Preallocating memory for a matrix of unknown size

Question

Simon 2013-6-24

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https://ww2.mathworks.cn/matlabcentral/answers/80041-preallocating-memory-for-a-matrix-of-unknown-size

回答： Zhiyu Xiao 2019-4-18

I have a number of points I need to store in a matrix - an m*2 to be specific. The problem being I do not know how many I need to store beforehand, but it typically ranges from 1500 - 15000. Is there a more efficient way of allocating memory than just using an arbitrarily big size that I know will definitely fit everything, or should I increment the matrix size by 1 with every step? theoretically I could run the code once before and just count the number of points, but I wont do that, because the code is too slow already.

So what's the best way to to this?

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Answer 1

Iain 2013-6-24

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At what point do you know the required size?

If it's "at the end", then you do have to do with the "arbitrarily big" option, but you can do that piece-wise. You can preallocate, say, 5000, and when you get to 5001, allocate an extra 5000...

If it's at "some defined point in my code", you can allocate at that point.

Alternatively, you could not store the results in RAM, if it's an issue, you should consider writing a file and popping each pair of values in.

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Answer 2

Hugo 2013-6-24

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You can use a buffer. I mean, suppose the matrix you want is M, then create M=[]; and a vector X=zeros(xsize,2), where xsize is a relatively small value compared with m (the number of rows of M). Then, fill X and when it is filled, just concatenate the matrix with M by doing M=[M; X]; and start filling X again from the first row on. That should save some time.