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Indexing via 3d array changed behaviour 2015a -> 2015b?

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Sven
Sven on 15 Oct 2015
Commented: Scott French on 21 Oct 2015
Hi all, just got 2015b, here's a strange one where indexing via a multidimensional array has a differently shaped output to previous versions:
Code:
a = 1:10;
b = reshape(4:6,1,1,[]);
a(b)
2015a and before:
ans(:,:,1) =
4
ans(:,:,2) =
5
ans(:,:,3) =
6
2015b:
ans =
4 5 6
I searched through the release notes and couldn't find anything that relates to this behaviour change. Did I miss something? Or is this a bug that I should report?
  1 Comment
David Young
David Young on 15 Oct 2015
I really hope someone answers this! It's very worrying that something as fundamental as array indexing might have quietly changed between versions. Of course the underlying problem is the inconsistent way indexing is applied to A(B) if both A and B are vectors. Since in this case isvector(b) returns 0, I think that the pre-2015b results are "correct", so to me it looks like a bug.

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Accepted Answer

Scott French
Scott French on 20 Oct 2015
This was an intentional change/bug fix. Prior to R2015b, there was an inconsistency in the MATLAB indexing behavior - if you try your experiment in R2015a, but with the value of "a" being stored in a field of a struct, or an element of a cell, you'd get a different answer:
>> s.a = 1:10
s =
a: [1 2 3 4 5 6 7 8 9 10]
>> b=reshape(4:6, 1, 1, []);
>> s.a(b)
ans =
4 5 6
It didn't seem reasonable that the same value could be indexed by the same index, but give different results depending on where the value came from. So we changed it to give the same result.
The rule that we apply is, for an expression R(I) where R and I both have only one non-singleton dimension, then the dimensions of the result have the length of I, and the orientation of R.
Scott
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