pandas 如何通过广播将pandas数据帧与numpy数组相乘
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how to multiply pandas dataframe with numpy array with broadcasting
提问by Wei Li
I have a dataframe of shape (4, 3) as following:
我有一个形状为 (4, 3) 的数据框，如下所示：
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: x = pd.DataFrame(np.random.randn(4, 3), index=np.arange(4))
In [4]: x
Out[4]:
0 1 2
0 0.959322 0.099360 1.116337
1 -0.211405 -2.563658 -0.561851
2 0.616312 -1.643927 -0.483673
3 0.235971 0.023823 1.146727
I want to multiply each column of the dataframe with a numpy array of shape (4,):
我想将数据框的每一列与形状为 (4,) 的 numpy 数组相乘：
In [9]: y = np.random.randn(4)
In [10]: y
Out[10]: array([-0.34125522, 1.21567883, -0.12909408, 0.64727577])
In numpy, the following broadcasting trick works:
在 numpy 中，以下广播技巧有效：
In [12]: x.values * y[:, None]
Out[12]:
array([[-0.32737369, -0.03390716, -0.38095588],
[-0.25700028, -3.11658448, -0.68303043],
[-0.07956223, 0.21222123, 0.06243928],
[ 0.15273815, 0.01541983, 0.74224861]])
However, it doesn't work in the case of pandas dataframe, I get the following error:
但是，它不适用于 Pandas 数据框，出现以下错误：
In [13]: x * y[:, None]
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-13-21d033742c49> in <module>()
----> 1 x * y[:, None]
...
ValueError: Shape of passed values is (1, 4), indices imply (3, 4)
Any suggestions?
有什么建议？
Thanks!
谢谢！
回答by Wei Li
I find an alternative way to do the multiplication between pandas dataframe and numpy array.
我找到了一种替代方法来在 pandas 数据帧和 numpy 数组之间进行乘法。
In [14]: x.multiply(y, axis=0)
Out[14]:
0 1 2
0 0.195346 0.443061 1.219465
1 0.194664 0.242829 0.180010
2 0.803349 0.091412 0.098843
3 0.365711 -0.388115 0.018941
回答by dagrha
I think you are better off using the df.apply()method. In your case:
我认为您最好使用df.apply()方法。在你的情况下：
x.apply(lambda x: x * y)
