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Topic: indexing surprise
Replies: 7   Last Post: Aug 4, 2011 9:53 AM

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Andreas Lobinger

Posts: 12
Registered: 7/29/11
Re: indexing surprise
Posted: Aug 3, 2011 11:26 AM
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Hello colleague,

"Ben" wrote in message <j1bokj$bo8$1@newscl01ah.mathworks.com>...
> "Andreas Lobinger" wrote in message <j1bnbb$777$1@newscl01ah.mathworks.com>...
> > Hello colleagues,
> > i'm now into debugging for some time (~ 1h) until i recognized that Matlab
> > has a quite surprising way of creating new variables.
> > ...
> > And now the punchline:
> >

> > >> a(idx_new) = par.height(net.bt(idx_new))
> >
> > a =
> >
> > 10.5000 10.5000 10.5000
> >
> > so although my righthand side is a column vector and my index to a is a
> > column vector , the result in the new created a is a row vector.


> The moral of the example: If you want to store your vector in a column, you need to specify that directly.

Which is what i do now in the program. I had for some time the impression that the size of the index defines the field.

> As to why that is, I'm guessing a bit here, so I hope someone corrects me if I'm off, but I think MATLAB stores things in a row because it's faster. (If you do indexing through a matrix row by col vs col by row, you will see different speeds for large matrices.

This may be, however my experiences with linear addressing (which i use quite a lot) is that a linear index goes first dimenstion -which means columnwise- first.

But all this maybe have some historical backwards compatibility reasons.

Wishing a happy day,
Andreas



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