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From: Walter Roberson on 10 Aug 2010 00:03
Juliette Salexa wrote:

> The forloop looks like:
>
> for i=1:100
> A=reshape(bsxfun(@times,reshape(sum(reshape(A,[],N^2),2),1,[]),I),[],1);
> B(i)=summation of certain elements of A
> end

The calculation of A is not dependent upon the value of i, and thus the
entire calculation of A can be taken out of the loop.
From: Juliette Salexa on 10 Aug 2010 07:47
Walter Roberson <roberson(a)hushmail.com> wrote in message <sc48o.3830$1v3.2484(a)newsfe20.iad>...
> Juliette Salexa wrote:
>
> > The forloop looks like:
> >
> > for i=1:100
> > A=reshape(bsxfun(@times,reshape(sum(reshape(A,[],N^2),2),1,[]),I),[],1);
> > B(i)=summation of certain elements of A
> > end
>
> The calculation of A is not dependent upon the value of i, and thus the
> entire calculation of A can be taken out of the loop.

Hello, I'm not sure what you mean here.

My forloop is like:

i=1:100
A=A+A^3
B(i)=A
end

The calculation doesn't explicitly depend on i, but if I take it out of the forloop, I won't be able to get the next value of B (i think..)
From: Matt Fig on 10 Aug 2010 09:41
Walter Roberson <roberson(a)hushmail.com> wrote in message
> The calculation of A is not dependent upon the value of i, and thus the
> entire calculation of A can be taken out of the loop.

Walter,
Though the value of A is not dependent directly on the loop index, the value of A may change with each index since the value of A at each iteration is dependent on the value of A in the previous iteration.

Juliette,
I think we may need to know more about what goes on with B in order to help optimize your code. Do the values in A tend asymptotically towards a value, or oscillate about some value?
From: Andy on 10 Aug 2010 10:03
> Although I'm convinced that I remember seeing a blog post by Loren Shure which described how to avoid this, but for some reason I can't find it anymore =(

Could it have been one of these?:

http://blogs.mathworks.com/loren/2007/03/22/in-place-operations-on-data/
http://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/
http://blogs.mathworks.com/loren/2006/03/01/more-on-expansion-arrayfun/

I also agree with Matt Fig that we need more information about B. If you only need certain elements of A in each iteration, it's possible there's a way to calculate only those elements, which could save lots of time and space.
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