How do I slice a numpy array to get both the first and last two rows

Question

As far as I can see it, it isn’t covered in the indexing, slicing and iterating scipy tutorial, so let me ask it here:

Say I’ve

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

How do I slice the array in order get both the first and last rows:

y: array([[1, 2],
          [3, 4],              
          [7, 8],
          [9, 0]])

Asked By: Mattijn

||

Answer 1

I don’t know if there’s a slick way to do that. You could list the indices explicitly, of course:

>>> x[[0,1,-2,-1]]
array([[1, 2],
       [3, 4],
       [7, 8],
       [9, 0]])

Or use r_ to help, which would probably be more convenient if we wanted more rows from the head or tail:

>>> x[np.r_[0:2, -2:0]]
array([[1, 2],
       [3, 4],
       [7, 8],
       [9, 0]])

Answered By: DSM

Answer 2

Alternatively,you can use indices to remove as

mask = np.ones(len(x), dtype=np.bool)
mask[2:3] = False # that you want to remove
y = x[mask]

Answered By: emesday

Question: