Creating Graphical User Interfaces

Plot Push Button Callback

This GUI uses only the Plot button callback; the edit text callbacks are not needed and have been deleted from the GUI M-file. When a user clicks the Plot button, the callback performs three basic tasks - it gets user input from the edit text components, calculates data, and creates the two plots.

Getting User Input

The three edit text boxes provide a way for the user to enter values for the two frequencies and the time vector. The first task for the callback is to read these values. This involves:

• Reading the current values in the three edit text boxes using the handles structure to access the edit text handles.
• Converting the two frequency values (f1 and f2) from string to doubles using str2double.
• Evaluating the time string using eval to produce a vector t, which the callback used to evaluate the mathematical expression.

The following code shows how the callback obtains the input.

• % Get user input from GUI
  f1 = str2double(get(handles.f1_input,'String'));
  f2 = str2double(get(handles.f2_input,'String'));
  t = eval(get(handles.t_input,'String'));
  

Calculating Data

Once the input data has been converted to numeric form and assigned to local variables, the next step is to calculate the data needed for the plots. See the fft function for an explanation of how this is done.

Targeting Specific Axes

The final task for the callback is to actually generate the plots. This involves

• Making the appropriate axes current using the axes command and the handle of the axes. For example,

• axes(handles.frequency_axes)
  

• Issuing the plot command.
• Setting any properties that are automatically reset by the plot command.

The last step is necessary because many plotting commands (including plot) clear the axes before creating the graph. This means you cannot use the Property Inspector to set the XMinorTick and grid properties that are used in this example, since they are reset when the callback executes plot.

When looking at the following code listing, note how the handles structure is used to access the handle of the axes when needed.

Plot Button Code Listing

• function plot_button_Callback(hObject, eventdata, handles)
  % hObject    handle to plot_button (see GCBO)
  % eventdata  reserved - to be defined in a future version of MATLAB
  % handles    structure with handles and user data (see GUIDATA)
  
  % Get user input from GUI
  f1 = str2double(get(handles.f1_input,'String'));
  f2 = str2double(get(handles.f2_input,'String'));
  t = eval(get(handles.t_input,'String'));
  
  % Calculate data
  x = sin(2*pi*f1*t) + sin(2*pi*f2*t);
  y = fft(x,512);
  m = y.*conj(y)/512;
  f = 1000*(0:256)/512;;
  
  % Create frequency plot
  axes(handles.frequency_axes) % Select the proper axes
  plot(f,m(1:257))
  set(handles.frequency_axes,'XMinorTick','on')
  grid on
  
  % Create time plot
  axes(handles.time_axes) % Select the proper axes
  plot(t,x)
  set(handles.time_axes,'XMinorTick','on')
  grid on
  

Design of the GUI

List Box Directory Reader