MATLAB: Contiguous memory and relational operators

permute

I have a 2D matrix of floats called data (approx size 1E5 x 2E2) and wish to test some conditions many times (>1e9 times) eg
data(i:j, h) <= k, where k is a float
This process is a real bottleneck in my code according to the profiler.
I have been reading http://www.mathworks.com/matlabcentral/answers/64457-why-are-relational-operators-so-slow-in-this-case
Here the author (Jan) seems to suggest running permute.m to make the memory contiguous before applying the inequality operator.
I am unclear how to order the permutation though. What have I missed? (or is the permute trick only valid in dimensions above 2?)
tmp = permute(data(i:j, h) , orderVec); %where does orderVec come from???
tmp <= k %this is now fast

Best Answer

The permute method is useful, when a large multi-dimensional array is indexed repeatedly. In the other thread it was P(M, 4, N) with large M and N. Then calling P(:,i,:) repeatedly consumes much more time than getting Q(:, :, i) after:
Q = permute(P, [1,3,2]);
In your case, the comparison could be performed once only:
comp = (data <= k);
And instead of comparing in a loop, the vector comp(i:j, h) can be used directly. But I assume this is not a dramatic improvement. More precise advices are possible if you post the relevant part of the code.
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