Rows and columns of NumPy arrays can be selected or modified using the square-bracket indexing notation in Python.
To select a row in a 2D array, use P[i]. For example, P[0] will return the first row of P.
To select a column, use P[:, i]. The : essentially means “select all rows”. For example, P[:, 1] will select all rows from the second column of P.
If you want to change the values of a row or column of an array, you can assign it to a new list (or array) of values of the same length.
To change the values in the first row, write:
>>> P[0] = [7, 8, 9]
>>> P
array([[7, 8, 9],
[4, 5, 6]])
To change the values in the second column, write:
>>> P[:, 1] = [7, 8]
>>> P
array([[1, 7, 3],
[4, 8, 6]])