Function in class not working which normally works

Question

Ronny 2017-9-12

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/356254-function-in-class-not-working-which-normally-works

评论： Steven Lord 2017-9-15

采纳的回答： Adam

在 MATLAB Online 中打开

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

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Stephen23 2017-9-12

在 MATLAB Online 中打开

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.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Adam 2017-9-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/356254-function-in-class-not-working-which-normally-works#answer_281207

编辑：Adam 2017-9-12

在 MATLAB Online 中打开

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.

7 个评论
显示 5更早的评论隐藏 5更早的评论

Steven Lord 2017-9-12

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 2017-9-15

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 2017-9-15

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.

请先登录，再进行评论。