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How can I average multiple matrices by element to create a new matrix of the same size?

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I have 31 matrices, each of size 72x144, and each representing a day of the month of January. I would like to have a 32nd matrix, also of size 72x144, which includes the average value for each element in the original 31 matrices. For example, if I have:
A=[1,2,3|2,3,4|3,4,5] B=[2,3,4|3,4,5|4,5,6] C=[3,4,5|4,5,6|5,6,7]
I want:
D=[2,3,4|3,4,5|4,5,6] (the average by element of the originals)
What is the easiest way for me to do this? I have found a few threads here which have answers I don't really understand, as I am relatively unfamiliar with writing codes, for MATLAB or otherwise. I need something easy to understand.
Thanks, Patrick
Patrick on 2 Oct 2013
Azzi, please see my comment in reply to Image Analyst's answer. I was mistakenly unclear about the file names, and that adds some value to the correct answer, I presume. Thanks for your help!

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Accepted Answer

Image Analyst
Image Analyst on 2 Oct 2013
meanMatrix = (matrix1 + matrix2 + matrix3 + .... etc...... + matrix 30 + matrix31)/31;
Patrick on 2 Oct 2013
Awesome, and thank you so much for the quick reply! However, is there an easier way than that, even? I should have been more clear - the matrix .txt files are for the month of January 2004, and are written in the format "WP20040101.txt", "WP20040102.txt", etc.
And, on that topic - I am ultimately going to need to find an average matrix of the same size using data for the whole year (366 files, since 2004 was a leap year). After "WP20040131.txt", I have "WP20040201.txt". The answer you have provided would require me to type the file names for 366 separate matrices.
Since they are in chronological order, is there an easier way to write the code?
Thanks, Patrick
Image Analyst
Image Analyst on 2 Oct 2013
Edited: Image Analyst on 2 Oct 2013
If you don't already have them in arrays, then you can use the code in the FAQ to read in the arrays from a bunch of files:
myFolder = 'C:\Documents and Settings\yourUserName\My Documents\My Data Files';
if ~isdir(myFolder)
errorMessage = sprintf('Error: The following folder does not exist:\n%s', myFolder);
filePattern = fullfile(myFolder, '*.txt');
txtFiles = dir(filePattern);
for k = 1:length(txtFiles)
baseFileName = txtFiles(k).name;
fullFileName = fullfile(myFolder, baseFileName);
fprintf(1, 'Now reading %s\n', fullFileName);
thisArray = csvread(fullFileName); % Or however you read them in.
if k == 1
sumArray = thisArray;
sumArray = sumArray
meanArray = sumArray / length(txtFiles);

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More Answers (3)

Azzi Abdelmalek
Azzi Abdelmalek on 2 Oct 2013

Matt J
Matt J on 2 Oct 2013
Read the matrices into the slices A(:,:,i) of a 3D array and do

Tope Oyelade
Tope Oyelade on 20 Apr 2021
Edited: Image Analyst on 20 Apr 2021
Just add a dot before the '/' as below and you are good to go!
meanMatrix = (matrix1 + matrix2 + matrix3 + .... etc...... + matrix30 + matrix31)./31
Bruno Luong
Bruno Luong on 18 Jul 2022
Edited: Bruno Luong on 18 Jul 2022
@Sebastian Garzon Replace 0 with NaN the use 'omitnan' argumant
A(A==0) = NaN;
meanA = mean(A, 3, 'omitnan')
Or non destructive way:
meanA = sum(A,3) ./ sum(A~=0,3)
Image Analyst
Image Analyst on 18 Jul 2022
@Sebastian Garzon with Bruno's way you'd need to create A as a 3-D matrix:
A = cat(3, matrix1, matrix2, matrix3, .... etc...... , matrix30, matrix31);
A(A==0) = NaN;
meanA = mean(A, 3, 'omitnan')
Here's another way:
countMatrix = (matrix1 ~= 0 + matrix2 ~= 0 + matrix3 ~= 0 + .... etc...... + matrix30 ~= 0 + matrix31 ~= 0);
sumMatrix = (matrix1 + matrix2 + matrix3 + .... etc...... + matrix30 + matrix31);
averageMatrix = sumMatrix ./ countMatrix;

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