How to determine if an output of a function-call is unused?

Question

Say I have a function foo that can return three values given an input:

function [a,b,c] = foo(input)

The calculations of variables b and c take a long time, so sometimes I may wish to ignore their calculation within foo. If I want to ignore both calculations, I simply call the function like this:

output1 = foo(input);

and then include nargout within foo:

if nargout == 1
    % Code to calculate "a" only
else
    % Code to calculate other variables

The problem occurs if I want to calculate the last output, but not the second. In that case my function call would be:

[output1,~,output3] = foo(input);

Now if I use nargout within foo to check how many outputs are in the function-call, it will always return 3 because the tilde operator (~) is considered a valid output. Therefore, I cannot use nargout to determine whether or not to calculate the second output, b, within foo.

Is there any other way to do this? I.e., is it possible to check which outputs of a function-call are discarded from within the function itself?

Answer 1

The commenters are basically right; this is not something that can be totally solved by the user unless The MathWorks adds functionality. However, I wrote a small function, istilde (Wayback Machine Archive), a while back that attempts to do what you ask. It works in many cases, but it's really a bit of hack and not a totally robust solution. For example, I did not attempt to get it to work for functions called from the Command Window directly (this could potentially be added with some work). Also, it relies on parsing the actual M-file, which can have issues. See the included demo file for how one might use istilde.

Feel free to edit my code for your needs – just don't use this in any production code due to the robustness issues. Any improvements would be welcome.