Displaying an error message a non-numerical input is inputted in GUI

To be more clear, I'm trying to use GUI in MATLAB for a project. In this project I want the user to input numbers, but if they input a character (like 'a' or 'b', or 'djos') I want there to be an error message to pop up.

Currently I am using:

m = get(handles.filter);
if ~ischar(m)
   errordlg('ERROR: Input must be numerical.')
end

I've also used commands "error" and "msgbox", as well as "isnumeric" and "isstring" to no avail. I can get the error message to pop up just fine, however it even gives me the error message for fractions or numbers like "1" and "1/4"!

What am I doing wrong?

Answer 1

[Walter Roberson]

m = get(handles.filter, 'String');
d = str2double(m);
if isnan(d)
   fprintf('Entry "%s" is not a valid number!\n', m);
   return
 end
 ...

However, if you want '1/4' to be permitted, then you are going to have to describe exactly what operations are to be permitted. Is '3+4' okay? '3*4' ? '3.*4' ? '3^4' ? '3^3/4' (and if so should that be 27/4 or 3^(3/4) ?). How about 'sin(1.23)' ? 'pi' ? 'besselk(3,.8)' ?

[Isaac Lee]

Walter, thank you for your help.

Do you mean that each and every single operation has to explicitly be permitted? This is only an input for a filter window using the "filter" function in MATLAB, so only whole integers and fractions will be necessary.

I tried inputting your code into my GUI code, however I get error messages for letters AND any numerical input (even 1, or 1/4, or 0.25).

[Walter Roberson]

Is '7 3/4' permitted ?

What is handles.filter ? Is it a uicontrol style 'edit' ?

[Isaac Lee]

For the input it should be something like "1/4 1/4 1/4 1/4" or "1 1 1 1" as this would be the filter window.

handles.filter is just the edit text object in which I input this filter window. I do not believe it is a uicontrol style 'edit' as I only use this data in order to filter my data.

[Walter Roberson]

m = get(handles.filter, 'String');
if ~all(ismember(m, ['0':'9', '/', ' ']))
  fprintf('only digits, spaces and / permitted\n');
  return
end
parts = regexp(m, '(?<num>-?\d+)/(?<den>\d+)|(?<num>\d+(?!=/))', 'names');
numers = str2double({parts.num});
denoms = cellfun(@(S) str2double([S repmat('1', 1, isempty(S))]), {parts.den});

Now numers and denoms are row vectors of corresponding values. In places the user did not specify a denominator, the value is 1. The numerators may be negative; the denominators may not be. Either one may be 0. They have not been reduced to lowest terms.

[Isaac Lee]

Thank you I think I got it!