MATLAB: How to sum every nth row

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If there's a matrix, for example:
A = [1 2 3 4 5 6 ; 1 3 5 7 9 11 ; 2 4 6 8 10 12]'
A =
     1     1     2
     2     3     4
     3     5     6
     4     7     8
     5     9    10
     6    11    12
     7    13    14
I'm trying to sum 3 rows at a time (like an attached jpg file).
So I want to get:
 B = 
     6     9     12
     15    27    30
     7     13    14
Thanks a lot!

Best Answer

 A = [1 2 3 4 5 6 7; 1 3 5 7 9 11 13; 2 4 6 8 10 12 14]';
 [n,col] = size(A);
 index = 1:n;
 elem = [repmat(3,1,floor(n/3))];
 endv = n-sum(elem);
 if(~endv)
     endv = [];
 end
 index = mat2cell(index,1,[elem,endv])';
 B = cell2mat(cellfun(@(x) sum(A(x,:),1),index,'un',0));
 B =
     6     9    12
    15    27    30
     7    13    14
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