Python列表未正确附加排列值

Question

I am running a permutation test by randomly shuffling rows and columns of a matrix array and storing the resulting shuffled matrix into a python list. The idea is that I could use the shuffled matrix for all my other permutation tests (other programs). The below is the code highlighting the issue

#Debug the issues with permutation tests
import numpy as np
temp=[[0.11101831,0.444,0.555,0.6666],[.1,.2,.3,.4],[.10,.20,.30,.40],[.9,.8,.7,.6],[.4,.5,.6,.7]]
a=np.array(temp)
saved=[None for i in range(2)]
for i in range(2):
    np.random.shuffle(a)
    np.random.shuffle(a.T)
    saved[i]=a
    print ("-----------------------")
    print (a)
    print (saved)

The output of this is below:

[[ 0.3         0.2         0.1         0.4       ]
 [ 0.7         0.8         0.9         0.6       ]
 [ 0.3         0.2         0.1         0.4       ]
 [ 0.555       0.444       0.11101831  0.6666    ]
 [ 0.6         0.5         0.4         0.7       ]]
[array([[ 0.3       ,  0.2       ,  0.1       ,  0.4       ],
       [ 0.7       ,  0.8       ,  0.9       ,  0.6       ],
       [ 0.3       ,  0.2       ,  0.1       ,  0.4       ],
       [ 0.555     ,  0.444     ,  0.11101831,  0.6666    ],
       [ 0.6       ,  0.5       ,  0.4       ,  0.7       ]]), []]
[[ 0.3         0.2         0.4         0.1       ]
 [ 0.555       0.444       0.6666      0.11101831]
 [ 0.3         0.2         0.4         0.1       ]
 [ 0.6         0.5         0.7         0.4       ]
 [ 0.7         0.8         0.6         0.9       ]]
[array([[ 0.3       ,  0.2       ,  0.4       ,  0.1       ],
       [ 0.555     ,  0.444     ,  0.6666    ,  0.11101831],
       [ 0.3       ,  0.2       ,  0.4       ,  0.1       ],
       [ 0.6       ,  0.5       ,  0.7       ,  0.4       ],
       [ 0.7       ,  0.8       ,  0.6       ,  0.9       ]]), array([[ 0.3       ,  0.2       ,  0.4       ,  0.1       ],
       [ 0.555     ,  0.444     ,  0.6666    ,  0.11101831],
       [ 0.3       ,  0.2       ,  0.4       ,  0.1       ],
       [ 0.6       ,  0.5       ,  0.7       ,  0.4       ],
       [ 0.7       ,  0.8       ,  0.6       ,  0.9       ]])]

As you can see that for i=0, the value of shuffle matrix a and first index of the list (saved) are the same. But when i=1, both saved[0] and saved[1] becomes the same. This should not happen since I am using the index of the list to assign the shuffled matrix. Is there anything here that I am missing?

Answer 1

You need to copy the array, not just a reference.

In your code above, saved[i]=a refer to a same array a for all i .

Change it to saved[i]=np.copy(a) .

Answer 2

np.random.shuffle is an in-place operation, so you need to either use copies, as explained by @liliscent ; or, my preference, just use a function which does not operate in place.

np.random.permutation is sufficient for your task:

import numpy as np

a = np.array([[0.11101831,0.444,0.555,0.6666],
             [.1,.2,.3,.4],
             [.10,.20,.30,.40],
             [.9,.8,.7,.6],
             [.4,.5,.6,.7]])

saved = []

for i in range(2):
    x = np.random.permutation(np.random.permutation(a).T)
    saved.append(x)

Even better, you can convert this into a list comprehension:

saved = [np.random.permutation(np.random.permutation(a).T) for _ in range(2)]

Example result:

# [array([[ 0.8       ,  0.444     ,  0.5       ,  0.2       ,  0.2       ],
#        [ 0.9       ,  0.11101831,  0.4       ,  0.1       ,  0.1       ],
#        [ 0.7       ,  0.555     ,  0.6       ,  0.3       ,  0.3       ],
#        [ 0.6       ,  0.6666    ,  0.7       ,  0.4       ,  0.4       ]]),
#  array([[ 0.4       ,  0.11101831,  0.1       ,  0.9       ,  0.1       ],
#        [ 0.6       ,  0.555     ,  0.3       ,  0.7       ,  0.3       ],
#        [ 0.7       ,  0.6666    ,  0.4       ,  0.6       ,  0.4       ],
#        [ 0.5       ,  0.444     ,  0.2       ,  0.8       ,  0.2       ]])]