Python:Pandas Dataframe如何将整个列与标量相乘 [英] Python: Pandas Dataframe how to multiply entire column with a scalar

问题描述

如何将数据框给定列的每个元素与标量相乘? (我曾尝试过寻找SO，但似乎找不到正确的解决方案)

How do I multiply each element of a given column of my dataframe with a scalar? (I have tried looking on SO, but cannot seem to find the right solution)

做类似的事情:

df['quantity'] *= -1 # trying to multiply each row's quantity column with -1

给我一​​个警告:

A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

注意:如果可能的话，我不想遍历数据框并执行类似的操作...因为我认为整个列上的任何标准数学运算都应该是可能的，而不必编写循环:

Note: If possible, I do not want to be iterating over the dataframe and do something like this...as I think any standard math operation on an entire column should be possible w/o having to write a loop:

for idx, row in df.iterrows():
    df.loc[idx, 'quantity'] *= -1

编辑:

我正在运行0.16.2熊猫

完整跟踪:

 SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self.obj[item] = s

推荐答案

经过一番研究后，答案如下:

Here's the answer after a bit of research:

df.loc[:,'quantity'] *= -1 #seems to prevent SettingWithCopyWarning 

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