On Thu, Oct 9, 2014 at 6:42 AM, suchith <suchithjn22@gmail.com> wrote:

How to extract individual columns from a numpy array? For example, consider this script

import numpy as np a = np.array([[1,2,3],[4,5,6],[7,8,9]]) a[0][:] a[:][0]

Now both a[:][0] and a[0][:] are outputting the same result, i.e np.array([1,2,3]). If I want to extract the array [[1],[4],[7]] then what should I do?

You want a[:, 0]. I'd recommend never writing expressions like a[0], where you give just 1 index into a 2d array -- numpy interprets such a thing as equivalent to a[0, :], so you should just write a[0, :] in the first place, it'll be more explicit and less confusing. (This also explains the problem you're having: a[:] is the same as a[:, :], i.e., it just returns all of 'a'. So a[:][0] is the same as a[0]. Similarly, a[0][:] returns all of a[0].) (The one time you might want to write something that looks like a[foo], with no commas inside the [], is where 'foo' is a 2d boolean mask.) -n -- Nathaniel J. Smith Postdoctoral researcher - Informatics - University of Edinburgh http://vorpus.org