英文:
Index numpy array by matrix of two arrays
问题
You can achieve the desired indexing into the matrix using NumPy like this:
result = weights[positions1[:, np.newaxis], positions2]
This will give you a result similar to the matrix you described:
array([[8, 8, 7],
[5, 5, 4],
[1, 1, 0]])
Each element of result corresponds to the values obtained by indexing weights with the combinations of positions1 and positions2 as specified in your question.
英文:
I have a 2D numpy array like
weights = np.array(
[
[1, 2, 3],
[4, 5, 6],
[7, 8, 9],
]
)
And i have two 1-D numpy arrays that i want to use to index into weights
positions1 = np.array([2, 1, 0])
positions2 = np.array([1, 1, 0])
If i want the results for stepping through the arrays together and indexing into the matrix that way i can do
print(weights[positions1[...], positions2[...]])
And get [8 5 1]
However now i want to index into the matrix with all possible combinations of positions1 and positions2 so that i get a matrix like
[
[weights[2, 1], weights[2, 1], weights[2, 0]],
[weights[1, 1], weights[1, 1], weights[1, 0]],
[weights[0, 1], weights[0, 1], weights[0, 0]],
]
So
[
[weights[pos1[0], pos2[0]], weights[pos1[0], pos2[1]], weights[pos1[0], pos2[2]]],
[weights[pos1[1], pos2[0]], weights[pos1[1], pos2[1]], weights[pos1[1], pos2[2]]],
[weights[pos1[2], pos2[0]], weights[pos1[2], pos2[1]], weights[pos1[2], pos2[2]]],
]
What would be the canonical way to do that in numpy?
I know that this is kinda like an outer product but i dont actually want to multiply the values in my array but just get a tuple of their values to index into the matrix with.
答案1
得分: 2
需要更改第一个索引器的形状：
weights[positions1[:, None], positions2]
或者对于通用版本，如Chrysophylaxs所指出的：
weights[np.ix_[positions1, positions2]]
输出：
array([[8, 8, 7],
[5, 5, 4],
[2, 2, 1]])
英文:
You need change the shape of the first indexer:
weights[positions1[:,None], positions2]
Or for a generalized version, as pointed out by Chrysophylaxs:
weights[np.ix_[positions1, positions2]]
Output:
array([[8, 8, 7],
[5, 5, 4],
[2, 2, 1]])
答案2
得分: 1
另一种可能的解决方案：
weights[*np.meshgrid(positions1, positions2, indexing='ij')]
如果上述在您的Python版本上不起作用，您可以使用@mozway建议的方法：
weights[tuple(np.meshgrid(positions1, positions2, indexing='ij'))]
输出：
array([[8, 8, 7],
[5, 5, 4],
[2, 2, 1]])
英文:
Another possible solution:
weights[*np.meshgrid(positions1, positions2, indexing='ij')]
If the above does not work on your Python version, you can use what @mozway suggests:
weights[tuple(np.meshgrid(positions1, positions2, indexing='ij'))]
Output:
array([[8, 8, 7],
[5, 5, 4],
[2, 2, 1]])
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