问题描述
在Python中，我想从1维数组减去一行行的2维数组。
In python, I wish to subtract line by line a 2-dim array from a 1-dim array.
我知道如何与一个for循环和索引的做到这一点，但我想这可能会更快使用numpy的功能。但是我没有找到一个方法来做到这一点。这是一个带'for'循环的例子：
I know how to do it with a 'for' loop and indexes but I suppose it may be quicker to use numpy functions. However I did not find a way to do it. Here is an example with a 'for' loop :
from numpy import *
x=array([[1,2,3,4,5],[6,7,8,9,10]])
y=array([20,10])
j=array([0, 1])
a=zeros([2,5])
for i in j :
... a[i]=y[i]-x[i]
这是一些不工作的范例，通过这种取代'for'循环：
And here is an example of something that does not work, replacing the 'for' loop by this:
a=y[j]-x[j,i]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: shape mismatch: objects cannot be broadcast to a single shape
豆你有什么建议？
Dou you have suggestions ?
推荐答案
的问题是， YX 有各自的形状（2）（2 5）。要做到正确的广播，你需要的形状（2,1）（2,5）。我们可以用 .reshape 做到这一点，只要元素的数量是preserved：
The problem is that y-x have the respective shapes (2) (2,5). To do proper broadcasting, you'll need shapes (2,1) (2,5). We can do this with .reshape as long as the number of elements are preserved:
y.reshape(2,1) - x
给出：
array([[19, 18, 17, 16, 15],
[ 4, 3, 2, 1, 0]])
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