I am currently working on a computer vision project with python and openCV. I have a 2D numpy array like this:
[100 38 18]
[134 332 16]
[136 200 16]
[136 288 15]
[138 160 17]
[138 246 15]
[140 76 12]
[140 116 12]
[142 34 14]
The 2D array is already sorted by the first column. This works fine. Now I need to sort pakets of 3 rows by the second column. This is the result I need to achieve:
[100 38 18]
[136 200 16]
[134 332 16]
[138 160 17]
[138 246 15]
[136 288 15]
[142 34 14]
[140 76 12]
[140 116 12]
How can I achieve this?
CodePudding user response:
Just by NumPy, not looping:
sort_ = np.argsort(np.split(a[:, 1], a.shape[0] // 3))
# [[0 2 1]
# [1 2 0]
# [2 0 1]]
sort_ = np.linspace(0, a.shape[0] - 3, a.shape[0] // 3, dtype=np.int64)[:, None]
# [[0 2 1]
# [4 5 3]
# [8 6 7]]
a = a[sort_.ravel()]
CodePudding user response:
Consider reshaping your data into 3d, then use for loop to sort each array and cast back into an np.array
np.array([sorted(i, key = lambda x: x[1]) for i in ar.reshape(3, -1, ar.shape[1])]).reshape(ar.shape)
array([[100, 38, 18],
[136, 200, 16],
[134, 332, 16],
[138, 160, 17],
[138, 246, 15],
[136, 288, 15],
[142, 34, 14],
[140, 76, 12],
[140, 116, 12]])
CodePudding user response:
I see no other way than to use a loop
let A
be your array
output = np.zeros((0, 3))
for i in range(int(A.shape[0]/3)):
output = np.vstack((output, A[3*i np.argsort(A[3*i:3*(i 1), 1])]))
Note: I'm assuming that your array has a number of lines which is a multiple of 3