why do i get this error when i try add a working code to guide

Hi, i get the error below when trying to solve a D.E. whit euler:

I have a code that solves D.E. whit euler metod, but i have to add that code to a GUI (guide) like this one, i made it from a blank guide:

The euler code is quite simple and it works in a script.

syms x
syms y
fprintf('\tResolucion de Ecuaciones Diferenciales por Metodo Euler');
f=inline(input('\n Ingrese la derivada:','s'));
x=input('\n Ingrese el valor de X inicial:');
xf=input('\n Ingrese el valor de X final:');
y=input('\n Ingrese el valor de Y inicial:');
h=input('\n Ingrese el paso:');
n=(xf-x)/h
disp(' x    y');
for i=1:n+1    
y1= feval(f,x,y);    
hy1=h*y1;
fprintf('\n%0.1f %0.4f ',x,y);    
y=y+hy1;    
x=x+h; 
end

The error i get is this:

Error using  :  (line 38)
Unable to compute number of steps from 1 to 5 - 2*x by 1.

Error in gui>bResolver_Callback (line 99)
        for i=1:n+1

Error in gui_mainfcn (line 95)
        feval(varargin{:});

Error in gui (line 42)
    gui_mainfcn(gui_State, varargin{:});

Error in
matlab.graphics.internal.figfile.FigFile/read>@(hObject,eventdata)gui('bResolver_Callback',hObject,eventdata,guidata(hObject))
 
Error while evaluating UIControl Callback.

and here is the GUI(guide) code

function varargout = gui(varargin)
% GUI MATLAB code for gui.fig
%      GUI, by itself, creates a new GUI or raises the existing
%      singleton*.
%
%      H = GUI returns the handle to a new GUI or the handle to
%      the existing singleton*.
%
%      GUI('CALLBACK',hObject,eventData,handles,...) calls the local
%      function named CALLBACK in GUI.M with the given input arguments.
%
%      GUI('Property','Value',...) creates a new GUI or raises the
%      existing singleton*.  Starting from the left, property value pairs are
%      applied to the GUI before gui_OpeningFcn gets called.  An
%      unrecognized property name or invalid value makes property application
%      stop.  All inputs are passed to gui_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 gui

% Last Modified by GUIDE v2.5 21-Jul-2019 19:03:38

% Begin initialization code - DO NOT EDIT
gui_Singleton = 1;
gui_State = struct('gui_Name',       mfilename, ...
                   'gui_Singleton',  gui_Singleton, ...
                   'gui_OpeningFcn', @gui_OpeningFcn, ...
                   'gui_OutputFcn',  @gui_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 gui is made visible.
function gui_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 gui (see VARARGIN)

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

% Update handles structure
guidata(hObject, handles);

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


% --- Outputs from this function are returned to the command line.
function varargout = gui_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;


% --- Executes on button press in bResolver.
function bResolver_Callback(hObject, eventdata, handles)
% hObject    handle to bResolver (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
clc;
opcion=get(handles.menuMetodo, 'Value');
%a=get(handles.Tecua, 'string');
a=inline(get(handles.Tecua, 'string'));
x=str2num(get(handles.Txo, 'string'));
y=str2num(get(handles.Tyo, 'string'));
xf=str2num(get(handles.Txf, 'string'));
h=str2num(get(handles.Tpaso, 'string'));
%disp(a)
%disp(x)
%disp( y )
%disp( xf )
%disp( h)
syms x y;
switch opcion
    case 1 %Método de Euler
        n=(xf-x)/h;
        %clc
        for i=1:n+1
            y1= feval(a,x,y);
            hy1=h*y1;
            fprintf('\n%0.1f %0.4f ',x,y);
            y=y+hy1;
            x=x+h;
        end
        set(handles.Tresul, 'string', char(y));
    case 2 %Método de Euler mejorado
    
    case 3 %Método de Runge-Kutta
        
end


    



function Tresul_Callback(hObject, eventdata, handles)
% hObject    handle to Tresul (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 Tresul as text
%        str2double(get(hObject,'String')) returns contents of Tresul as a double


% --- Executes during object creation, after setting all properties.
function Tresul_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Tresul (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 Txf_Callback(hObject, eventdata, handles)
% hObject    handle to Txf (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 Txf as text
%        str2double(get(hObject,'String')) returns contents of Txf as a double


% --- Executes during object creation, after setting all properties.
function Txf_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Txf (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 Tpaso_Callback(hObject, eventdata, handles)
% hObject    handle to Tpaso (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 Tpaso as text
%        str2double(get(hObject,'String')) returns contents of Tpaso as a double


% --- Executes during object creation, after setting all properties.
function Tpaso_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Tpaso (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 selection change in menuMetodo.
function menuMetodo_Callback(hObject, eventdata, handles)
% hObject    handle to menuMetodo (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: contents = cellstr(get(hObject,'String')) returns menuMetodo contents as cell array
%        contents{get(hObject,'Value')} returns selected item from menuMetodo


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

% Hint: popupmenu 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 Tcond_Callback(hObject, eventdata, handles)
% hObject    handle to Tcond (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 Tcond as text
%        str2double(get(hObject,'String')) returns contents of Tcond as a double


% --- Executes during object creation, after setting all properties.
function Tcond_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Tcond (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 bLeer.
function bLeer_Callback(hObject, eventdata, handles)
% hObject    handle to bLeer (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)



function Tecua_Callback(hObject, eventdata, handles)
% hObject    handle to Tecua (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 Tecua as text
%        str2double(get(hObject,'String')) returns contents of Tecua as a double


% --- Executes during object creation, after setting all properties.
function Tecua_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Tecua (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 Txo_Callback(hObject, eventdata, handles)
% hObject    handle to Txo (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 Txo as text
%        str2double(get(hObject,'String')) returns contents of Txo as a double


% --- Executes during object creation, after setting all properties.
function Txo_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Txo (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 Tyo_Callback(hObject, eventdata, handles)
% hObject    handle to Tyo (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 Tyo as text
%        str2double(get(hObject,'String')) returns contents of Tyo as a double


% --- Executes during object creation, after setting all properties.
function Tyo_CreateFcn(hObject, eventdata, handles)
% hObject    handle to Tyo (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

Answer 1

[Walter Roberson]

x=str2num(get(handles.Txo, 'string'));
y=str2num(get(handles.Tyo, 'string'));

Those will result in x and y being defined as numeric values (probably, but not always. str2double() would be better.)

syms x y;

Those are equivalent to

x = sym('x');
y = sym('y');

so whatever numeric values x and y had will be erased and replaced with symbolic values.

switch opcion
    case 1 %Método de Euler
        n=(xf-x)/h;

x is symbolic so n ends up symbolic

        for i=1:n+1

with n being symbolic, 1:n+1 tries to use the : operator with a symbolic upper bound, which fails.

[Santiago Gomez]

Thank you very much, you answer help me a lot :D