# 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])`

See also: "numpy.arange" and "reshape" to allocate memory

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[1] 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[0] for row in a]`

So the result looks like this:

`[1, 2, 3]`