structures, cells or high dimensional arrays

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Patrick Mboma
Patrick Mboma 2013-6-24
Dear all,
I have to deal with arrays of dimension 4 or 5. So far, I can only think of 3 ways to represent such arrays.
  1. Using matrices only, the representation of the i-j-k-l-m can be written as A(i,j,k,l,m)
  2. using structures, it would be A(i).V(j,k,l,m)
  3. finally using cells, it is A{i}(j,k,l,m)
My problem is that I have non-vectorizable loops around those elements and I was wondering 1) which one of those representations has the greatest speed, 2) whether there are more efficient representations and 3) whether it is possible to accelerate operations involving such arrays.
Thanks, Patrick

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Matt J
Matt J 2013-6-24
编辑：Matt J 2013-6-24
Why is calling say P faster than calling P(:,:)?
The indices (:,:) have to be processed in the second case, just like any other indexing expression, e.g., P(:,1:2:end).
Perhaps the MATLAB developers could optimize things by giving special treatment to the colons-only syntax (:,:,...,:) but why would they bother? Who would ever use P(:,:) when just P is enough?
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James Tursa
James Tursa 2013-6-24
编辑：James Tursa 2013-6-25
@Patrick: FYI,
P(:,:,j)=A % This copies the A data into P
Q{j}=A % This makes a shared data copy of A and puts the address of it into Q
R(j).V=A % This makes a shared data copy of A and puts the address of it into R
Subsequent operations that you do will typically drive which one is fastest for your application. E.g., doing P(:,:,j) downstream will cause a data copy of the entire contents of P(:,:,j) to be made if it appears on the right-hand-side of an assignment or as an argument to a function, whereas doing Q{j} downstream will only cause a shared data copy of Q{j} to be made. So for read-only purposes you may find that Q{j} is faster. But if you are only modifying the contents downstream and P(:,:,j) only appears on the left-hand-side of assignments, then P(:,:,j) may end up being faster for you. The "fastest" question can only be answered by your testing and how you are accessing the data downstream.

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Patrick Mboma
Patrick Mboma 2013-6-24
I plan to formally test the three versions. But a more simple question is the following: suppose we just have 2 dimensions. Why is calling say P faster than calling P(:,:)?
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Sean de Wolski
Sean de Wolski 2013-6-24
编辑：Sean de Wolski 2013-6-24
passing P, passes a reference to P and does not make a memory copy. Indexing into P using P(:,:) copies the memory into a new array. You can see this by:
Opening the task manager (Windows) and looking at performance -> physical memory usage.
Then run:
A = magic(10000);
You'll see a jump in memory. If you then run:
B = A;
No change, since B is merely a reference to A.
C = A(:,:)
It jumps again.

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