Submatrices
Start by defining a matrix
A = [1 2 3; 4 5 6; 7 8 9; 10 11 12]
A =
1 2 3
4 5 6
7 8 9
10 11 12
The dimensions of a matrix are given by
[r c] = size(A)
r =
4
c =
3
We can use this to define a new matrix
B = ones(size(A))
B =
1 1 1
1 1 1
1 1 1
1 1 1
Note: Indexing in MATLAB always begins with 1
To access a single element of 
type
A(2,3)
ans =
6
The second column of is given by
A(:,2)
ans =
2
5
8
11
The third row of is given by
A(3,:)
ans =
7 8 9
Here's a submatrix (the second and third rows of the first three columns)
A(2:3,1:3)
ans =
4 5 6
7 8 9
We can replace parts of a matrix
B(2,:) = [2 4 6]
B =
1 1 1
2 4 6
1 1 1
1 1 1
or
A(1:2,1:2) = B(2:3,2:3)
A =
4 6 3
1 1 6
7 8 9
10 11 12
We can also easily remove certain rows or columns of a matrix by assigning an empty vector '[]':
A([2 4],:) = []
A =
4 6 3
7 8 9