How do I get my code to recognise an empty function input?

function P = poly_input(y)

    while
        if  isempty(y) == 1
            y = input('Invalid, no value entered. Enter a value: ', 's');
        end
    end

end

Basically, i'd like for a re-entry of number message to appear if no value is entered for y!

Please and thanks!

[Jessica Parry]

Trouble is... if you've not spotted... y is the value entered in the function (at the top of the question). So if y in the poly_input(y) is blank, then the message appears!

[Image Analyst]

Get rid of your while loop and say

if isempty(y)
  % Then use my code with the while loop.
end
% Continue with the rest of the function now since y is valid.

[Jessica Parry]

sadly this still isnt working how i hoped it would when typing in the command window the function name and a blank matlab still produces an error;

>> poly_input()

Error using Untitled (line 2)

Not enough input arguments.

[Image Analyst]

See my comment about how to handle nargin (number of input arguments) being zero.

Answer 1

[Wayne King]

isempty() should work

    y = input('What is your favorite color? ','s');
    if isempty(y)
       y = input('Invalid, no value entered. Enter a value: ', 's');
    end

Answer 2

[Image Analyst]

Try this:

while true
  % Ask user for a number.
  defaultValue = 45;
  titleBar = 'Enter a value';
  userPrompt = 'Enter the number ';
  caUserInput = inputdlg(userPrompt, titleBar, 1, {num2str(defaultValue)});
  if isempty(caUserInput) % Doesn't work.  Not empty if user input is null.
    return;
  end; % Bail out if they clicked Cancel.
  numericalValue = str2double(cell2mat(caUserInput));
  % Check for a valid integer.
  if isnan(numericalValue)
    % They didn't enter a number.
    % They clicked Cancel, didn't enter anything
    % or entered a character, symbols, or something else not allowed.
    message = sprintf('I said it had to be a number.\nTry again.');
    uiwait(warndlg(message));
  else
    break;
  end
end

[Image Analyst]

That's different! If you're going to pass in null from the command line, you should check nargin. Look it up in the help for how to handle it.

if nargin == 0
  % Use my while code
end

[Matt Butts]

If I'm not mistaken, this will also require a modification to the function definition:

function P = poly_input(varargin)
if nargin < 1
   y = input('Invalid, no value entered. Enter a value: ', 's');
else
y = varargin{1};
end

[Steven Lord]

Nope. This works just fine. [Whether it's preferred or not is a different question. Personally I'd probably have poly_input use narginchk to throw an error if you didn't call it with 1 input.]

function P = poly_input(y)
if nargin < 1
  y = input('Enter a value for y: ');
end
P = [y 0];

Call it like this and you will be prompted to enter a value.

P = poly_input()

Answer 3

[Garret Mans]

I know I'm only a few years late to this thread, but I'm confused why this question got so overcomplicated.

You just need the input and an initial while loop to catch the empty input. Maybe theres something I'm missing but as far as the question asked, this solves the problem just fine:

function gimmiStuff(y)

    while isempty(y)
        y = input('You gave me empty stuff. Gimmi stuff: ');
    end
    
    % Continue with function
    .
    .
    .
    
end