Trying to make a simple calculator/multiplyer. Only works for the first time.

Good day! I'm tryin to make a simple multiplyer that would take 2 input values,sum them up and multiply the sum by 2. The gui has 2 edit windows and 1 push button to request the answer at the static text window. The program works only for the first time,as soon as I try to change the previously input values, it gives me an error in the command window and won't work. I have also changed the string in the edit boxes to 0. Those 0 are string values,not actual values. I tryed the following script to turn them into values,but it just changes them permamently.

% --- Executes just before EditBoxTrial is made visible.
function EditBoxTrial_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to EditBoxTrial (see VARARGIN)

% Choose default command line output for EditBoxTrial
handles.output = hObject;
num = 0;
handles.edit1 = num;
handles.edit2 = num;

% Update handles structure
guidata(hObject, handles);

% UIWAIT makes EditBoxTrial wait for user response (see UIRESUME)
% uiwait(handles.figure1);

How do I fix two of those problems? The full program code is

function varargout = EditBoxTrial(varargin)
% EDITBOXTRIAL MATLAB code for EditBoxTrial.fig
%      EDITBOXTRIAL, by itself, creates a new EDITBOXTRIAL or raises the existing
%      singleton*.
%
%      H = EDITBOXTRIAL returns the handle to a new EDITBOXTRIAL or the handle to
%      the existing singleton*.
%
%      EDITBOXTRIAL('CALLBACK',hObject,eventData,handles,...) calls the local
%      function named CALLBACK in EDITBOXTRIAL.M with the given input arguments.
%
%      EDITBOXTRIAL('Property','Value',...) creates a new EDITBOXTRIAL or raises the
%      existing singleton*.  Starting from the left, property value pairs are
%      applied to the GUI before EditBoxTrial_OpeningFcn gets called.  An
%      unrecognized property name or invalid value makes property application
%      stop.  All inputs are passed to EditBoxTrial_OpeningFcn via varargin.
%
%      *See GUI Options on GUIDE's Tools menu.  Choose "GUI allows only one
%      instance to run (singleton)".
%
% See also: GUIDE, GUIDATA, GUIHANDLES

% Edit the above text to modify the response to help EditBoxTrial

% Last Modified by GUIDE v2.5 10-Jul-2012 09:24:44

% Begin initialization code - DO NOT EDIT
gui_Singleton = 1;
gui_State = struct('gui_Name',       mfilename, ...
                 'gui_Singleton',  gui_Singleton, ...
                 'gui_OpeningFcn', @EditBoxTrial_OpeningFcn, ...
                 'gui_OutputFcn',  @EditBoxTrial_OutputFcn, ...
                 'gui_LayoutFcn',  [] , ...
                 'gui_Callback',   []);
if nargin && ischar(varargin{1})
    gui_State.gui_Callback = str2func(varargin{1});
end

if nargout
    [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});
else
    gui_mainfcn(gui_State, varargin{:});
end
% End initialization code - DO NOT EDIT

% --- Executes just before EditBoxTrial is made visible.
function EditBoxTrial_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to EditBoxTrial (see VARARGIN)

% Choose default command line output for EditBoxTrial
handles.output = hObject;

% Update handles structure
guidata(hObject, handles);

% UIWAIT makes EditBoxTrial wait for user response (see UIRESUME)
% uiwait(handles.figure1);

% --- Outputs from this function are returned to the command line.
function varargout = EditBoxTrial_OutputFcn(hObject, eventdata, handles) 
% varargout  cell array for returning output args (see VARARGOUT);
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Get default command line output from handles structure
varargout{1} = handles.output;

function edit_box1_Callback(hObject, eventdata, handles)
num=str2double(get(handles.edit_box1, 'String'));
handles.edit_box1=num;
guidata(hObject,handles);
% hObject    handle to edit_box1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit_box1 as text
%        str2double(get(hObject,'String')) returns contents of edit_box1 as a double

%  --- Executes during object creation, after setting all properties.
function edit_box1_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit_box1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

function edit_box2_Callback(hObject, eventdata, handles)
num=str2double(get(handles.edit_box2,'String'));
handles.edit_box2=num;
guidata(hObject,handles);
% hObject    handle to edit_box2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of edit_box2 as text
%        str2double(get(hObject,'String')) returns contents of edit_box2 as a double

% --- Executes during object creation, after setting all properties.
function edit_box2_CreateFcn(hObject, eventdata, handles)
% hObject    handle to edit_box2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end

% --- Executes on button press in push_button1.
function push_button1_Callback(hObject, eventdata, handles)
c=(handles.edit_box1+handles.edit_box2)*2;
str=sprintf('Answer is: %d',c);
set (handles.static_text, 'String',str);
% hObject    handle to push_button1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
  % handles    structure with handles and user data (see GUIDATA)

Answer 1

[John Petersen]

You're reassigning handles.edit_box1 which loses your handle. Instead, use

x=str2double(get(handles.edit_box1,'String'));
setappdata(handles.figure1,'x',x);

and

y=str2double(get(handles.edit_box2,'String'));
setappdata(handles.figure1,'y',y);

in both the edit boxes, then in the pushbutton function use

x = getappdata(handles.figure1, 'x');
y = getappdata(handles.figure1, 'y');
c = 2*(x + y);