In my application, I have an array like this:
[10,20,30,
40,50,60,
70,80,90,
0.1,0.2,0.3,
0.4,0.5,0.6,
0.7,0.8,0.9,
1,2,3,
4,5,6,
7,8,9]
I want to reverse every 9 numbers so that my array looks like this:
[90,80,70,
60,50,40,
30,20,10,
0.9,0.8,0.7,
0.6,0.5,0.4,
0.3,0.2,0.1,
9,8,7,
6,5,4,
3,2,1]
Can someone tells me how to do this efficiently?
Perhaps something like:
n = a.shape[0]
a.reshape((n//9,9))[:,::-1].reshape((n,))
array([ 90. , 80. , 70. , 60. , 50. , 40. , 30. , 20. , 10. ,
0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1,
9. , 8. , 7. , 6. , 5. , 4. , 3. , 2. , 1. ])
But this relies on there being a multiple of 9 elements in your array. It leaves the original array unchanged. To alter the original a in-place you can use resize:
a.resize((n//9,9))
a[:,::-1] = a
a.resize((n,))
This works. Not sure if it is the most effivient way:
import numpy
a = numpy.array([10,20,30,
40,50,60,
70,80,90,
0.1,0.2,0.3,
0.4,0.5,0.6,
0.7,0.8,0.9,
1,2,3,
4,5,6,
7,8,9])
# reshape and transpose
b = a.reshape(-1,9).T
# reverse
b = b[::-1]
# convert back to flat array
print b.T.flatten()
# in one line
print (a.reshape(-1,9).T[::-1].T).flatten()
