pandas groupby mean with nan

问题:

I have the following dataframe:

date id  cars 2012 1    4   2013 1    6 2014 1    NaN     2012 2    10  2013 2    20  2014 2    NaN   

Now, I want to get the mean of cars over the years for each id ignoring the NaN's. The result should be like this:

date id  cars  result 2012 1    4      5 2013 1    6      5 2014 1    NaN    5 2012 2    10     15 2013 2    20     15 2014 2    NaN    15 

I have the following command:

df["result"]=df.groupby("id")["cars"].mean() 

The command runs without errors, but the result column only has NaN's. What did I do wrong?

回答1:

Use transform, this returns a series the same size as the original:

df["result"]=df.groupby("id")["cars"].transform('mean') print (df)    date  id  cars  result 0  2012   1   4.0     5.0 1  2013   1   6.0     5.0 2  2014   1   NaN     5.0 3  2012   2  10.0    15.0 4  2013   2  20.0    15.0 5  2014   2   NaN    15.0 

文章来源: pandas groupby mean with nan