[英]Use of slicing numpy object in a list
I have a problem using the numpy slicing. 我在使用numpy切片时遇到问题。 I don't even know how to give this problem a name or title.
我什至不知道如何给这个问题起一个名字或标题。
Below are a segment of test code. 以下是一段测试代码。
import numpy
input_items = []
output_items = []
input_items.insert(0, numpy.array([1, 2, 3], dtype=numpy.float32))
output_items.insert(0, numpy.array([0, 0, 0], dtype=numpy.float32))
in0 = input_items[0]
out = output_items[0]
print "Before, input_items[0] : {0}".format(input_items[0])
print "Before, output_items[0]: {0}".format(output_items[0])
out[:] = in0 * 2
#out = in0 * 2
print "After, input_item[0] : {0}".format(input_items[0])
print "After, output_item[0] : {0}".format(output_items[0])
If I use out[:] = in0 * 2
, I will get: 如果我使用
out[:] = in0 * 2
,我将得到:
Before, input_items[0] : [ 1. 2. 3.]
Before, output_items[0]: [ 0. 0. 0.]
After, input_items[0] : [ 1. 2. 3.]
After, output_items[0] : [ 2. 4. 6.]
If I use out = in0 * 2
, I will get: 如果使用
out = in0 * 2
,我将得到:
Before, input_items[0] : [ 1. 2. 3.]
Before, output_items[0]: [ 0. 0. 0.]
After, input_items[0] : [ 1. 2. 3.]
After, output_items[0] : [ 0. 0. 0.]
In the code, I have assign output_items[0]
to out
, but obviously the use of out
or out[:]
can affect the result of output_items[0]
. 在代码中,我已将
output_items[0]
分配给out
,但是显然使用out
或out[:]
会影响output_items[0]
的结果。 Could anyone figure it out? 有人能弄清楚吗?
Thanks. 谢谢。
out[:] = in0 * 2
changes your original array, because in numpy slicing is a view on the original array (which is NOT a copy) , so you get a reference to it and change it out[:] = in0 * 2
更改原始数组,因为在numpy 切片中是原始数组的视图 (不是副本) ,因此您可以获取对其的引用并进行更改
out = in0 * 2
doesn't change any original array, because you are simply assigning a computed result to out
(the result is stored in a fresh new separate array), therefore isn't a reference to output_items
or input_items
out = in0 * 2
不会更改任何原始数组,因为您只是将计算结果分配给out
(结果存储在新的单独数组中),因此不是对output_items
或input_items
的引用
If you need to copy an array, you can use numpy.copy()
, not just assign out = output_items[0]
If you assign without a copy, you are still modifying the same array, so that change will reflect elsewhere (eg between out
and output_items[0]
如果您需要复制一个数组,则可以使用
numpy.copy()
,而不仅仅是分配out = output_items[0]
如果您没有复制就进行分配,则您仍在修改同一数组,这样更改将反映在其他地方(例如, out
和output_items[0]
So eg if you do out = output_items[0].copy()
, now out has a fresh new array copied from the values in output_items[0]
, but won't be affecting it 因此,例如,如果执行
out = output_items[0].copy()
,则out会从output_items[0]
的值复制一个新的新数组,但不会影响它
ref http://docs.scipy.org/doc/numpy/reference/generated/numpy.copy.html 参考http://docs.scipy.org/doc/numpy/reference/generated/numpy.copy.html
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