MATLAB SIMULINK 7 - GRAPHICAL USER INTERFACE Manuale Utente Scaricare il pdf (Pagina 271)

100

101

UI That Accepts Parameters and Generates Plots

9-25

a MATLAB expression. Because you can type many things that eval cannot handle,

the first task is to make sure that eval succeeded. The t_input_Callback uses try,

catch blocks to do the following:

• Call eval with the t_input string inside the try block.

• If eval succeeds, perform additional tests within the try block.

• If eval generates an error, pass control to the catch block.

• In that block, the callback disables the Plot button and changes its String to

'Cannot plot t'.

The remaining code in the try block makes sure that the variable t returned from eval

is a monotonically increasing vector of numbers with no more than 1000 elements. If t

passes all these tests, the callback enables Plot button and sets its String to 'Plot'. If

it fails any of the tests, the callback disables the Plot button and changes its String to

an appropriate short message. Here are the try and catch blocks from the callback:

function t_input_Callback(hObject, eventdata, handles)

% Disable the Plot button ... until proven innocent

set(handles.plot_button,'Enable','off')

try

t = eval(get(handles.t_input,'String'));

if ~isnumeric(t)

% t is not a number

set(handles.plot_button,'String','t is not numeric')

elseif length(t) < 2

% t is not a vector

set(handles.plot_button,'String','t must be vector')

elseif length(t) > 1000

% t is too long a vector to plot clearly

set(handles.plot_button,'String','t is too long')

elseif min(diff(t)) < 0

% t is not monotonically increasing

set(handles.plot_button,'String','t must increase')

else

% All OK; Enable the Plot button with its original name

set(handles.plot_button,'String','Plot')

set(handles.plot_button,'Enable','on')

return

end

% Found an input error other than a bad expression

% Give the edit text box focus so user can correct the error

uicontrol(hObject)

catch EM

% Cannot evaluate expression user typed

set(handles.plot_button,'String','Cannot plot t')

% Give the edit text box focus so user can correct the error

uicontrol(hObject)

end

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