How to access the ith column of a NumPy multidimensional array?

python arrays numpy
lpl picture lpl · Dec 15, 2010 · Viewed 797.1k times · Source

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?

Answer

mtrw picture mtrw · Dec 15, 2010 
>>> 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.

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