Switch expression must be a scalar or string constant -- Matlab GUI

Hello to everyone,

I am creating a GUI where a user can select with radio button three different type of steels. For the first two type, I don't have a problem because I call "304" and "316". The third option instead is a string value called "STR18" and when I try to execute my script Matlab says " Switch expression must be a scalar or string constant". I think that the problem could be in the variable "steeltype", but I am not sure. How can I solve my problem? How can I use, at the same time, numbers and string for the switch function ?

Thanks in advance

function pushbutton1_Callback(hObject, eventdata, handles)

 st= get(handles.bottoni,'SelectedObject'); 
 steeltype=str2num(get(st,'String'));
 riduzione= str2num(get(handles.riduzione,'String'));
 e=riduzione;

 switch steeltype 
   case 1 
   ......
   case 2 
   ......
   case 3 
   ......
otherwise 
end

[Stephen23]

Why convert your perfectly nice strings into numbers? Get rid of the str2num and use strings to specify the switch cases.

Answer 1

[Adam]

Just keep them all as strings. You can't convert STR18 to a number so don't convert the others either, just switch on the strings instead.

Incidentally you should use str2double rather than str2num.

[Giuseppe Napoli]

Ok, but Matlab still doesn't recognize the variable STR18 ("Undefined function or variable 'STR18' "). In the GUI, in the string field, I wrote STR18 and with the "steeltype= get(st,'String'); " Matlab can read the STR18.What am I doing wrong ? Thanks again

P.S : I changed, in the string field, STR18 to 18 and magically everithig works fine. So there is something in the variable declaration.

[Stephen23]

"but Matlab still doesn't recognize the variable STR18"

Which is quite reasonable as you do not have any variable named STR18. You might have a string containing the characters 'STR18', but that is a totally different thing entirely.

Please show use the complete error message, this means all of the red text.

And note that once you get rid of the useless str2num the case will need to use the correct string, e.g.:

switch steeltype 
   case 'STR18'
       ...
   case 'blahblah'
       ...
end

This is just like the switch documentation explains and the examples clearly show. Did you read it?

[Adam]

STR18 is not a variable so I don't see why it would be expected to be, it is a string that will be contained in the 'steeltype' variable.

Changing to just '18' would work because it can be converted to a number so all your options are numeric.

[Giuseppe Napoli]

Ok, I solved all my problem. Since I removed the str2num all the case have to put between the two apostrophe ('STR18').

Thanks all for your help. Have a nice evening.

[Jan]

@Giuseppe: If the answer has helped you to solve the problem, please accept it.