替换numpy数组值python [英] Replacing numpy array values python

问题描述

我在尝试替换numpy数组中的值时遇到了可怕的时光，遇到了我希望有人能解释的非常奇怪的行为.本质上，我想在遗传算法中进行交叉操作.这是一个简单的例子.我有一个2 X 10的数组，并希望将第1行直到第5列中的所有值与第2行直到第5列中的值交换.这是代码:

I am having a terrible time trying to replace values in a numpy array and running up against a very strange behavior I was hoping someone could explain. Essentially I want to do a crossing over operation in a genetic algorithm. Here's a simple example. I have a 2 X 10 array, and want all the values in row 1 up to column 5 to be swapped with the values in row 2 up to column 5. Here's the code:

z=np.random.uniform(low=0,high=1,size=(2,10))
zcopy = z
print z

[[ 0.77488523  0.39966358  0.63233664  0.77093136  0.04102615  0.98984184
0.43402537  0.0910648   0.28037032  0.76654885]
[ 0.49980878  0.28161905  0.71972029  0.01208004  0.87851569  0.16853681
0.96325992  0.90886083  0.12344231  0.83665396]]

z[1,range(4)] = zcopy[0,range(4)]
print z

[[ 0.77488523  0.39966358  0.63233664  0.77093136  0.04102615  0.98984184
0.43402537  0.0910648   0.28037032  0.76654885]
[ 0.77488523  0.39966358  0.63233664  0.77093136  0.87851569  0.16853681
0.96325992  0.90886083  0.12344231  0.83665396]]

如您所见，它只是将第1行的所有内容复制到两行中.但是，如果我不指定另一个数组的子集，而只是给它说整数，那么它会完美地工作

As you can see it's just copied all of row 1 into both rows. But, if I don't specify a subset of another array but just give it say integers it works perfectly

z[1,range(4)] = range(4)
print z
[[ 0.77488523  0.39966358  0.63233664  0.77093136  0.04102615  0.98984184
0.43402537  0.0910648   0.28037032  0.76654885]
[ 0.          1.          2.          3.          0.87851569  0.16853681
0.96325992  0.90886083  0.12344231  0.83665396]]

我很困惑.有谁知道如何解决这个问题?

I'm rather perplexed. Does anyone have any idea how to work around this?

推荐答案

这里似乎有两个问题.

There seem to be two questions here.

1. 为什么zcopy = z不进行复制?"
2. 为什么z[1,range(4)] = zcopy[0,range(4)]不交换前四列中的值?"

1. "Why doesn't zcopy = z make a copy?"
2. "Why doesn't z[1,range(4)] = zcopy[0,range(4)] swap the values in the first four columns?"

第一个问题的答案是，在Python中为变量名称分配值不会产生副本. Python中的变量只是对象的标签；给对象一个新的标签根本不会改变对象本身.如果要复制numpy数组，则可以使用copy方法，该方法将返回该数组的新副本.如:

The answer to the first question is that assigning a value to a variable name in Python doesn't make a copy. A variable in Python is just a label for an object; giving the object a new label doesn't change the object itself at all. If you want to make a copy of a numpy array, specifically, you can use the copy method, which returns a new copy of the array. As in:

zcopy = z.copy()

第二个问题的答案是您的代码仅分配一组值:z[1, range(4)].如果要更改两行，则必须分配给两行！在许多语言中，您都可以使用tmp变量来执行此操作，但是python提供了一种无需临时变量即可交换值的绝佳方法:

The answer to the second question is that your code only assigns one set of values: z[1, range(4)]. If you want to change both rows, you have to assign to both rows! In many languages you'd do this with a tmp variable, but python provides an elegant way to swap values without needing temporary variables:

>>> z[1, range(4)], z[0, range(4)] = z[0, range(4)], z[1, range(4)]

nye17 的答案比较简洁，但它的作用相同.

nye17's answer is a bit cleaner, but it does the same thing.

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