Asked by Margaux
on 4 Jan 2013

Aloha,

I need to process 72 hours of sampling at a rate of 25.6 kHz. The MatLab files I am given have the form of a structure, with string fields containing instrument information, units, etc. The numerical data itself is contained in 262144x2 double fields named *StreamSegment000000_000255*, *StreamSegment000256_000511*, and so forth, each of which containing 10.24 seconds of data. The file I am currently working on has roughly 2 hours of data, divided into 703 10.24-second fields of size 262144x2 double and a 2.84-second field of size 72704x2 double (of course, files will have different sizes). The first thing I need to compute is 30-second RMS for all 72 hours. Here is my approach:

S=load('REC0049.mat'); cell0=struct2cell(S); C=cell0(3:end,:); % crops out the first 2 arrays of junk information

Now cell2mat(C) will of course lead me "??? Error using ==> cat; Out of memory", so I tried to break down to several matrices.

for i=1:number eval(sprintf('M%d = cell2mat(C((1+3*(i-1)):(3*i),:));', i)); end

If number=235, obtaining 235 matrices each containing 3 of these 10.24-second fields would allow me to easily compute 30.72-second rms (close enough) for these 2 hours. Of course, this works just fine for small "numbers", but I quickly encounter "*??? Error using ==> cat Out of memory" again.

I could try nested for-loops and progressively clear out the memory, but I am sure there are solutions that are a lot more elegant and efficient.

Thank you!

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Answer by Matt J
on 4 Jan 2013

Edited by Matt J
on 4 Jan 2013

Accepted answer

So if I understood you, the fields named StreamSegment000000_000255, StreamSegment000256_000511 are the fields of S when you do

S=load('REC0049.mat');

In other words, you generated REC0049.mat by using eval to auto-name 703 different variables and saved them to a .mat file? I hope you know that that's a bad idea, as described here.

Instead, you could use MATFILE to write the segments to and from the elements of a cell array C in REC0049.mat.

C=cell(1,number); save REC0049 C;

matObj = matfile('REC0049.mat','Writable',true);

for i=1:number*3

matObj.C(1,i)={data_chunk_to_write};

end

Then later

rms=zeros(1,number); for i=1:number

rms(i) = computeRMS( cell2mat(matObj.C(1, (1+3*(i-1)):(3*i),:) ) )

end

Answer by Malcolm Lidierth
on 5 Jan 2013

Depending on the MAT-file version, there may be something that could help in:

Opportunities for recent engineering grads.

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