Input: How to differntiate between numbers and strings?

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!

[Muhammad Usman Saleem]

share your code? then will get your problem

[Andreas Donauer]

You will not be able to do this with the input-function. Reason is, that whatever you enter gets evaluated. Meaning, your code says:

x = input('your prompt text');

If you/user now enters 'xyz', Matlab will look for the variable xyz in your workspace - and not find it. Therefore the input crashes and the input function just repeats itself. You can not even trigger a try/catch statement, so no way to do what you ask for.

Maybe consider the function "inputdlg" instead?

[Stephen23]

@Andreas Donauer: Why is this not possible ? It works perfectly:

>> str = input('please enter a number: ','s')
please enter a number: 123
str = 123
>> str = input('please enter a number: ','s')
please enter a number: xx
str = xx

and then convert to numeric as required.

Answer 1

[Guillaume]

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

[mtmtmt]

Thank you so much! I have implented option three, it works perfectly:)