how to assign part of a matrix to other matrix in matlab

Question

I have a 30 x 30 matrix, called A, and I want to assign B as the leftmost 30 x 20 block of A how can I do that?

Is this the correct way to do i

Answer 1

No the correct way is

B = A(:, 1:20);

where : is shorthand for all of the rows in A.

Matrix indexing in MATLAB uses round brackets, (). Square brackets, [], are used to declare matrices (or vectors) as in

>> v = [1 2 3; 4 5 6; 7 8 9]
v =

     1     2     3
     4     5     6
     7     8     9

excaza provides a very good link on Matrix Indexing in MATLAB which should help you. There is also Matrix Indexing.

Answer 2

A_new = A(:,1:20)

takes all the rows from A with this part A(:,) and the first 20 columns with this part A(,1:20)

A_newis now 30x20

You can also iterate over elements in two loops, but the above answer is easiest