Numpy提取子矩陣

結論

想要從numpy的矩陣或者ndarray中提取子矩陣還是使用np.ix_函數最快。另外想不到ndarray速度比matrix要快一下。水平有限，多多斧正。

import numpy as np

# ndarray
mtx = np.random.random((300, 300))
a = [1,3,5,7,8,9,12,16,23,45,64,72,89]
b = [1,3,5,7,8,9,12,16,23,45,64,72,89]

# 矩陣
M = np.matrixlib.matrix(mtx)

indices = np.ix_(a,b)

c = M[indices]

c.shape

%timeit c = mtx[indices]

%timeit c = M[indices]

%timeit c = mtx[:, b][a, :]

%timeit c = mtx[a, :][:, b]

100000 loops, best of 3: 2.75 µs per loop
100000 loops, best of 3: 3.99 µs per loop
100000 loops, best of 3: 10.7 µs per loop
100000 loops, best of 3: 8.56 µs per loop