How to speed up the following Matlab script? Can anyone suggest me to speed up the process?

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SA 2021-5-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/825570-how-to-speed-up-the-following-matlab-script-can-anyone-suggest-me-to-speed-up-the-process

评论： SA 2021-5-13

Array.mat

In the following scripts, I get expected result but the process is very slow. Is there any other way to make the execution faster/fastest ?

clc; close all; clear all;
load Array.mat; % B is the name
per_group = 1000;
[row,col]=size(B); % val would be a matrix of size (C*C)
limit=row/per_group;
value=cell(1,limit);
count=0;
val = zeros(col); % Memory Pre-allocation
for kk = 1:per_group:row
  count=count+1; 
  group_end = min(row, kk+per_group-1);
  for jj= 1:col
    f1 = A(kk:group_end,jj);
    for i=(jj+1):col
      f2= A(kk:group_end,i);
      [Pyy,freq]=cpsd(f2,f2,hanning(512),[],512,500);
      [Pxy,freq]=cpsd(f1,f2,hanning(512),[],512,500);
      [Pxx,freq]=cpsd(f1,f1,hanning(512),[],512,500);
      coh=(Pxy)./sqrt(Pxx.*Pyy);
      realCoh=real(coh(42));
      val(jj,i)=realCoh;
    end      
  end
  value{count}=val+val'+eye(col)
end

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Answer 1

dpb 2021-5-9

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https://ww2.mathworks.cn/matlabcentral/answers/825570-how-to-speed-up-the-following-matlab-script-can-anyone-suggest-me-to-speed-up-the-process#answer_695695

编辑：dpb 2021-5-9

在 MATLAB Online 中打开

Since you're doing all the work only to return coherence, look at

doc mscohere

to return the coherence directly with all the intermediares buried in the call instead of separate.

Alternatively, you could use the array syntax of cpsd and pass arrays instead of three calls with vectors--

...
[P,freq]=cpsd([f1 f2 f1],[f1 f2 f2],hanning(512),[],512,500);
coh=P(:,3)./sqrt(P(:,1).*P(:,2));
realCoh=real(coh(42));

The burying of "magic numbers" in code makes things hard to change/debug; if that number would change, you have to change the code and find all occurrences to ensure everything changes together. If it were a variable, then only change data value and (hopefully) the variable is used everywhere.

Of course, this value and the inevitable comparison to Deep Thought and The Hitchhiker's Guide to the Galaxy, is purely coincidental, I'm sure.

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dpb 2021-5-9

You caught the typo I had just corrected from the cut'n paste. The Answer is fixed now.

"As I've a very BIG data array (execution time takes some hours!)..."

I'd strongly suggest to then take a small section of the file and test the alternatives and time them rather than only working with the one huge dataset.

Make sure you get the right answer with the revised code compared to the original (again, just use one or a very few sections) first and you can also do the timing comparisons at the same time to see which option(s) hold out the most hope for improvement in speed.

I'd guess mscohere probably would still win, but the array input form should be able to run it a closer second, anyways, I'd think.

I'd intended to comment but forgot -- I'd also make my block size a power of 2 so is multiple of the fft blocksize.

You might also consider the overlap amount and whether you could use larger FFT size and still get required information. 512/1024 is awful short segment from an extremely long time record; is there a real reason for that?

We "know nuthink!" about the application so no idea about what might/might not make sense.

SA 2021-5-12

编辑：SA 2021-5-13

@dpb

I have a cell array of N*N size, each cell is a vector of 10 elements.

{0×0 double} {10×1 double} {10×1 double} {10×1 double}

{0×0 double} {0×0 double} {10×1 double} {10×1 double}

{0×0 double} {0×0 double} {0×0 double} {10×1 double}

{0×0 double} {0×0 double} {0×0 double} {0×0 double}

I want to extract individual elements of the cell sequentially and form a matrix like in the following. How can I form the matrix, A_1? Can you help me? @dpb

A_1 is a matrix consist of first element of each cell (as follows)=

0 x1 x2 x3

0 0 y1 y2

0 0 0 z3

0 0 0 0 ]; then the 2nd element of of every cell will form another 4*4 matrix=A_2,

3rd element of every cell will form 4*4 matrix=A_3,.....and so on up to A_10. With these A_1, A_2,...,A_10 matrices, I want to add its transpose+ Identity matrix(i.e. eye(4)). Finally, it should be like this

FullMatrix(1) = A_1+A_1’+eye(4)= 1 x1 x2 x3

X1 1 y1 y2

X2 y1 1 z3

X3 y2 z3 1;

FullMatrix(2), FullMatrix(3),......,FullMatrix(10). I've tried but failed to do it. Your support is highly appriciated @dpb. Thanks & Regards

dpb 2021-5-13

I don't follow that at all -- in the 4x4 cell array there is only one 10x1 vector, not 10 vectors.

Cut the size down to something manageable to write out by hand to illustrate because I have no idea what you're tyring to describe verbally, sorry.

You might want to look at cell2mat and kron in conjunction with cellfun in order to write something, I don't know....

SA 2021-5-13

Ohh, I got the solution from the following link https://www.mathworks.com/matlabcentral/answers/829123-extract-elements-of-each-cell-to-form-a-matrix-and-store-it-can-anyone-give-me-suggestions?s_tid=mlc_ans_email_view#comment_1518608. Thank you very much.

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