A matrix consists of a one or more rows vectors of the same lenght or one or more column vectors of the same lenght.
To define a matrix in MATLAB, each element of a row must be folowed by a space or comma and each row must be delimited by either a semi-colon or a new line.
| >> A=[1 2 3;0 -2 6;8 10 14] A = 1 2 3 0 -2 6 8 10 14 >> B=[5 3 1 12 9 10 4 7 0] B = 5 3 1 12 9 10 4 7 0 |
You can easily change the value of a matrix element using MATLAB's indexing commands by typing the following in the command line:
| >> A(1,2)=0 A = 1 0 3 0 -2 6 8 10 14 |
As you can see, the second element of the first row of the matrix has now the value 0.
To extract a specific column of a matrix in MATLAB, use the semi-colon:
| >> A(:,2) ans = 0 -2 10 |
The snippet above selects the entire second column of the A matrix previous defined and it essentially means "every row, second column".
Selecting a specific row of a matrix in MATLAB is done in a similar manner:
| >> A(end,:) ans = 8 10 14 |
This will extract the entire last row of the matrix.
Accessing a submatrix in MATLAB is done as follows:
| >> A(2:3,1:2) ans = 0 -2 8 10 |
This passage will select rows 2 to 3 and columns 1 to 2.
If you want to extract scattered elements from a matrix, you'll need to use linear indexing, that is indexing the elements of the matrix as if they were part of a long column vector.
In the following example, each element of the matrix has the same value as its index.
To determine the number of rows and columns of a matrix, use size.
If you want to extract scattered elements from a matrix, you'll need to use linear indexing, that is indexing the elements of the matrix as if they were part of a long column vector.
In the following example, each element of the matrix has the same value as its index.
| >> M=[1 5 9 13;2 6 10 14;3 7 11 15;4 8 12 16] M = 1 5 9 13 2 6 10 14 3 7 11 15 4 8 12 16 >> M(7) ans = 7 |
To determine the number of rows and columns of a matrix, use size.
| >> C=[11 3 2;25 6 14] C = 11 3 2 25 6 14 >> size(C) ans = 2 3 |
MATLAB has a few build-in functions you can use to create special matrices.
For a matrix with all its elements having the value zero, use zeros.
| >> zeros(2,2) ans = 0 0 0 0 |
For a matrix with all its elements having the value one, use ones.
| >> ones(2,3) ans = 1 1 1 1 1 1 |
To create a m x n identity matrix, use the eye command.
| >> I=eye(3) I = 1 0 0 0 1 0 0 0 1 |
For a diagonal matrix, first define a vector containing the elements on the diagonal of the matrix, then use diag.
| >> d=[1 2 3] d = 1 2 3 >> diag(d) ans = 1 0 0 0 2 0 0 0 3 |
For a similar article on vectors, see this article.
No comments:
Post a Comment