Extract one or more columns from a matrix in Octave
In this lesson I'll explain how to extract a column or more columns from a two-dimensional array (matrix) in Octave.
I'll give you a practical example.
Create a matrix.
>> M = [ 1 2 3 ; 4 5 6 ; 7 8 9 ]
M =
1 2 3
4 5 6
7 8 9
It is a 3x3 square matrix with three rows and three columns.
To extract the first column of the matrix type M(:, 1)
- In the first parameter type the colon symbol: to take all the rows of the matrix
- In the second parameter type 1 to take only the first column of the matrix
>> M(:,1)
This command extracts all the values in the first column of the matrix
ans =
1
4
7
To extract the second column of the matrix type M(:, 2)
Leave the colon: in the first parameter and type 2 in the second parameter to get all the elements of the second column of the matrix.
>> M(:,2)
ans =
2
5
8
Finally, to extract the third column of the matrix type M(:, 3)
This will extract only the third column of the matrix.
>> M(:,3)
ans =
3
6
9
If you want to extract only some rows of a column, type the range of rows to consider in the first parameter.
For example, to extract only the first two rows of the third column, type M(1:2,3)
>> M(1:2,3)
ans =
3
6
If the rows to be extracted are not close to each other, type the list of rows in square brackets in the second parameter, separating them from each other with a comma or a space.
For example, type M([1 3],2) to extract the first and third rows of the second column
>> M([1 3],2)
ans =
2
8
You can also extract two or more columns from the matrix.
For example, type M(:, 1: 2) to extract the first two columns of the matrix
In the second parameter type the 1: 2 column range you want to take
>> M(:,1:2)
ans =
1 2
4 5
7 8
To extract two or more columns separated from each other, type in the second parameter the list of columns to be extracted in square brackets separating them from each other with a space or a comma.
For example, type M(:, [1 3]) to extract the first and third columns of the matrix
>> M([1 3],:)
ans =
1 3
4 6
7 9
This way you can extract the columns of the matrix even if they are not close to each other.
