I have a 2D NumPy array and I hope to expand its size on both dimensions by copying the bottom row and right column.
For example, from 2x2:
[[0,1],
[2,3]]
to 4x4:
[[0,1,1,1],
[2,3,3,3],
[2,3,3,3],
[2,3,3,3]]
What's the best way to do it?
Thanks.
Here, the hstack and vstack functions can come in handy. For example,
In [16]: p = array(([0,1], [2,3]))
In [20]: vstack((p, p[-1], p[-1]))
Out[20]:
array([[0, 1],
[2, 3],
[2, 3],
[2, 3]])
And remembering that p.T is the transpose:
So now you can do something like the following:
In [16]: p = array(([0,1], [2,3]))
In [22]: p = vstack((p, p[-1], p[-1]))
In [25]: p = vstack((p.T, p.T[-1], p.T[-1])).T
In [26]: p
Out[26]:
array([[0, 1, 1, 1],
[2, 3, 3, 3],
[2, 3, 3, 3],
[2, 3, 3, 3]])
So the 2 lines of code should do it...
Make an empty array and copy whatever rows, columns you want into it.
def expand(a, new_shape):
x, y = a.shape
r = np.empty(new_shape, a.dtype)
r[:x, :y] = a
r[x:, :y] = a[-1:, :]
r[:x, y:] = a[:, -1:]
r[x:, y:] = a[-1, -1]
return r
