MATLAB: Aggregate every Nth row of a matrix

Question

Hi everyone

I want to aggregate every Nth row of my X by Y matrix, such that the matrix transforms into a N by Y matrix

Example: aggregate every 3rd row of a 6 by 3 matrix A, transforming A into a 3 by 3 matrix B.

A=
1    6    8
8    6    4    
2    5    12
6    8    7
14    8    6
7    21    5
B=
7    14    15
22  14    10
9     26    17

I have a 2464×44 matrix (F), of which I want to aggregate every 56th row, such that the matrix transforms into a 56×44 matrix. I know I can find the sum of every 56th element through the command: sum(F(1:56:end,:)1);

I just don't know how do this for each row and how to create the matrix that I need.

Thanks a lot in advance!!

Answer 1

Try this

A = [ ...
    1    6    8
    8    6    4
    2    5    12
    6    8    7
    14   8    6
    7    21   5];
n = 3;
B = zeros(n, size(A,2));
for i=1:n
    B(i,:) = sum(A(i:n:end, :));
end

Result

>> B
B =
     7    14    15
    22    14    10
     9    26    17

Alternative solution without for-loop

n = 3;
m = size(A, 1)/n;
idx = repmat(1:n, 1, m);
splitapply(@(x) sum(x), A, idx.')