Check which output arguments are requested within a function

I have a custom function that returns multiple arguments, say:

    function [arg1, arg2, arg3, arg4] = myfun(x,y,z)
     % Code
    end

Now, we can call that function e.g. in such way:

    [A, B, ~, D] = myfun(3,2,1)

If we make such call, no value has to be assigned to variable arg3 within myfun. I want to know if there is a way to check within the function body which output arguments have been called (same as arg1, arg2, arg4 above), and which outputs have been neglected (i.e. arg3 which is called with tilde).

There is a number of functions dealing with i/o arguments, but I haven't been able to find a solution to this question in the documentation of 2015a version.

[Adam]

Have you nosied into the code of Matlab functions that do this to see what they do? I don't know off-hand. They may just calculate all results anyway or they may have a more intelligent method.

Certainly nargout is not of use here since you are requesting non-contiguous output arguments.

Answer 1

[Stephen23]

Currently there is no inbuilt way to detect this:

https://www.mathworks.com/matlabcentral/newsreader/view_thread/325735.html

https://www.mathworks.com/matlabcentral/answers/176207-mex-and-matlab-check-if-an-output-is-skipped-unused-i-e-marked-with-tilde

Although it would be possible with some extremely inefficient and non-robust file parsing, e.g.:

https://stackoverflow.com/questions/19498911/how-to-determine-if-an-output-of-a-function-call-is-unused-matlab

Answer 2

[Jan]

You can implement this manually:

function Out = myfun(x,y,z, Want)
% Want: cell string containing the names of the wanted outputs:
if any(strcmpi(Want, 'A'))
  Out.A = rand(1);
end

if any(strcmpi(Want, 'B'))
  Out.B = 'any string';
end
...
end

And automatic solution with identifying the "~" would be much nicer. Note that Matlab does have this information during the runtime already, but there is no way to provide this detail to the called function. I assume this is built into the design of Matlab, because this would require forwarding information to a function, which is used in the future -- after leaving the function.

You can send an enhancement request to MathWorks.