How do you extract a column from a multi-dimensional array? How do you extract a column from a multi-dimensional array? python python

# How do you extract a column from a multi-dimensional array?

``>>> import numpy as np>>> A = np.array([[1,2,3,4],[5,6,7,8]])>>> Aarray([[1, 2, 3, 4],    [5, 6, 7, 8]])>>> A[:,2] # returns the third colummarray([3, 7])``

Example: (Allocating a array with shaping of matrix (3x4))

``nrows = 3ncols = 4my_array = numpy.arange(nrows*ncols, dtype='double')my_array = my_array.reshape(nrows, ncols)``

Could it be that you're using a NumPy array? Python has the array module, but that does not support multi-dimensional arrays. Normal Python lists are single-dimensional too.

However, if you have a simple two-dimensional list like this:

``A = [[1,2,3,4],     [5,6,7,8]]``

then you can extract a column like this:

``def column(matrix, i):    return [row[i] for row in matrix]``

Extracting the second column (index 1):

``>>> column(A, 1)[2, 6]``

Or alternatively, simply:

``>>> [row for row in A][2, 6]``

If you have an array like

``a = [[1, 2], [2, 3], [3, 4]]``

Then you extract the first column like that:

``[row for row in a]``

So the result looks like this:

``[1, 2, 3]``