Matlab: Behavior of the unique function

Question

I'm currently migrating code from R2012a to R2013b.

I noticed that the unique function behavior has changed:

R2012a

>> size(unique([]))

ans =

     0     0

R2013b

>> size(unique([]))

ans =

     0     1

It seems counter-intuitive to me that a 0x0 matrix would become a 0x1 matrix after removing doublons, which is essentially what the unique function does. Does anybody has a rationale for this?

This is possibly related: [Iterating an empty matrix using a for loop](http://stackoverflow.com/questions/17950940/iterating-an-empty-matrix-using-a-for-loop) — Dennis Jaheruddin, Dec 05 '13 at 11:15

score 7 · Answer 1 · answered Dec 05 '13 at 10:56

7

The behaviour has changed with R2013a, if you need the old behaviour use:

size(unique([],'legacy'))

If you need code for both versions, I would recommend to write some function which calls unique(x,'legacy') for new versions and unique(x) for old versions.

btw: same issue with union, intersect, setdiff, setxor and ismember

answered Dec 05 '13 at 10:56

Daniel

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Here are [the release notes](http://www.mathworks.nl/help/matlab/release-notes.html), expand all of 2013a and search for `unique`. – Dennis Jaheruddin Dec 05 '13 at 11:26
@DennisJaheruddin I looked through the release notes and couldn't find a mention to it. Am I blind? – Eric Salemi Dec 05 '13 at 12:46
1

Ok found it under "mathematics". Makes sense. – Eric Salemi Dec 05 '13 at 12:53

score 1 · Answer 2 · edited Dec 05 '13 at 12:39

1

I don't know whether this is the reason, but it does come with an advantage.

Now you will see that unique(M) gives the same output as unique(M(:)), even if M is empty.

Example:

M = magic(5);
isequal(size(unique(M)), size(unique(M(:)))); 
M = [];
isequal(size(unique(M)), size(unique(M(:))));

The latter returns false on old versions of matlab, this may be confusing.

edited Dec 05 '13 at 12:39

Rody Oldenhuis

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answered Dec 05 '13 at 11:21

Dennis Jaheruddin

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Matlab: Behavior of the unique function

2 Answers2

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