数组中的减法会影响结果

Question

当我将一个矩阵的数组保存到另一个变量中时，当我改变矩阵的值时，另一个变量的值也被改变了。不知道为什么。

import numpy as np

a = np.array([[1,2,3],[4,5,6]])
b = a[1,:]
print a
print b

a[1,:] = a[1,:] - a[0,:]
print a
print b

结果是

[[1 2 3]
 [4 5 6]]
[4 5 6]

[[1 2 3]
[3 3 3]]
[3 3 3]

在这个脚本中，当a改变时b的值也改变了。

Answer 1

您需要阅读更多关于浅拷贝的内容：

Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations.

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

示例：

>>> lst1 = ['a','b',['ab','ba']]
>>> lst2 = lst1[:]
>>> lst2[0] = 'c'
>>> lst2[2][1] = 'd'
>>> print(lst1)
['c', 'b', ['ab', 'd']]

现在，您必须使用deepcopy，例如：

>>>from copy import deepcopy
>>>lst1 = ['a','b',['ab','ba']]
>>>lst2 = deepcopy(lst1)
>>>lst2[2][1] = "d"
>>>lst2[0] = "c";
>>>print lst2
>>>print lst1
['c', 'b', ['ab', 'd']]
['a', 'b', ['ab', 'ba']]

您可以阅读有关 Shallow and Deep Copy 的更多信息。

数组中的减法会影响结果

Subtraction in array affects the result

python

numpy

subtraction