# preallocate vector or not

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Salvatore Mazzarino 2012 年 9 月 22 日
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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### 回答 (3 件)

Thomas 2012 年 9 月 22 日

Preallocation helps speed up code and reduces memory fragmentation..
Another good link on improving speed..

#### 0 件のコメント

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José-Luis 2012 年 9 月 22 日

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.

#### 2 件のコメント

Azzi Abdelmalek 2012 年 9 月 22 日
not fair comparison, why
noAlloc = [noAlloc 1]
and not
Alloc(ii) = 1
Elapsed time is 0.058491 seconds.
Elapsed time is 0.010952 seconds.
José-Luis 2012 年 9 月 22 日
Because that's how I understood how Salvatore intended to build his results matrix.

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Azzi Abdelmalek 2012 年 9 月 22 日
I think, if you have an idea about your vector size, your code will be faster, if you preallocate:
A=zeros(1,m) % for example

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