# How to separate two matrices from a single matrice?

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Yamina chbak on 8 Feb 2023
Commented: Yamina chbak on 8 Feb 2023
Hi, I would like to extract two matrix B and C from a matrix A to get A=B+C, where A=sparse(N,N) with N is a number.
Since index of A has two types : index = index_1 + index_2,
so the matrix B and C should be like
B= [A(i,j) , for iindex_1 and jindex;
0 otherwise]
C = [A(i,j) , for iindex_2 and jindex;
0 otherwise]
For example A= sparse(9,9):
A = [ 1 2 3
5 6 7
8 9 10];
index = {1,2,...,9}, index_1 = {2,4,6,8}, index_2={1,3,5,9}
how to write in matlab
B = sparse(9,9); C=sparse(9,9);
B = A(index_1,:) and C = A(index_2,:)
to get
B = [ 0 2 0
5 0 7
0 9 0];
C = [ 1 0 3
0 6 0
8 0 10];

the cyclist on 8 Feb 2023
Edited: the cyclist on 8 Feb 2023
Here is one way:
% Inputs
A = [ 1 2 3
5 6 7
8 9 10];
index_1 = [2,4,6,8];
index_2 = [1,3,5,9];
% Get the dimensions of A
[m,n] = size(A);
% Initialize the B and C as sparse arrays that are the size of A
[B,C] = deal(sparse(m,n));
B(index_1) = A(index_1)
B =
(2,1) 5 (1,2) 2 (3,2) 9 (2,3) 7
C(index_2) = A(index_2)
C =
(1,1) 1 (3,1) 8 (2,2) 6 (3,3) 10
##### 2 CommentsShowHide 1 older comment
Yamina chbak on 8 Feb 2023
Now i can fix it , just edit code from @the cyclist
B(index_1,[1:n]) = A(index_1,[1:n])
C(index_2,[1:n]) = A(index_2,[1:n])

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