[英]Extracting classes from numpy array
I am having a numpy array of data我有一个 numpy 数据数组
data = np.random.random((5, 5))
and another numpy array of equal shape masking the first one with classes from 0
to n
:和另一个形状相同的 numpy 数组,用
0
到n
的类屏蔽第一个数组:
>>> mask
array([[3, 3, 1, 1, 0],
[2, 0, 1, 2, 2],
[0, 1, 0, 0, 3],
[2, 1, 1, 0, 2],
[0, 2, 3, 0, 2]])
What's the best way to compute a two-dimensional array with n
rows, where each row contains all elements from data
with class row_idx
(described by mask
)?计算具有
n
行的二维数组的最佳方法是什么,其中每行包含来自data
的所有元素 class row_idx
(由mask
描述)?
You can't do it better than O(n^2)
as you should iterate through all mask
array.你不能比
O(n^2)
做得更好,因为你应该遍历所有mask
数组。 As the number of elements in each class can be different the result can't be numpy array (rows have different sizes).由于每个 class 中的元素数量可能不同,因此结果不能是 numpy 数组(行具有不同的大小)。 So I don't think you can avoid python loop here with pure numpy functions.
所以我认为你不能用纯 numpy 函数来避免 python 循环。 I suggest this
O(n^2)
solution:我建议这个
O(n^2)
解决方案:
ans = [[] for i in range(mask.max() + 1)]
for k, v in np.ndenumerate(mask):
ans[v].append(data[k])
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