Matt J
Posts:
4,976
Registered:
11/28/09
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Re: Temporary output when assigning into slices?
Posted:
Jan 23, 2013 4:05 PM
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"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <kdpg30$6gl$1@newscl01ah.mathworks.com>...
>
> > A(:,1)=B*x;
> >
> > Is this equivalent to
> >
> > z=B*x; %memory allocated here
> > A(:,1)=z;
> >
> > Or, does the output of B*x get directly generated in the memory locations occupied by A(:,1)?
>
> On recent MATLAB versions (2010a?), MATLAB is smart and puts directly the output in-place.
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OK. That's strangely inconsistent with things I'm seeing, but so be it.
> If the resulting is not in a contiguous memory, similar optimization on memory space will slow down the speed. I suspect that is the main reason why the optimization cannot be carried out in the later example.
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Don't you mean, that optimization cannot be carried out in the *latter* example? I can see for example why
A=B(1,idx)*C
might have to extract B(1,idx) to a continguous memory block when C is a matrix. In that case the elements in B(1,idx) will be accessed many times.
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