How to add a "if not a number" expression to GUI error dialog

Hi, I'm making a GUI and is trying to add error dialogs to it. I have an input pressure P, and was wishing to set minimum and maximum boundaries for the GUI to either work or show error dialogs. However when I used P=NaN to make an expression for a non-numerical input (like P=apple where apple is not defined numerically) I got error messages telling me that "the expression to the left of the equals sign is not a valid target for an assignment". What could I do instead?

Here is my code:

P = str2num(get(findobj('tag','gP'),'string'));      
if P>550.7921
elseif P<0
elseif P=NaN
  errordlg('No solution','Error')
  return
end

[Stephen23]

As an aside: use str2double rather than str2num: this guarantees only numeric scalars in the output, and avoids the eval inside str2num.

Answer 1

[M]

Are you searching for somethink like isnan ?

if isnan(P)
   errordlg('...');
end