Python与矩阵

image.png

Python与C++编程中的矩阵，都是向上图这样坐标的。行表示为i，列表示为j。例如，上图中 matrix[1][0] 的值为 4。

先用python描述一下这个矩阵：

>>> import numpy
>>> m=numpy.array([[1,2,3],[4,5,6],[7,8,9]])
>>> m
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])
>>>

1 某一点

>>> m[1,0]
4

2 某一行

>>> m[1]
array([4, 5, 6])

3 某一列

>>> m[:,1]
array([2, 5, 8])

4 切小块

>>> m[1:,1:]
array([[5, 6],
       [8, 9]])

5 横遍历

>>> for cell in m.flatten():
...     print(cell, end=' ')
... 
1 2 3 4 5 6 7 8 9 >>>

>>> for cell in numpy.nditer(m, order='C'):
...     print(cell, end=' ')
... 
1 2 3 4 5 6 7 8 9 >>>

6 竖遍历

>>> for cell in m.T.flatten():
...     print(cell, end=' ')
... 
1 4 7 2 5 8 3 6 9 >>> 

>>> for cell in numpy.nditer(m, order='F'):
...     print(cell, end=' ')
... 
1 4 7 2 5 8 3 6 9 >>> 

7 3x2 变 2x3 矩阵

>>> m=numpy.array([[1,2],[3,4],[5,6]])
>>> m
array([[1, 2],
       [3, 4],
       [5, 6]])
>>> m.reshape(2,3)
array([[1, 2, 3],
       [4, 5, 6]])

参考

https://www.pythoninformer.com/python-libraries/numpy/index-and-slice/
https://www.pluralsight.com/guides/numpy-arrays-iterating