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Very basic object oriented question

Asked by Kevin Bachovchin on 12 Feb 2014
Latest activity Edited by per isakson
on 13 Feb 2014
Hello,
I'm trying to call a method of a class in order to calculate one of the properties of the class, but I'm getting an error.
classdef Module
properties
Parameters;
StateVariables;
ControllableInputs;
StateSpace;
end
methods
function this =Module(Parameters,StateVariables,ControllableInputs)
% class constructor
if(nargin > 0)
this.Parameters = Parameters;
this.StateVariables = StateVariables;
this.ControllableInputs = ControllableInputs;
this.StateSpace = FindStateSpace(Parameters,StateVariables,ControllableInputs);
end
end
function StateSpace = FindStateSpace(Parameters,StateVariables,ControllableInputs)
....
end
end
end
This is producing an error "Undefined function 'FindStateSpace' for input arguments of type 'sym'." Can someone please tell me what I'm doing incorrectly, and how to fix this problem? I think this is probably a very very basic question, but I'm a novice with MATLAB object oriented programming.
Thank you, Kevin

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2 Answers

Answer by Eric
on 12 Feb 2014
Edited by Eric
on 12 Feb 2014
 Accepted Answer

I don't believe you're allowed to call non-static class methods from within the constructor itself. Until the constructor returns, the variable "this" is not known to be of class Module.
Since you're passing in Parameters, StateVariables, and ControllableInputs as inputs to the FindStateSpace method, I think an easy work around is to make it a static method.
Before the final "end" statement of this file, do something like:
methods(Static)
function StateSpace = FindStateSpace(Parameters, StateVariables, ControllableInputs)
....
end
end
In the constructor, use the static method as follows:
this.StateSpace = Module.FindStateSpace(Parameters,StateVariables,ControllableInputs);
Good luck,
Eric

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Thanks, I had a feeling there was a really easy solution.

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Answer by per isakson
on 12 Feb 2014
Edited by per isakson
on 13 Feb 2014

Try this:
>> mm = my_class
mm =
my_class with properties:
prop: 17
where
classdef my_class
properties
prop
end
methods
function this = my_class( )
this.prop = this.my_method();
end
function val = my_method( this )
val = 17;
end
end
end
.
AFAIK: You can /should not have a property and a method with the same name, StateSpace
.
And static
mm =
my_class with properties:
prop: 17
where
classdef my_class
properties
prop
end
methods
function this = my_class( )
this.prop = my_class.my_method();
end
end
methods( Static = true )
function val = my_method()
val = 17;
end
end
end

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