# Calculating a mean matrix from a large number of matrices

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Aaron Smith il 13 Mar 2017
Commentato: Aaron Smith il 15 Mar 2017
I have been trying to calculate an average matrix from 50 matrices. Using squeeze and cat, i have calculated the mean to two matrices. My question was; how is it possible to get the mean of 50 matrices. I have a code to open the files from a folder and reformat and bin them.
myFolder = uigetdir('C:\Users\c13459232\Documents\MATLAB');
if ~isdir(myFolder)
errorMessage = sprintf('Error: the following folder does not exist: \n%s', myFolder);
uiwait(warndlg(errorMessage));
return;
end
outFolder = fullfile(myFolder, 'output');
mkdir(outFolder);
filePattern = fullfile(myFolder, '*.asc');
Files = dir(filePattern);
for k = 1 : length(Files)
baseFileName = Files(k).name;
FileName = fullfile(myFolder, baseFileName);
fid = fopen(FileName);
Cell = textscan( fid, '%d', 'delimiter', ';')
fclose(fid);
Data = cell2mat(Cell);
N = 1024;
skip = 2;
Finish0 = reshape(Data, N, [])';
Finish1 = Finish0(1:skip:end, 1:skip:end);
newFileName = fullfile(outFolder, ['finish_' sprintf('%03d', k) '.txt']);
dlmwrite(newFileName, Finish0, ';');
eval(['finish_' sprintf('%03d', k) ' = Finish1;']);
disp([':: Now writing ' newFileName]);
end
Where could i insert squeeze(cat()) to average all of these matrices. The example I found to get the mean of two matrices is:
c2 = squeeze(mean(cat(3,finish_002,finish_004), 3));
Would you suggest a variation on this code or is there a better way to get an average matrix?
##### 9 CommentiMostra 7 commenti meno recentiNascondi 7 commenti meno recenti
Stephen23 il 14 Mar 2017
@Aaron Smith: using indexing is going to be much easier to work with.
If you want to know why, read that link I gave you.
Guillaume il 14 Mar 2017
Modificato: Guillaume il 14 Mar 2017
"You see, when each full matrix is saved as a single cell in a cell array, I'm not sure if this will give me the freedom i need to plot each one individually and to bin each one individually"
It's the exact opposite. Having all the matrices as cells of a cell array gives you the freedom to treat each them individually any way you want. Use a fixed index in your cell array, and the freedom to operate on all them at once. Having individually named matrices you sacrifice the latter and don't gain anything in return.
As a rule, if you're numbering variables you're doing something wrong. These numbered variables are obviously related so they should be stored together in a container of some sort (matrix, cell array, containers.map, whatever).
%finish: cell array of matrices
meanofallmatrices = mean(cat(3, finish{:}), 3); %no need for squeeze
And since all the matrices are obviously the same size, you could store them directly as a 3D array instead of cell array (with again no loss of freedom)
Stephen is right, eval is completely the wrong approach. Here be dragons!

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### Risposte (1)

Thorsten il 13 Mar 2017
Modificato: Thorsten il 14 Mar 2017
In the for-loop, you can just use
if k == 1,
M = 1/length(Files)*Data;
else
M = M + 1/length(Files)*Data;
end
Then after the loop, M is the mean of the matrices.
##### 10 CommentiMostra 8 commenti meno recentiNascondi 8 commenti meno recenti
Guillaume il 15 Mar 2017
Modificato: Guillaume il 15 Mar 2017
You've got a closing bracket in the wrong location, the second 3 is an argument to mean, not cat.
mean(cat(3, finishCell{:}), 3);
%bracket goes here ------^
That second 3 tells mean to average across pages instead of the default across rows as you saw.
Aaron Smith il 15 Mar 2017
Ah, I see.
Thanks a million Guillaume

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