This works:
P = -1;
while P<0
caUserInput = inputdlg('Enter a positive numeric value for an initial amount of money');
if isempty(caUserInput),return,end; % Bail out if they clicked Cancel.
% Convert to floating point from string.
usersValue = str2double(caUserInput{1})
% Check for a valid integer.
if isnan(usersValue)
% They didn't enter a number.
% They clicked Cancel, or entered a character, symbols, or something else not allowed.
message = sprintf('I said it had to be a positive number!');
uiwait(warndlg(message));
else
% Else the entry is okay.
P = usersValue;
% But check if it's negative.
if P < 0
message = sprintf('I said it had to be a positive number!');
uiwait(warndlg(message));
else
break;
end
end
end
uiwait(msgbox('Done with while loop'));
