如何使用 numpy.reshape 交换数组的轴

Question

I have a huge empty array (300000,80,80) that I want to swap its axes using numpy.reshape . I've tried numpy.rollaxis , numpy.swapaxes and numpy.transpose . they worked like a charm but they slowed down fancy indexing up ahead.

also I've tried C or F order in creating empty array but nothing changed.

so, how can I use numpy.reshape in order to change axis order like this :

(300000,80,80) -> (80,80,300000) without using numpy.rollaxis or etc.

every idea would be appreciated.

here is my code:

patch = np.ones([3,80,80])
image = np.empty([300000,80,80], dtype='uint8', order='C')

for i in range(0,300000,3):
  image[i:i+3] = patch

# if i use np.rollaxis, next fancy indexing execute too slow.
pt = ([...], [...]) #some tuple
ij = ([...], [...]) #some tuple

transformed[pt] = image[ij]

Answer 1

reshape cannot work the same as transpose/swapaxes .

I'll try to illustrate.

In [1]: arr = np.arange(6).reshape(2,3)
In [2]: arr
Out[2]: 
array([[0, 1, 2],
       [3, 4, 5]])

arr is actually a view of the source arange , and order of elements in the shared databuffer is:

In [3]: arr.ravel()
Out[3]: array([0, 1, 2, 3, 4, 5])

transpose also a view , but with different shape , strides and order

In [4]: tarr = np.transpose(arr)
In [5]: tarr
Out[5]: 
array([[0, 3],
       [1, 4],
       [2, 5]])
In [6]: tarr.ravel()
Out[6]: array([0, 3, 1, 4, 2, 5])      # order C
In [7]: tarr.ravel(order='F')
Out[7]: array([0, 1, 2, 3, 4, 5])
In [8]: arr.strides
Out[8]: (24, 8)
In [9]: tarr.strides
Out[9]: (8, 24)

To go across the columns of tarr it steps 24 bytes, or 3 elements - from 0 to 3, from 1 to 4 etc.

Because it is a view the transpose is fast. But subsequent operations often require a copy, which for large arrays is much slower.

If we try to just reshape, we get:

In [10]: np.reshape(arr,(3,2))
Out[10]: 
array([[0, 1],
       [2, 3],
       [4, 5]])
In [11]: np.reshape(arr,(3,2)).ravel()
Out[11]: array([0, 1, 2, 3, 4, 5])
In [12]: np.reshape(arr,(3,2)).strides
Out[12]: (16, 8)

The shape matches tarr , but the strides don't. The [0,1,2] line has been split.