Resample pandas dataframe only knowing result measurement count

问题

I have a dataframe which looks like this:

Trial    Measurement    Data
    0              0      12 
                   1       4
                   2      12
    1              0      12
                   1      12
    2              0      12
                   1      12
                   2     NaN
                   3      12

I want to resample my data so that every trial has just two measurements So I want to turn it into something like this:

Trial    Measurement    Data
    0              0       8 
                   1       8
    1              0      12
                   1      12
    2              0      12
                   1      12

This rather uncommon task stems from the fact that my data has an intentional jitter on the part of the stimulus presentation.

I know pandas has a resample function, but I have no idea how to apply it to my second-level index while keeping the data in discrete categories based on the first-level index :(

Also, I wanted to iterate, over my first-level indices, but apparently

for sub_df in np.arange(len(df['Trial'].max()))

Won't work because since 'Trial' is an index pandas can't find it.

回答1:

Well, it's not the prettiest I've ever seen, but from a frame looking like

>>> df
   Trial  Measurement  Data
0      0            0    12
1      0            1     4
2      0            2    12
3      1            0    12
4      1            1    12
5      2            0    12
6      2            1    12
7      2            2   NaN
8      2            3    12

then we can manually build the two "average-like" objects and then use pd.melt to reshape the output:

avg = df.groupby("Trial")["Data"].agg({0: lambda x: x.head((len(x)+1)//2).mean(), 
                                       1: lambda x: x.tail((len(x)+1)//2).mean()}) 
result = pd.melt(avg.reset_index(), "Trial", var_name="Measurement", value_name="Data")
result = result.sort("Trial").set_index(["Trial", "Measurement"])

which produces

>>> result

                   Data
Trial Measurement      
0     0               8
      1               8
1     0              12
      1              12
2     0              12
      1              12

来源：https://stackoverflow.com/questions/20107958/resample-pandas-dataframe-only-knowing-result-measurement-count