uicontrol callback function too many input arguments - matlab

Question

My goal is to get the user's input from a uicontrol text box, do operations on the input and then display the output to another text box. MATLAB gives me the error:
Error using
UnitConverter/lbs2kg
Too many input arguments.
Error in
UnitConverter>#(varargin)app.lbs2kg(varargin{:})
(line 22)
'Callback',#app.lbs2kg,'String',app.inputMass);
Error while evaluating UIControl Callback
Here is my code:
classdef UnitConverter < handle
properties
Figure % Graphics handles
DispInputMass
DispOutputMass
inputMass %Variables/Class Properties
outputMass
end
methods
function app = UnitConverter
% This is the "constructor" for the class
% It runs when an object of this class is created
app.Figure = figure('Name','Unit Converter') ;
app.DispInputMass = uicontrol('Style','edit',...
'Callback',#app.lbs2kg,'String',app.inputMass);
app.DispOutputMass = uicontrol(app.Figure,'Style','edit','Position'...
,[168 100 47 26],'String','kg');
end
function lbs2kg(app,evt)
app.inputMass = get(app.DispInputMass,'string');
app.outputMass = app.inputMass*.453;
set(app.DispOutputMass,'string',app.outputMass);
end
end
end

Answer 1

The callback method actually has 3 inputs - MATLAB is throwing this error because it is trying to send three inputs to your callback which is written to only accept 2. The 3 inputs are (in order): the main object (app), the object sending the event (uicontrol) and the event (matlab.ui.eventdata.ActionData).
You can change the code to the following to get it to work:
function lbs2kg(app, obj, evt)
app.inputMass = get(app.DispInputMass,'string');
app.outputMass = app.inputMass*.453;
set(app.DispOutputMass,'string',app.outputMass);
end
Additionally you can change the first line of the function to the following:
function lbs2kg(varargin)
Breakpoint the code at the first line of the callback and investigate the contents of varargin. For more help about varargin see here (http://www.mathworks.com/help/matlab/ref/varargin.html)