Python扩展库numpy提供了大量的矩阵运算,本文进行详细描述。
>>> import numpy as np
>>> a_list = [3, 5, 7]
# 创建矩阵
>>> a_mat = np.matrix(a_list)
>>> a_mat
matrix([[3, 5, 7]])
# 矩阵转置
>>> a_mat.T
matrix([[3],
[5],
[7]])
# 矩阵形状
>>> a_mat.shape
(1, 3)
# 元素个数
>>> a_mat.size
3
# 创建矩阵
>>> b_mat = np.matrix((1, 2, 3))
>>> b_mat
matrix([[1, 2, 3]])
# 矩阵相乘
>>> a_mat * b_mat.T
matrix([[34]])
# 元素平均值
>>> a_mat.mean()
5.0
# 所有元素之和
>>> a_mat.sum()
15
# 最大值
>>> a_mat.max()
7
# 横向最大值
>>> a_mat.max(axis=1)
matrix([[7]])
# 纵向最大值
>>> a_mat.max(axis=0)
matrix([[3, 5, 7]])
# 创建二维矩阵
>>> c_mat = np.matrix([[1, 5, 3], [2, 9, 6]])
>>> c_mat
matrix([[1, 5, 3],
[2, 9, 6]])
# 纵向排序后的元素序号
>>> c_mat.argsort(axis=0)
matrix([[0, 0, 0],
[1, 1, 1]], dtype=int64)
# 横向排序后的元素序号
>>> c_mat.argsort(axis=1)
matrix([[0, 2, 1],
[0, 2, 1]], dtype=int64)
>>> d_mat = np.matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
# 矩阵对角线元素
>>> d_mat.diagonal()
matrix([[1, 5, 9]])
# 矩阵平铺
>>> d_mat.flatten()
matrix([[1, 2, 3, 4, 5, 6, 7, 8, 9]])
# 特征值与特征向量
>>> np.linalg.eig([[1,1],[2,2]])
(array([ 0., 3.]), array([[-0.70710678, -0.4472136 ],
[ 0.70710678, -0.89442719]]))
>>> x = np.matrix([[1,2], [3,4]])
# 逆矩阵
>>> y = np.linalg.inv(x)
>>> x * y
matrix([[ 1.00000000e+00, 1.11022302e-16],
[ 0.00000000e+00, 1.00000000e+00]])
>>> y * x
matrix([[ 1.00000000e+00, 4.44089210e-16],
[ 0.00000000e+00, 1.00000000e+00]])
>>> d_mat = np.matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
>>> e, v = np.linalg.eig(d_mat)
>>> v*np.diag(e)*np.linalg.inv(v)
matrix([[ 1., 2., 3.],
[ 4., 5., 6.],
[ 7., 8., 9.]])
# 协方差
>>> np.cov([1,1,1,1,1])
array(0.0)
>>> x = [-2.1, -1, 4.3]
>>> y = [3, 1.1, 0.12]
>>> X = np.vstack((x,y))
# 协方差
>>> print(np.cov(X))
[[ 11.71 -4.286 ]
[ -4.286 2.14413333]]
>>> print(np.cov(x, y))
[[ 11.71 -4.286 ]
[ -4.286 2.14413333]]
>>> print(np.cov(x))
11.709999999999999
# 二维矩阵
>>> x = np.matrix(np.arange(0,10).reshape(2,5))
>>> x
matrix([[0, 1, 2, 3, 4],
[5, 6, 7, 8, 9]])
# 所有元素之和
>>> x.sum()
45
# 纵向求和
>>> x.sum(axis=0)
matrix([[ 5, 7, 9, 11, 13]])
# 横向求和
>>> x.sum(axis=1)
matrix([[10],
[35]])
# 平均值
>>> x.mean()
4.5
>>> x.mean(axis=1)
matrix([[ 2.],
[ 7.]])
>>> x.mean(axis=0)
matrix([[ 2.5, 3.5, 4.5, 5.5, 6.5]])