Extract array from arrays of arrays
I have arrays like this:
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 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 someone explain why?
The only solution I found was to explicitly list every index ( arr[0][1]...
) that I didn't like.
It is an array 3D
and you are trying to select the second element of the second axis and retrieve all elements along the other axes. So, this is as simple as -
arr[:,1,:]
We can omit the enum :
for the rear axles, so it simplifies to -
arr[:,1]
Example 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. ]])
I don't know anything about numpy, so there might be an easier way to do this. But a simple list comprehension will work:
[a[1] for a in arr]