NumPy - 副本和视图

简述

在执行函数时，其中一些返回输入数组的副本，而一些返回视图。当内容物理存储在另一个位置时，它被称为Copy. 另一方面，如果提供了相同内存内容的不同视图，我们称其为View.

没有复制

简单的赋值不会复制数组对象。相反，它使用与原始数组相同的 id() 来访问它。这id()返回 Python 对象的通用标识符，类似于 C 中的指针。

此外，任何一方的任何变化都会反映在另一方中。例如，一个人的形状变化也会改变另一个人的形状。

例子


import numpy as np 
a = np.arange(6) 
print 'Our array is:' 
print a  
print 'Applying id() function:' 
print id(a)  
print 'a is assigned to b:' 
b = a 
print b  
print 'b has same id():' 
print id(b)  
print 'Change shape of b:' 
b.shape = 3,2 
print b  
print 'Shape of a also gets changed:' 
print a

它将产生以下输出 -


Our array is:
[0 1 2 3 4 5]
Applying id() function:
139747815479536
a is assigned to b:
[0 1 2 3 4 5]
b has same id():
139747815479536
Change shape of b:
[[0 1]
 [2 3]
 [4 5]]
Shape of a also gets changed:
[[0 1]
 [2 3]
 [4 5]]

查看或浅拷贝

NumPy 有ndarray.view()方法是一个新的数组对象，它查看原始数组的相同数据。与之前的情况不同，新数组的维度变化不会改变原始数组的维度。

例子


import numpy as np 
# To begin with, a is 3X2 array 
a = np.arange(6).reshape(3,2) 
print 'Array a:' 
print a  
print 'Create view of a:' 
b = a.view() 
print b  
print 'id() for both the arrays are different:' 
print 'id() of a:'
print id(a)  
print 'id() of b:' 
print id(b)  
# Change the shape of b. It does not change the shape of a 
b.shape = 2,3 
print 'Shape of b:' 
print b  
print 'Shape of a:' 
print a

它将产生以下输出 -


Array a:
[[0 1]
 [2 3]
 [4 5]]
Create view of a:
[[0 1]
 [2 3]
 [4 5]]
id() for both the arrays are different:
id() of a:
140424307227264
id() of b:
140424151696288
Shape of b:
[[0 1 2]
 [3 4 5]]
Shape of a:
[[0 1]
 [2 3]
 [4 5]]

数组的切片创建一个视图。

例子


import numpy as np 
a = np.array([[10,10], [2,3], [4,5]]) 
print 'Our array is:' 
print a  
print 'Create a slice:' 
s = a[:, :2] 
print s

它将产生以下输出 -


Our array is:
[[10 10]
 [ 2 3]
 [ 4 5]]
Create a slice:
[[10 10]
 [ 2 3]
 [ 4 5]]

深拷贝

这ndarray.copy()函数创建一个深层副本。它是数组及其数据的完整副本，不与原始数组共享。

例子


import numpy as np 
a = np.array([[10,10], [2,3], [4,5]]) 
print 'Array a is:' 
print a  
print 'Create a deep copy of a:' 
b = a.copy() 
print 'Array b is:' 
print b 
#b does not share any memory of a 
print 'Can we write b is a' 
print b is a  
print 'Change the contents of b:' 
b[0,0] = 100 
print 'Modified array b:' 
print b  
print 'a remains unchanged:' 
print a

它将产生以下输出 -


Array a is:
[[10 10]
 [ 2 3]
 [ 4 5]]
Create a deep copy of a:
Array b is:
[[10 10]
 [ 2 3]
 [ 4 5]]
Can we write b is a
False
Change the contents of b:
Modified array b:
[[100 10]
 [ 2 3]
 [ 4 5]]
a remains unchanged:
[[10 10]
 [ 2 3]
 [ 4 5]]