What does a tilde (~) inside square brackets mean?

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Delvin
Delvin 2013-4-18
评论： Adam Danz 2021-1-3
[~, Palette] = kmeans(reshape(B(:),M*N,3),8,'E','s','S','U');
Specifically, what does the ~ inside the square brackets represent (e.g. a matrix with multiple LHS assignment)?

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采纳的回答

Tanguy
Tanguy 2013-4-18
The function kmean can be used with two outputs :
[IDX,C] = kmeans(X,k)
Here you use the brackets to define the two outputs (it is not an array). Here, IDX will be the first output (a vector containing the cluster indices of each point) and C will be the second one (a matrix containing the cluster centroid locations).
So the brackets don't mean that you have just one ouput (an array), but they are used to gather the outputs.
And whatever happens, IDX will be the first output and C the second one.
But if you just want to know C (and you don't care IDX), it is not usefull to assign this value to a variable.
So, when you use [~,palette], that means that you just want the second output of your function, and do not care the first one.
Use this is helpfull for you (you don't have many useless variables in your workspace), and is faster!
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Walter Roberson
Walter Roberson 2021-1-3
Does this '~' save the time for calculating IDX, or it calculates both IDX and C but only output C?
As far as the community experts have been able to tell, the implementation is that an output slot is still created for the ~ variables, but that no symbol table entry is created and the output is released afterwards -- the same way that for 1 + 2 + 3 an output slot must be created for the (1+2) part but the slot is unnamed and will be released.
As far as the community experts have been able to find, there is no documented method or flag on a variable that could be used by a function to determine that a particular ~ output is going to be thrown away.
You can use nargout to probe what the index of the last programmed output is, but you cannot tell whether any of them are ~ or not.
Adam Danz
Adam Danz 2021-1-3
> Does this '~' save the time for calculating IDX, or it calculates both IDX and C but only output C?
This discussion focuses on that topic:
https://www.mathworks.com/matlabcentral/answers/580671
TL;DR: no but there are ways to communicate which output variables should be produced in your function.

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更多回答（2 个）

Walter Roberson
Walter Roberson 2013-4-18
It is equivalent to
[temp, Palette] = kmeans(reshape(B(:),M*N,3),8,'E','s','S','U');
clear temp
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Delvin
Delvin 2013-4-18
What is "temp" ? Is it short for temporary as in temporary variable? Also, what does the square brackets mean ie is this an array ? Thanks
Walter Roberson
Walter Roberson 2019-5-9
[ThIsVArIAblEiZnOTuzED, Palette] = kmeans(reshape(B(:),M*N,3),8,'E','s','S','U');
clear ThIsVArIAblEiZnOTuzED
and the [] mean that multiple outputs are being returned from the function. It is not an array.

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Ankur Bhardwaj
Ankur Bhardwaj 2017-5-24
Whether it is supported in Matlab Version 2009 or not.
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Steven Lord
Steven Lord 2017-5-24
This functionality was introduced in release R2009b. So it depends what you mean by "Version 2009" -- release R2009a no, release R2009b yes.

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