using fopen inside of a class

Hello there,

I am a graduate student looking to make a gcode writing script for our machine. I figured the easiest way would be to create a class with readable methods for me and my partner to share. We have previously been using fopen() to open a text file and write the gcode using fprintf statements, and I was hoping to translate that into a class; But I'm having problems with assigning the file to the object property.

Here's the class definition:

classdef Welder
    properties
        gcodefile
        txtout = fopen(gcodefile,'w');
    end
    
    methods
        function startUp(obj,Amperage)
            obj.txtout = fopen(obj.gcodefile,'w');
            fprintf(obj.txtout,"G0 G90 G54 G17 G40 G49 G80\nG21\nM3 S%.0f\nG4 P1.0\n",Amperage);
        end
        
        function moveX(obj,X)
        fprintf(obj.txtout,'G1 X%.2f\n',X);
        end
        
    end    
    

And here's the code in which I create an object of the welder class:

ArcNid = ARCNID;

% Startup
ArcNid.gcodefile = "GCodeOut.txt";
ArcNid.startUp(row,S);
ArcNid.turnArcOn(S,PF);

However, running this code yields the error message -

Error using U_Notch_SQR_Maker

Invalid default value for property 'txtout' in class 'ARCNID':

Unrecognized function or variable 'gcodefile'.

I've tried moving the definition of the txtout property around, but when changing the value inside of another method (I.E, the startUp method), the class property doesn't change, and any subsequent methods yield a similar 'unknown definition' error. Alternatively, I'm unsure if txtout should even be a property; I'm unused to matlab classes/etc., but I thought fopen returned essentially an interger file identifier, which sounds like it should then be a property.

Thanks in advance for the help!!

[Walter Roberson]

Your class does not initialize the property gcodefile

Your class tries to assign txtout the result of fopen() . But you also assign the same thing in startUp.

[Sam Vellequette]

Hi Walter,

About the double declaration - I'd like to be able to change the file if necessary, so in this slightly improved version I have gcodefile defined to be a string, and would like to be able to change Arcnid.gcodefile, run Arcnid.startUp, and have .startUp update the property txtout. Yet, when I run this, txtout does not change - it remains the same value.

Below is the slightly changed code:

classdef ARCNID
    properties
        gcodefile {mustBeText} = 'GCodeOut.txt'
        txtout 
    end
    methods
        function startUp(obj,row,Amperage)
            obj.txtout = fopen(obj.gcodefile,'w');
            fprintf(obj.txtout,"(Code Produced from Excel Line: %.0f)\n",row+1);
            fprintf(obj.txtout,"G0 G90 G54 G17 G40 G49 G80\nG21\nM3 S%.0f\nG4 P1.0\n",Amperage);
        end
    end
end

txtout (I believe) is a public property by default, so any idea why it won't budge?

Answer 1

[Steven Lord]

txtout (I believe) is a public property by default, so any idea why it won't budge?

Your class is a value class. Value classes and handle classes behave differently in MATLAB. In particular, "If you pass this variable to a function to modify it, the function must return the modified object as an output argument."

So as you've written it, your startUp function modifies the copy of the object that was created for the method to operate on. When your method exits, because you're not returning that copy, the copy gets discarded along with the rest of the method workspace.

The easiest solution, if you want your class to be a value class, is to return the copy as an output argument.

See this documentation page for some discussion of the pros and cons of making your class a value class or a handle class.

[Sam Vellequette]

thanks Steven!