Created
November 16, 2022 09:04
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numpy cumsum from a specified center
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| import numpy as np | |
| def center_cumsum(a:np.ndarray, pivot:tuple) -> np.ndarray: | |
| """ | |
| Compute the cumsum but pivoting a specified index | |
| """ | |
| assert isinstance(a, np.ndarray) | |
| assert a.ndim == 2 | |
| a_cum = np.zeros_like(a, dtype=a.dtype) | |
| assert len(pivot) == 2 | |
| i, j = pivot | |
| assert (i >= 0) and (i < a.shape[0]) | |
| assert (j >= 0) and (j < a.shape[1]) | |
| quad1 = a[:i+1,j:] | |
| a_cum[:i+1,j:] = quad1.cumsum(axis=1)[::-1].cumsum(axis=0)[::-1] | |
| quad2 = a[:i+1,:j+1] | |
| a_cum[:i+1,:j+1] = quad2[::-1, ::-1].cumsum(axis=1).cumsum(axis=0)[::-1, ::-1] | |
| quad3 = a[i:,:j+1] | |
| a_cum[i:,:j+1] = quad3[:, ::-1].cumsum(axis=1).cumsum(axis=0)[:, ::-1] | |
| quad4 = a[i:,j:] | |
| a_cum[i:,j:] = quad4.cumsum(axis=1).cumsum(axis=0) | |
| return a_cum |
Author
SheikSadi
commented
Nov 16, 2022
import matplotlib.pyplot as plt
from center_cumsum import center_cumsum
a = np.random.randint(0, 255, (9,9))
plt.imshow(a, cmap="Greys")
_ = plt.axis("off")
a_cum = center_cumsum(a, [4, 4])
plt.imshow(a_cum, cmap="Greys")
_ = plt.axis("off")
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