Extract array from arrays of arrays

Question

I have this arrays:

arr = np.array([[[ -1.,  -1.,  -1.,  0.,   0.,   0.],
                [ 0.1,  0.1,  0.1,  2.,   3.,   4.]], # <-- this one

               [[ -1.,  -1.,  -1.,  0.,   0.,  -1.],
                [ 0.1,  0.1,  0.1, 16.,  17.,  0.1]], # <-- and this one

               [[ -1.,  -1.,  -1.,  0.,   0.,   0.],
                [ 0.1,  0.1,  0.1,  4.,   5.,   6.]], # <-- and this one

               [[  0.,   0.,   0., -1.,   0.,   0.],
                [  1.,   2.,   3., 0.1,   1.,   2.]], # <-- and this one

               [[ -1.,  -1.,   0.,  0.,   0.,   0.],
                [ 0.1,  0.1,   1.,  9.,  10.,  11.]]]) # <-- and the last one

I want to extract the 2nd array in each array, and the result should be as follows:

res = [[ 0.1,  0.1,  0.1,  2.,   3.,   4.],
       [ 0.1,  0.1,  0.1, 16.,  17.,  0.1],
       [ 0.1,  0.1,  0.1,  4.,   5.,   6.],
       [  1.,   2.,   3., 0.1,   1.,   2.],
       [ 0.1,  0.1,   1.,  9.,  10.,  11.]]

I want to get the res in one line of code, I tried this but it didn't work

arr[:][1] # select the element 1 in each array
# I got
array([[ -1. ,  -1. ,  -1. ,   0. ,   0. ,  -1. ],
       [  0.1,   0.1,   0.1,  16. ,  17. ,   0.1]])

Can anyone explain why?

The only solution that I found is to explicitly indicate each index (arr[0][1]...), which I didn't like.

res = [x[1] for x in arr]?

depperm
– depperm

2017-04-26 12:42:43 +00:00
Commented Apr 26, 2017 at 12:42 — depperm
– depperm, Commented Apr 26, 2017 at 12:42

Divakar · Accepted Answer · 2017-04-26 12:49:51Z

7

That's a 3D array and you are trying to select the second element of the second axis and extracting all elements along the rest of the axes. So, its as simple as -

arr[:,1,:]

We can skip listing the : for the trailing axes, so it further simplifies to -

arr[:,1]

Sample run -

In [360]: arr
Out[360]: 
array([[[ -1. ,  -1. ,  -1. ,   0. ,   0. ,   0. ],
        [  0.1,   0.1,   0.1,   2. ,   3. ,   4. ]],

       [[ -1. ,  -1. ,  -1. ,   0. ,   0. ,  -1. ],
        [  0.1,   0.1,   0.1,  16. ,  17. ,   0.1]],

       [[ -1. ,  -1. ,  -1. ,   0. ,   0. ,   0. ],
        [  0.1,   0.1,   0.1,   4. ,   5. ,   6. ]],

       [[  0. ,   0. ,   0. ,  -1. ,   0. ,   0. ],
        [  1. ,   2. ,   3. ,   0.1,   1. ,   2. ]],

       [[ -1. ,  -1. ,   0. ,   0. ,   0. ,   0. ],
        [  0.1,   0.1,   1. ,   9. ,  10. ,  11. ]]])

In [361]: arr[:,1]
Out[361]: 
array([[  0.1,   0.1,   0.1,   2. ,   3. ,   4. ],
       [  0.1,   0.1,   0.1,  16. ,  17. ,   0.1],
       [  0.1,   0.1,   0.1,   4. ,   5. ,   6. ],
       [  1. ,   2. ,   3. ,   0.1,   1. ,   2. ],
       [  0.1,   0.1,   1. ,   9. ,  10. ,  11. ]])

edited Apr 26, 2017 at 12:49

answered Apr 26, 2017 at 12:43

Divakar

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1 Comment

kmario23 Over a year ago

Congratulations!

Daniel Roseman · Accepted Answer · 2017-04-26 12:42:56Z

2

I don't know anything about numpy, so there might be an easier way of doing it. But a simple list comprehension would work:

[a[1] for a in arr]

answered Apr 26, 2017 at 12:42

Daniel Roseman

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