Loop how to sum part of matrix

if I have a 9x3 matrix A=[123;123;456;212;121;111;312;141;010] AND i want to get matrix B dimension 3x3 (see photo) so that cell 1,1 would have a sum of 1+2+3 (so in first coulmn of A matrix taking 1,1 cell and 4,1 cell and 7,1 cell) and in the entry of matrix B 2,1 it would be 1+1+1 (taking from matrix A cell 2,1 and 5,1 and 8,1) and in matrix B the entry 3,1 would take from A 4+1+0 so 3,1 cell and 6,1 cell and 9,1 cell and the same logic for column 2 of B and column 3 of B to get at the end B=[647;385;577]

This was just an example in reality I need the same logic for much larger matrix dimension (from 1435x35 getting it into 35x35) so I would be really grateful for loop command or some other help to get the logic of this....

Answer 1

[Cris LaPierre]

Reshape your matrix to a 3x3x3 and then sum along the 3rd dimension.

[NINA BAJEC]

could you please help me with the reshape command and how then to sum every 3rd dimension in a row... I also need to get 3x3 B matrix and this summing over 3rd dimension would give me 3x1 outcome?

[Cris LaPierre]

The documentation I linked to contains examples. Take a stab at it yourself first. If you get stuck, share your code and a specific question about what you are not understanding.