仅使用广播从矩阵的指定列向量中减去列向量 [英] Subtract a column vector from matrix at specified vector of columns using only broadcast
问题描述
我想使用另一个向量从numpy矩阵中减去列向量,该向量是需要从主矩阵中减去第一个列向量的列的索引.例如.
I want to subtract a column vector from a numpy matrix using another vector which is index of columns where the first column vector needs to be subtracted from the main matrix. For eg.
M = array([[ 1, 2, 1, 1],
[ 2, 1, 1, 1],
[ 1, 1, 2, 1],
[ 2, 1, 1, 1],
[ 1, 1, 1, 2]]) # An example matrix
V = array([1, 1, 1, 1, 1]) # An example column vector
I = array([0, 3, 2, 3, 1, 3, 3]) # The index maxtrix
现在我想在I中给定的列号上从M中减去V. 例如. I [0]为0,因此从矩阵M的第一列(零索引)中减去V.
Now I want to subtract V from M at column numbers given in I. For eg. I[0] is 0, so subtract V from first column (zero index) of matrix M.
类似地,I [1] = 3,从矩阵M的第四列(三个索引)中减去V.
Similarly I[1] = 3, subtract V from fourth column (three index) of matrix M.
操作结束时,由于3在I中出现4次,因此将从第三列中减去V,即M-4的最后一列.
At the end of operation, since 3 occurs 4 times in I, so V will be subtracted from third column i.e. last column of M- 4 times.
我只需要广播,不循环就可以做到这一点.
I need to do this using only broadcast, no loops.
我尝试了以下操作:
M[:, I] - V[np.newaxis, :].T
但是它最终广播的结果矩阵的列数比M中的列数还要多.
but it ends up broadcasting resultant matrix to have more columns than there are in M.
推荐答案
一个人可以使用bincount
和outer
>>> M - np.outer(V, np.bincount(I, None, M.shape[1]))
array([[ 0, 1, 0, -3],
[ 1, 0, 0, -3],
[ 0, 0, 1, -3],
[ 1, 0, 0, -3],
[ 0, 0, 0, -2]])
或subtract.at
>>> out = M.copy()
>>> np.subtract.at(out, (np.s_[:], I), V[:, None])
>>> out
array([[ 0, 1, 0, -3],
[ 1, 0, 0, -3],
[ 0, 0, 1, -3],
[ 1, 0, 0, -3],
[ 0, 0, 0, -2]])
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