开启方法

内容部分先不用管，我这边只是做个测试，过几天会讲这些的，先忽略

矩阵系列¶

1.创建特殊矩阵¶

1.1.创建全为0的矩阵¶

np.zeros(tuple)

$$\begin{bmatrix} 0&0&0 \\ 0&0&0 \\ 0&0&0 \end{bmatrix}$$

import numpy as np

help(np.zeros)

Help on built-in function zeros in module numpy.core.multiarray:

zeros(...)
zeros(shape, dtype=float, order='C')

Return a new array of given shape and type, filled with zeros.

Parameters
----------
shape : int or sequence of ints
Shape of the new array, e.g., ``(2, 3)`` or ``2``.
dtype : data-type, optional
The desired data-type for the array, e.g., `numpy.int8`.  Default is
`numpy.float64`.
order : {'C', 'F'}, optional
Whether to store multidimensional data in C- or Fortran-contiguous
(row- or column-wise) order in memory.

Returns
-------
out : ndarray
Array of zeros with the given shape, dtype, and order.

See Also
--------
zeros_like : Return an array of zeros with shape and type of input.
ones_like : Return an array of ones with shape and type of input.
empty_like : Return an empty array with shape and type of input.
ones : Return a new array setting values to one.
empty : Return a new uninitialized array.

Examples
--------
>>> np.zeros(5)
array([ 0.,  0.,  0.,  0.,  0.])

>>> np.zeros((5,), dtype=int)
array([0, 0, 0, 0, 0])

>>> np.zeros((2, 1))
array([[ 0.],
[ 0.]])

>>> s = (2,2)
>>> np.zeros(s)
array([[ 0.,  0.],
[ 0.,  0.]])

>>> np.zeros((2,), dtype=[('x', 'i4'), ('y', 'i4')]) # custom dtype
array([(0, 0), (0, 0)],
dtype=[('x', '<i4'), ('y', '<i4')])

# 一维
np.zeros(5) # 完整写法：np.zeros((5,))

array([0., 0., 0., 0., 0.])

# 可以指定类型
np.zeros(5,dtype=int)

array([0, 0, 0, 0, 0])

# 二维
np.zeros((2,5))

array([[0., 0., 0., 0., 0.],
[0., 0., 0., 0., 0.]])

# 三维 ==> 可以这么理解，2个2*5（2行5列）的矩阵
np.zeros((2,2,5))

array([[[0., 0., 0., 0., 0.],
[0., 0., 0., 0., 0.]],

[[0., 0., 0., 0., 0.],
[0., 0., 0., 0., 0.]]])

################### 扩展部分 ########################

# 建议用元组，官方文档都是元组，而且shape返回类型就是元组
array1 = np.zeros([2,3])
print(array1)
type(array1)
print(array1.shape) # shape返回类型就是元组

[[0. 0. 0.]
[0. 0. 0.]]
(2, 3)

1.2.创建全为1的矩阵¶

np.ones(tuple)

np.zeros(tuple)

$$\begin{bmatrix} 1&1&1 \\ 1&1&1 \\ 1&1&1 \end{bmatrix}$$

help(np.ones)

Help on function ones in module numpy.core.numeric:

ones(shape, dtype=None, order='C')
Return a new array of given shape and type, filled with ones.

Parameters
----------
shape : int or sequence of ints
Shape of the new array, e.g., ``(2, 3)`` or ``2``.
dtype : data-type, optional
The desired data-type for the array, e.g., `numpy.int8`.  Default is
`numpy.float64`.
order : {'C', 'F'}, optional
Whether to store multidimensional data in C- or Fortran-contiguous
(row- or column-wise) order in memory.

Returns
-------
out : ndarray
Array of ones with the given shape, dtype, and order.

See Also
--------
zeros, ones_like

Examples
--------
>>> np.ones(5)
array([ 1.,  1.,  1.,  1.,  1.])

>>> np.ones((5,), dtype=int)
array([1, 1, 1, 1, 1])

>>> np.ones((2, 1))
array([[ 1.],
[ 1.]])

>>> s = (2,2)
>>> np.ones(s)
array([[ 1.,  1.],
[ 1.,  1.]])

# 一维
np.ones(5) # 完整写法 np.ones((5,))

array([1., 1., 1., 1., 1.])

# 可以指定类型
np.ones(5,dtype=int)

array([1, 1, 1, 1, 1])

# 二维，传一个shape元组
np.ones((2,5))

array([[1., 1., 1., 1., 1.],
[1., 1., 1., 1., 1.]])

# 三维 可以理解为两个二维数组
np.ones((2,2,5))

array([[[1., 1., 1., 1., 1.],
[1., 1., 1., 1., 1.]],

[[1., 1., 1., 1., 1.],
[1., 1., 1., 1., 1.]]])

1.3.单位矩阵¶

先普及一个数学基础：任何

矩阵

单位矩阵

本身

单位矩阵是个方阵，从左上角到右下角的对角线（称为主对角线）上的元素均为1。其他全都为0，eg:

$$\begin{bmatrix} 1&0&0 \\ 0&1&0 \\ 0&0&1 \end{bmatrix}$$

用

np.eye()

扩展：

np.eye(rows,columns=rows)

help(np.eye)

Help on function eye in module numpy.lib.twodim_base:

eye(N, M=None, k=0, dtype=<class 'float'>, order='C')
Return a 2-D array with ones on the diagonal and zeros elsewhere.

Parameters
----------
N : int
Number of rows in the output.
M : int, optional
Number of columns in the output. If None, defaults to `N`.
k : int, optional
Index of the diagonal: 0 (the default) refers to the main diagonal,
a positive value refers to an upper diagonal, and a negative value
to a lower diagonal.
dtype : data-type, optional
Data-type of the returned array.
order : {'C', 'F'}, optional
Whether the output should be stored in row-major (C-style) or
column-major (Fortran-style) order in memory.

.. versionadded:: 1.14.0

Returns
-------
I : ndarray of shape (N,M)
An array where all elements are equal to zero, except for the `k`-th
diagonal, whose values are equal to one.

See Also
--------
identity : (almost) equivalent function
diag : diagonal 2-D array from a 1-D array specified by the user.

Examples
--------
>>> np.eye(2, dtype=int)
array([[1, 0],
[0, 1]])
>>> np.eye(3, k=1)
array([[ 0.,  1.,  0.],
[ 0.,  0.,  1.],
[ 0.,  0.,  0.]])

# 定义一个2行的单位矩阵（列默认和行一致）
np.eye(2)

array([[1., 0.],
[0., 1.]])

np.eye(3,dtype=int)

array([[1, 0, 0],
[0, 1, 0],
[0, 0, 1]])

# 定义一个5行5列的单位矩阵
np.eye(5)

array([[1., 0., 0., 0., 0.],
[0., 1., 0., 0., 0.],
[0., 0., 1., 0., 0.],
[0., 0., 0., 1., 0.],
[0., 0., 0., 0., 1.]])