How to check if one of output variables is not called

If I have a function which can return multiple outputs, how can I tell from inside the function that some of the output variables are not called? One application is skipping a long calculation of an unused output variable.

Here is a non-working example. Can it be made to work?

% This would perform both calculations
[addIt, multIt] = test_empty_function_outputs1(2,3); 
% This would only perform the addition to get the 1st output
[addIt, ~] = test_empty_function_outputs1(2,3); 
% This would only perform the multiplication to get the 2nd output
[~, multIt] = test_empty_function_outputs1(2,3); 

function varargout = test_empty_function_outputs1(x,y)
if ~isempty(varargout{1}) % Only calculate if 1st output is called
    % Gives an error: Undefined function or variable 'varargout'
    varargout{1} = x+y;
end
if ~isempty(varargout{2}) % Only calcluate if 2nd output is called
    varargout{2} = x*y;
end

Answer 1

[Stephen23]

You can use nargout to detect how many output arguments are requested:

if nargout>0
    varargout{1} = x+y;
end
if nargout>1
    varargout{2} = x*y;
end

There is no direct way to detect which of those outputs are allocated to variables:

https://www.mathworks.com/matlabcentral/answers/379731-how-to-check-if-output-argument-is-ignored

https://www.mathworks.com/matlabcentral/answers/353759-check-which-output-arguments-are-requested-within-a-function

[KAE]

Sorry I missed those answers. Checking for non-called outputs seems useful so I will put in a feature request.

[KAE]

In response to my feature request, Matlab support provided a workaround adding some flag arguments to control which output should be calculated:

function [out1, out2, out3] = example(in1, in2, flag)
%define all outputs%
out1 = -1;
out2 = -1;
out3 = -1;
%calculate the output according to flag%
if flag(1) == 1
    out1 = 1;
end
if flag(2) == 1
    out2 = 1;
end
if flag(3) == 1
    out3 = 1;
end
end
To invoke the function: 
 
%here, skip output2%
[a, ~, c] = example(11, 22, [1,0,1])