<s>[INST] numpy: access an array by column Suppose I have:
test = numpy.array([[1, 2], [3, 4], [5, 6]])

test[i] gets me ith line of the array (eg [1, 2]). How can I access the ith column? (eg [1, 3, 5]). Also, would this be an expensive operation?
 [/INST] >>> test[:,0]
array([1, 3, 5])

Similarly, 
>>> test[1,:]
array([3, 4])

lets you access rows.  This is covered in Section 1.4 (Indexing) of the NumPy reference.  This is quick, at least in my experience.  It's certainly much quicker than accessing each element in a loop.
 </s>