Calculating a mean matrix from a large number of matrices

Question

Aaron Smith 2017-3-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/329566-calculating-a-mean-matrix-from-a-large-number-of-matrices

评论： Aaron Smith 2017-3-15

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); 
fprintf(1, 'Now reading %s\n', FileName); 
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?

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Stephen23 2017-3-14

@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 2017-3-14

编辑：Guillaume 2017-3-14

在 MATLAB Online 中打开

"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).

This would make your question dead easy to solve:

%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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Answer 1

Thorsten 2017-3-13

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https://ww2.mathworks.cn/matlabcentral/answers/329566-calculating-a-mean-matrix-from-a-large-number-of-matrices#answer_258453

编辑：Thorsten 2017-3-14

在 MATLAB Online 中打开

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.

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Aaron Smith 2017-3-15

编辑：Aaron Smith 2017-3-15

在 MATLAB Online 中打开

myFolder = uigetdir('C:\Users\c13459232\Documents\MATLAB'); % Generate command window to choose a folder
if ~isdir(myFolder) % if the directory is not a valid path
errorMessage = sprintf('Error: the following folder does not exist: \n%s', myFolder); % print this error message
uiwait(warndlg(errorMessage)); % block the execution of program and wait to resume
return;
end
outFolder = fullfile(myFolder, 'output'); % build full file name from parts in folder 'Output'
mkdir(outFolder); % create folder
filePattern = fullfile(myFolder, '*.asc'); % Call all files with '.asc' from the chosen folder
Files = dir(filePattern);  % list folder contents
finishCell = cell(length(Files), k);
for k = 1 : length(Files) % for all files files in the folder
baseFileName = Files(k).name;
FileName = fullfile(myFolder, baseFileName); %
fprintf(1, 'Now reading %s\n', FileName); % generates statements to indicate files are being read
fid = fopen(FileName); % open the file from chosen folder
Cell = textscan( fid, '%d', 'delimiter', ';'); % scanning data from files
fclose(fid); % close file from chosen folder
Data = cell2mat(Cell); % convert the cell data to matrix
N = 1024; % Number of numbers per row
skip = 2; % skip every second value to bin the matrix in half
Finish0 = reshape(Data, N, [])'; % reshape the data into the correct format
Finish1 = Finish0(1:skip:end, 1:skip:end);
finishCell{k} = Finish0;
newFileName = fullfile(outFolder, ['finish_' sprintf('%03d', k) '.txt']); % finish_001.asc, finish_002.asc, ...
dlmwrite(newFileName, Finish0, ';'); % create new files
eval(['finish_' sprintf('%03d', k) ' = Finish1;']); % create matrices in the workspace
disp([':: Now writing ' newFileName]); % generates statement to say the data is being written to file
end

This code is the original I had written using eval. I added the cell array lines to simply test how it works. This works completely fine, creates the 50 separate files (eval) and creates the cell array. Now, below is a new code I've written with eval removed and the if loop above added.

myFolder = uigetdir('C:\Users\c13459232\Documents\MATLAB'); % Generate command window to choose a folder
if ~isdir(myFolder) % if the directory is not a valid path
errorMessage = sprintf('Error: the following folder does not exist: \n%s', myFolder); % print this error message
uiwait(warndlg(errorMessage)); % block the execution of program and wait to resume
return;
end
 filePattern = fullfile(myFolder, '*.asc'); % Call all files with '.asc' from the chosen folder
Files = dir(filePattern);  % list folder contents
finishCell = cell(length(Files), k);
for k = 1 : length(Files) % for all files files in the folder
baseFileName = Files(k).name;
FileName = fullfile(myFolder, baseFileName); %
fid = fopen(FileName); % open the file from chosen folder
Cell = textscan( fid, '%d', 'delimiter', ';'); % scanning data from files
fclose(fid); % close file from chosen folder
Data = cell2mat(Cell); % convert the cell data to matrix
N = 1024; % Number of numbers per row
Finish0 = reshape(Data, N, [])'; % reshape the data into the correct format
finishCell{k} = Finish0;
if k == 1,
M = 1/length(Files)*Finish0;
else
M = M + 1/length(Files)*Finish0;
end
end

I'm not sure if I have written the new code correctly but I can fix that later. The problem is an error occurred which said "undefined function or variable k". After trying this new code, this even began happening with the original code which worked fine previously. Where might this problem with k have cone from?

Aaron Smith 2017-3-15

编辑：Aaron Smith 2017-3-15

在 MATLAB Online 中打开

The problem must be the k before the loop. I must have never tried the code to create a cell array with an empty workspace and k still had a value assigned from the previous code. That's about the greenest of rookie mistakes.

myFolder = uigetdir('C:\Users\c13459232\Documents\MATLAB'); % Generate command window to choose a folder
if ~isdir(myFolder) % if the directory is not a valid path
errorMessage = sprintf('Error: the following folder does not exist: \n%s', myFolder); % print this error message
uiwait(warndlg(errorMessage)); % block the execution of program and wait to resume
return;
end
 filePattern = fullfile(myFolder, '*.asc'); % Call all files with '.asc' from the chosen folder
Files = dir(filePattern);  % list folder contents
finishCell = cell(length(Files));
for k = 1 : length(Files) % for all files files in the folder
baseFileName = Files(k).name;
FileName = fullfile(myFolder, baseFileName); %
fid = fopen(FileName); % open the file from chosen folder
Cell = textscan( fid, '%d', 'delimiter', ';'); % scanning data from files
fclose(fid); % close file from chosen folder
Data = cell2mat(Cell); % convert the cell data to matrix
N = 1024; % Number of numbers per row
Finish0 = reshape(Data, N, [])'; % reshape the data into the correct format
finishCell{k} = Finish0;
if k == 1,
M = 1/length(Files)*Finish0;
else
M = M + 1/length(Files)*Finish0;
end
end

This code doesn't present any errors but I'm fairly sure the way i'm using finish0 inside my if loop is wrong though.

Guillaume 2017-3-15

编辑：Guillaume 2017-3-15

在 MATLAB Online 中打开

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 2017-3-15

Ah, I see.

Thanks a million Guillaume

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Calculating a mean matrix from a large number of matrices

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Calculating a mean matrix from a large number of matrices

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