# How to concatenate matrices to calculate mean

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Joydeb Saha on 20 Oct 2020
Commented: Jan on 20 Oct 2020
I have 40 matrices DJF1979_1980_125 to DJF2019_2020_125. Each matrix is 3x721x1440. I want a mean of all the matrices, that will be a single matrix. Please help.

KSSV on 20 Oct 2020
Read about mean, you can specify the dimension along which you want mean.
Stephen Cobeldick on 20 Oct 2020
"I have 40 matrices DJF1979_1980_125 to DJF2019_2020_125."
Accessing lots of numbered variables forces you into writing slow, ineffiicient, complex, buggy code:
The much better approach is to load data into an output variable
and access its fields. Or use indexing with any type of array.

KSSV on 20 Oct 2020
iwant = mean(A,3) ;

Joydeb Saha on 20 Oct 2020
workspace
Joydeb Saha on 20 Oct 2020
I think I need to write M = mean(A,3);
Joydeb Saha on 20 Oct 2020
I think I need to write M = mean(A,1);

Jan on 20 Oct 2020
As Steven has mentioned already, this is the main problem:
"I have 40 matrices DJF1979_1980_125 to DJF2019_2020_125"
Do not use strange names of variables to store importand data, bevause this impedes the processing. If you load these variables from files, store them in an array instead:
List = dir('*.mat');
Data = cell(size(List));
for k = 1:numel(List)
end
Then you can apply the mean() function either is another loop, or by cellfun, or concatenate the varibales in a 4th dimension at first:
AllData = cat(4, Data{:});
MeanData = mean(AllData, 3);

Joydeb Saha on 20 Oct 2020
I am getting this error :
Error using cat
Names of fields in structure arrays being concatenated do not match. Concatenation of structure arrays requires that these arrays have the same set of fields.
Stephen Cobeldick on 20 Oct 2020
If every mat file contain exactly one variable then you can do this inside the loop:
fnm = fullfile(List(k).folder, List(k).name);
Jan on 20 Oct 2020
@Jaydeb Saha: I do not know the contents of these files. Maybe you want to import a specific variable only?