Creating a submatrix from a matrix

I want a code to create a matrix which consist of rows and columns of another matrix.

i.e. A (4X4) = [ 1 2 3 4; 5 6 7 8; 1 3 5 7; 2 4 6 8; ]

The submatrix B consist of the { 1, 2, 4 }rows of A and the { 2,3 }columns of A:

Β (3Χ2) = [ 2 3; 6 7; 4 6; ]

Any help could be useful.

Thanks in advance!

[said mohamed]

Using the matrix A = [5 1 11; 7 13 3; 8 5 2], the matrix B is constructed as B = [A A A; A A A; A A A]. Which of the following is the result of the operation K = L * J, made using the submatrices of matrix B, L = B (1: 3,3: 5) and J = B (2: 4,2: 3)?

Answer 1

[Azzi Abdelmalek]

A= [ 1 2 3 4; 5 6 7 8; 1 3 5 7; 2 4 6 8; ]
B=A([1 2 4],[2 3])

[Konstantinos]

Thanks a lot!

[Mahadi Hasan]

thanks a lot brother.

[Rajib Das]

Instead of specifying particular rows and columns can I specify range of rows and columms

[Stephen23]

@Rajib Das:

A(2:4,2:3)

[Himalsha Dharmapala]

Thanks

[said mohamed]

Using the matrix A = [5 1 11; 7 13 3; 8 5 2], the matrix B is constructed as B = [A A A; A A A; A A A]. Which of the following is the result of the operation K = L * J, made using the submatrices of matrix B, L = B (1: 3,3: 5) and J = B (2: 4,2: 3)?

Answer 2

[VANSHUL CHOUDHARY]

A = rand(4,3);
%  Get those elements of A that are located in rows 3 to 4 and
% column 2 to 3.
sub_matrix = A(3:4,2:3);

[Justin Paulan]

For example if the matrix is 5x5 and i want to creat a submatrix that is 2x4 it easy to just use how you showed. But i have to creat a submatrix that is 2x4 that has the first 2 rows and columns from the same position in the 5x5 but has the last two rows and columns from the middle position of the 5x5... How do i do that? Matix A is the 5x5 and i wanna get to A4 by using your method.

[Dyuman Joshi]

@Justin Paulan, experiment with the methods shown in the Answers in this thread.

[Ioannis Aggelos]

wow that was helpful (it wasnt)

[Gabriele Bunkheila]

Please note:

1. With matrices, the first dimesion is always the number of row, the second is the number of columns. So in this case A is 5x5 (size(A) would return [5,5]) and A4 is 4x2 (size(A4) would return [4,2]).
2. A4 here seems composed of two "stacked" (or vertically concatenated) 2x2 sub-matrices of A

A possible way to obtain A4 from A is the following;

A = [1:5; 0.5*(-10:-6); 0.1*0:4; 10:-1:6; 2*(1:5)]

A = 5×5

1.0000 2.0000 3.0000 4.0000 5.0000 -5.0000 -4.5000 -4.0000 -3.5000 -3.0000 0 1.0000 2.0000 3.0000 4.0000 10.0000 9.0000 8.0000 7.0000 6.0000 2.0000 4.0000 6.0000 8.0000 10.0000

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rows1 = [1, 2];
cols1 = [1, 2];
rows2 = [3, 4];
cols2 = [3, 4];
A4 = [A(rows1, cols1); A(rows2, cols2)]

A4 = 4×2

1.0000 2.0000 -5.0000 -4.5000 2.0000 3.0000 8.0000 7.0000

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@Ioannis Aggelos I hope this helps.