preallocate vector or not

Question

Salvatore Mazzarino 2012-9-22

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I'm doing a simulation and I initialize a vector that will contain during the simulation integer values. At the beginning I don't know its length so I can't initialize it to any value but I initialize it in this way:

vector = [];

as empty vector Starting my simulation integer values will be added depending on the results of this simulation. Matlab return me a suggestion to preallocate it for speed. This way it's correct or there is another way to do ?

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Answer 1

Thomas 2012-9-22

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编辑：Thomas 2012-9-22

Preallocation helps speed up code and reduces memory fragmentation..

The following link will help you understand how..

http://www.mathworks.com/support/solutions/en/data/1-18150/

Another good link on improving speed..

http://web.cecs.pdx.edu/~gerry/MATLAB/programming/performance.html

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Answer 2

José-Luis 2012-9-22

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编辑：José-Luis 2012-9-22

在 MATLAB Online 中打开

You should preallocate for speed, just try:

numVals = 10^5;
noAlloc = [];
Alloc = NaN * ones(numVals,1);
tic
for ii = 1:numVals
    noAlloc = [noAlloc 1];
end
toc
tic
for ii = 1:numVals
    Alloc(ii) = 1;
end
toc

I get:

Elapsed time is 10.128797 seconds. (No allocation)

Elapsed time is 0.000335 seconds. (Pre-allocation)

A difference of four orders of magnitude. I would recommend to allocate a vector as large as you think you might need, and then save your values in it, something like:

your_res = NaN * ones(reasonable_val,1);
counter = 1;
for ii = 1:your_simulations
 if you_get_result
    your_res(counter) = your_val;
    counter = counter + 1;
    if counter > reasonable_val %You could skip this, but it will return an error if counter > reasonable_val
       your_res = [your_res; NaN*ones(reasonable_val,1)];
    end
 end
end
your_res(counter:end) = [];

Alternatively, a cell array does not require contiguous memory allocation (well, all elements inside a cell are in contiguous memory locations), so it might be a (slower) alternative to your problem, as it does not require reallocating your array if it overruns an empty memory block.