Question

I have an array like:

a = array([[1,2,3],[3,4,5],[4,5,6]])

What’s the most efficient way to slice out a 1×2 array out of this that has only the first two columns of “a”?

i.e.

array([[2,3],[4,5],[5,6]]) in this case.

Answer 1

Two dimensional numpy arrays are indexed using a[i,j] (not a[i][j]), but you can use the same slicing notation with numpy arrays and matrices as you can with ordinary matrices in python (just put them in a single []):

>>> from numpy import array
>>> a = array([[1,2,3],[3,4,5],[4,5,6]])
>>> a[:,1:]
array([[2, 3],
       [4, 5],
       [5, 6]])