在python中翻转稀疏数组的行

Question

I have a sparse array, say:

from scipy import sparse
a = sparse.lil_matrix((2,3),)
a[0] = [1, 2, 3]
a[1, 2] = 5

so it looks like:

(0, 0)  1.0
(0, 1)  2.0
(0, 2)  3.0
(1, 2)  5.0

I was wondering - is there an easy way to flip the rows (something like numpy.fliplr equivalent)? ...so I would get the output as:

(0, 0)  3.0
(0, 1)  2.0
(0, 2)  1.0
(1, 0)  5.0

Answer 1

One way would be to convert the array to csr format, and then manipulate the row indices:

from scipy import sparse
a = sparse.lil_matrix((2,3),)
a[0] = [1, 2, 3]
a[1, 2] = 5

a = a.tocsr()
a.indices = -a.indices + a.shape[1] - 1
print(a)

yields

  (0, 2)    1.0
  (0, 1)    2.0
  (0, 0)    3.0
  (1, 0)    5.0

Answer 2

You could do multi-index assignment:

ii = [2,1,0,3]
a[:,1] = a[ii,:]

Where ii is an array of indexes you have to create somehow.