How to extract all columns but one from an array (or matrix) in python?

Question

Given a numpy 2d array (or a matrix), I would like to extract all the columns but the i-th.

E. g. from

1 2 3 4 2 4 6 8 3 6 9 12 

I would like to have, e.g.

1 2 3 2 4 6 3 6 9 

or

1 2 4 2 4 8 3 6 12 

I cannot find a pythonic way to do this. I now that you can extract given columns by simply

a[:,n] 

or

a[:,[n,n+1,n+5]] 

But what about extracting all of them but one?

Answer 1

Since for the general case you are going to be returning a copy anyway, you may find yourself producing more readable code by using np.delete:

>>> a = np.arange(12).reshape(3, 4) >>> np.delete(a, 2, axis=1) array([[ 0,  1,  3],        [ 4,  5,  7],        [ 8,  9, 11]]) 

Answer 2

Use a slice that excludes the last element.

In [19]: a[:,:-1] Out[19]:  array([[1, 2, 3],        [2, 4, 6],        [3, 6, 9]]) 

If you want something other than the last element I'd just build a list to select with.

In [20]: selector = [x for x in range(a.shape[1]) if x != 2] In [21]: a[:, selector] Out[21]:  array([[ 1,  2,  4],        [ 2,  4,  8],        [ 3,  6, 12]]) 

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html