How to extract a column from a matrix in Python

To read a column of a matrix object, previously created with the instruction array() of numpy, you can scroll the elements in the rows of the matrix or transpose the matrix.

Example

m=array([[1,2,3],[4,5,6],[7,8,9]])

This object is equivalent to a 3x3 matrix (3 rows x 3 columns)

Let's try to extract the first column with Python.

This problem has the following solutions:

Solution 1

The matrix can be transposed

m.T

The previous instruction creates an object with columns organized in lists

It is the transposed matrix of m with lines instead of columns.

array([[1, 4, 7],
[2, 5, 8],
[3, 6, 9]])

Now, to read the first column, you can simply extract the first row of the transposed matrix.

The first column is the number 0.

m.T[0]

The command returns the elements of the first column of the matrix.

array([1, 4, 7])

It is the first column of the matrix m.

Solution 2

To select a column of the matrix you can select the elements of the i-th column by scrolling the rows.

This instruction scrolls through all the rows of the matrix m and reads the second element of each using the row[1] function.

[row[1] for row in m]

The output result is as follows:

[2, 5, 8]

It is the second column of the initial matrix.

Similarly, the third column can be read by row [2].

The firest column can be read by row[0].

Note. The program must read all the rows of the matrix to extract a single column.

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