What does it mean when ~ is used as one of the outputs for a function?

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I have seen a few scripts where ~ is used as one of the output variables for a function/command? What is this supposed to mean and how is it useful? For example, in one of the lines for the script for the function divergence.m, I find the line [px, ~] = gradient(u);

Accepted Answer

James Tursa
James Tursa on 28 Oct 2016
Edited: James Tursa on 28 Oct 2016
It means that the particular output that would ordinarily be returned to a variable at that spot is instead discarded automatically. E.g.,
[px, ~] = gradient(u);
is essentially equivalent to:
[px, dummy] = gradient(u); clear dummy
James Tursa
James Tursa on 28 Oct 2016
All good points. I will add that the ~ does not prevent the function from actually calculating that particular output argument. It is in fact calculated (the function still thinks it needs to return two outputs) ... it is just automatically discarded per the syntax. So using ~ is more of a coding convenience rather than a resource or time saver. (Is there a way for the function to detect when the caller has used ~ for an output?)

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More Answers (1)

LauraLee Austin
LauraLee Austin on 28 Oct 2016
It indicates that you are not using that output variable.
[FX,FY] = gradient(F)
Let's say you wanted the FY variable but not FX:
[~,FY] = gradient(F)


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