how to fix too many output argument?

I am working on a GUI wherein the users can input the mass, gravity and drag force and in return, it will graph the differential equation. Can you suggest any methods on how can i callback the string and put it into the equation? I tried using this:

    a = str2num(get(handles.mass_second,'String'));
    b = str2num(get(handles.mass_gravity,'String'));
    c = str2num(get(handles.drag_second, 'String'));
    
    function rk= f(t,y);
        mass = a;
        gravity =b;
        rk= a*b - (c/a)*y; 
    end

but it gives me the error 'Too many output arguments'. I really want to learn matlab. thanks in advance :)

[Geoff Hayes]

Ma - please copy and paste the full error message to this question? Also, are you nesting your f function within a callback (?)? How are you calling f?

Answer 1

[Gareth]

Dear Ma.Cathyrine Ravina,

There are several ways of doing this:

• Use a Live Script (with control slider bar)
• Create a MATLAB function and pass in the parameters you want
• Create a GUI (if using a newer version of MATLAB I would recommend app designer and package as an APP for others)
• Use the Symbolic Toolbox in MATLAB to calculate this (symbolicaly)

Please note I am using the latest version of MATLAB R2018b.

Live Script

Live Script is a MATLAB File that ends in mlx, which is designed for explaining concepts.

mass = 10;
gravity =9.5;
drag_second = 20;
t = 0:.1:10;

rk = mass*gravity - (drag_second/mass) * t;

plot(rk,t)

Please note that you would add the slider after mass, gravity and drag_second.

MATLAB Function

This is what you started to do and gave an error. As you need to pass in the variable, a, b.

function rk= myfunction(a,b);
    t = 0:.01:10; % defining values of t, starting a 0 and going to 10, with 0.01 steps
    rk= a*b - (c/a)*t; 
end

Appdesigner

This is the new way MathWorks has of creating APPS, or User Interfaces.

1. Type appdesigner in MATLAB
2. Follow the tutorial
3. Drag 3 Edit Numeric Fields into your canvas (give them the right name, e.g. mass,gravity,drag_second)
4. Drag 1 Axes (for visualizition)
5. Drag 1 Push button and call it Run (this is optional but easiest to get you going)
6. Select Push button, right click it, Callback, Add PushButtonFcn Callback - this will execute your MATLAB Code when pushed.
7. Add the following code:

mass = app.MassEditField.Value;  %gets the numeric value from user interface
gravity = app.GravityEditField.Value;
drag_second = app.drag_secondEditField.Value;
t = 0:.01:10; % Calculate time vector same as before
rk = mass*gravity - (drag_second/mass) * t;
plot(app.UIAxes,rk); % updates the chart

If you are happy with your App, feel free to package it and share it with others.

Hope that this helps you get started.

[Ma.Cathyrine Ravina]

Thank you so much. This really helps me a lot :D