关于NumPy数组切片何时是引用以及何时是副本的困惑 [英] Confusion about when NumPy array slices are references and when they are copies

问题描述

我有以下代码:

import numpy as np
arr = np.arange(10)
slice = arr[2:5]
slice[:] = 12
print(arr)
slice = slice / 2
print(arr)

输出将是:

[ 0  1 12 12 12  5  6  7  8  9]
[6. 6. 6.]
[ 0  1 12 12 12  5  6  7  8  9]

因此，围绕 slice 的第一次只是对 arr 的一部分的引用，因此对其进行修改也将更改数组，但是第二次围绕它已成为的副本.数组的该部分并对其进行修改在 arr 中没有区别.为什么会这样?是什么使 slice = slice/2 与众不同?

So the first time around slice is just a reference to part of arr so modifying it also changes the array, but the second time around it has become a copy of that part of the array and modifying it makes no difference in arr. Why does this happen? What makes slice = slice / 2 different?

推荐答案

使用切片对象建立索引始终会返回数组的视图(引用).修改切片将修改原始数组.在第二个示例中，您将其分配给slice对象.那不会修改对象.将使用指定的值(在本例中为slice/2)创建一个新对象.如果这是所需的行为，则可以使用/= 在适当的位置修改该对象，或者在切片中建立索引([:] )，numpy解释为修改这些索引处的条目.

Indexing with a slice object always returns a view (reference) of an array. Modifying the slice will modify the original array. In your second example you assign to the slice object. That does not modify the object. A new object is created with the specified values, in this case slice / 2. You can use /= to modify the object in place if that's the desired behavior, or index into the slice ([:]) which numpy interprets as modifying the entries at those indices.

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