(file, name)     % Gets the variable name from file
            data = netcdf.open(file,'NOWRITE');
            data = netcdf.getVar(data, netcdf.inqVarID(data, name));
        end

        function data = getVar_std_tot(run, name)  % Get the standard output multiyear average
            file = strcat(run.CASE, '_totavg_standard.nc');
            disp(file)
            disp(name)
            data = getVar(file, name);
        end
    end
end

[Stephen23]

That error message:

Undefined function 'get_Var' for input arguments of type 'char'

gives a function name get_Var that does not exist in your code.

Answer 1

[Adam]

Class functions must take the object of the class as their first argument unless they are static. You are missing this.

function data = getVar_std_tot(obj,run, name)  % Get the standard output multiyear average
    file = strcat(run.CASE, '_totavg_standard.nc');
    disp(file)
    disp(name)
    data = getVar(file, name);
end

although your function doesn't seem to need the object and nor do you do anything with the returned value so not sure what you are really wanting to do. Make it static if you don't need to object/instance of the class.

[Ronny]

Okay that was the issue. Thanks. I don't understand what's the reason behind the fact that you always have to hand the object as the first input argument but that's another question.

[Adam]

In any language a class function needs to know the object of the class that it is acting on. In C++ this is hidden from you as the programmer, but is still there. In Matlab, as in other languages you must explicitly pass the object into the function (I have only really ever programmed in Matlab and C++ so I'm not familiar with many languages, but I think there are other scripting languages where the programmer must explicitly pass the object too).

[Steven Lord]

Be careful with your terminology. There are differences between a method of an object (which as Adam said must be called with an instance of that object unless it is a Static method) and a class-related function.

The getVar_std_tot function in this class needs to access properties of the object, so it seems reasonable that it should be a method of the class. Since it only accesses that one property you could instead define it as a class-related function and have whoever calls it pass the properties into it rather than having getVar_std_tot access them internally, but if it were to access more than just that one property I'd leave it as a method. And if it needed to call a method on the object, I would definitely leave it as a method.

The getVar function in this class does not need to access properties of this object, and I assume it's not intended to be called by other code outside of this class itself. If that is the case, I would make it a class-related function by defining it after the end of the classdef block. See myUtilityFcn in the example in the class-related functions section of the documentation page to which I linked above.

[Adam]

I had never actually realised that defining a function after the classdef block was even valid syntax. I guess I've never needed to try, I've always used Static Private functions, but that does add a little more 'noise' than doing it after the classdef block.

[per isakson]

Shouldn't

data = getVar(file, name);

be

data = obj.getVar(file, name);

?

[Adam]

Yes, it should actually, and the definition of getVar updated accordingly. I only looked at the signature and glanced quickly at the code inside it!

[Steven Lord]

per,

Yes, if you want to keep getVar as a non-Static method rather than a class-based function it needs to accept an instance of the object as an input. Technically it doesn't have to be the first input, but at least one input has to be an instance of the object.

Why might you want to have a non-Static method where the first input isn't an instance of the class? See the example on the operator overloading documentation page. Using the Adder class as defined on that page you could calculate both Adder(1:10) + 1 and 2 + Adder(1:10). The plus method on the Adder class handles both cases.