MATLAB: Input: How to differntiate between numbers and strings

characterinputnumbersstring

Hello everyone,
I am currently writing a code that asks the user to enter a number, let's say 1, 2, or 3. However, I would like to include the case that the user accidently enters characters like 'xyz'. Matlab then displays by standard:
Error using input
Undefined function or variable 'xyz'.
What I want Matlab now to do is to just display a message like:
Invalid! Please enter 1, 2, or 3.
How can I do this?
Thanks a lot for your help!

Best Answer

You have several options:
1. Continue using input as is, but just wrap it in a try, catch block.
isokinput = false;
while ~isokinput
   try 
      userinput = input('enter some numbers');
      isokinput = true;  %this line only gets executed if input does not raise an error
   catch
      fprintf('Invalid expression. Please retry\n\n');
   end
end
2. Take the input as string, check that the string is valid. For example to check that only digit were entered (using regexp):
isokinput = false;
while ~isokinput
    userinput = input('enter some numbers', 's');
    if isempty(regexp(userinput, '^\d+$'))  %check that only digits have been entered 
       fprintf('Invalid expression. Please retry\n\n');
    else
       isokinput = true;
    end
end
3. Again, take the input as string. Attempt to convert it to number and complain if it doesn't:
isokinput = false;
while ~isokinput
    userinput = input('enter some numbers', 's');
    usernumber = str2double(userinput);  %will return nan if it can't convert the number
    if isnan(usernumber)   
         fprintf('Invalid expression. Please retry\n\n');
    else
       isokinput = true;
    end
end
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