# Mean of certain elements within a matrix

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Malik Khan
on 11 Dec 2019

Commented: RAMPUNIT KUMAR
on 1 Sep 2021

I have a matrix which is a 372x6. Rather than find a mean for a specific row or a specific column, I want to find the mean of elements that lay in different rows as

Although the elements are from different rows, I have a pattern for the order in which they are to be taken. The last 2 elements (column 5 and 6) is to be averaged with the second third and fourth of the following row. The mean then needs to be found for each line. To make this easier to visualise I have written out a matrix for which we need to find 2 means. We need one mean to be of (e f h i j) and the other (k l m n o). For my case however I need to do this 371 times as we have 372 lines (obviously we can’t find mean for the first row).

(a b c d e f)

(g h i j k l)

(l m n o p q)

##### 1 Comment

RAMPUNIT KUMAR
on 1 Sep 2021

I too have a doubt, like for matrix

(a b c d e f g h I j k l m n o p q r)

I need to find the mean of a,b then c,d then e,f then g,h and so on upto last element. How could we do that if size is big enough.

### Accepted Answer

Stijn Haenen
on 11 Dec 2019

I think this will work:

for i=1:371

Mean_i(i)=mean([matrix(i,5:6),matrix(i+1,2:4)]);

end

##### 0 Comments

### More Answers (2)

Bandar
on 11 Dec 2019

Edited: Bandar
on 11 Dec 2019

A=[1 2 3 4 5 6; 7 8 9 10 11 12;13 14 15 16 16 18;19 20 21 22 23 24]

[row,~]=size(A);

for i=1:row-1

c = A(i,[5 6]);

r = A(i+1,[2 3 4]);

B = [c r]

mean(B)

end

##### 1 Comment

RAMPUNIT KUMAR
on 1 Sep 2021

Andrei Bobrov
on 11 Dec 2019

Edited: Andrei Bobrov
on 11 Dec 2019

Here A - your array (m x 6)

[m,n] = size(A);

i = repmat((0:m-1)',1,n-1);

i(:,end-1:end) = circshift(i(:,end-1:end),-1);

i = [zeros(m,1),i];

lo = i ~= 0;

out = accumarray(i(lo),A(lo),[],@mean);

or

[m,n] = size(A);

i = [zeros(m,1),reshape(circshift(repelem(0:m-1,n-1),-2),n-1,[])'];

out = accumarray(i(i>0),A(i>0),[],@mean);

##### 1 Comment

RAMPUNIT KUMAR
on 1 Sep 2021

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