Python pandas dataframe: find max for each unique values of an another column

问题

I have a large dataframe (from 500k to 1M rows) which contains for example these 3 numeric columns: ID, A, B

I want to filter the results in order to obtain a table like the one in the image below, where, for each unique value of column id, i have the maximum and minimum value of A and B. How can i do?

EDIT: i have updated the image below in order to be more clear: when i get the max or min from a column i need to get also the data associated to it of the others columns

回答1:

Sample data (note that you posted an image which can't be used by potential answerers without retyping, so I'm making a simple example in its place):

df=pd.DataFrame({ 'id':[1,1,1,1,2,2,2,2],
                   'a':range(8), 'b':range(8,0,-1) })

The key to this is just using idxmax and idxmin and then futzing with the indexes so that you can merge things in a readable way. Here's the whole answer and you may wish to examine intermediate dataframes to see how this is working.

df_max = df.groupby('id').idxmax()
df_max['type'] = 'max'
df_min = df.groupby('id').idxmin()
df_min['type'] = 'min'

df2 = df_max.append(df_min).set_index('type',append=True).stack().rename('index')

df3 = pd.concat([ df2.reset_index().drop('id',axis=1).set_index('index'), 
                  df.loc[df2.values] ], axis=1 )

df3.set_index(['id','level_2','type']).sort_index()

                 a  b
id level_2 type      
1  a       max   3  5
           min   0  8
   b       max   0  8
           min   3  5
2  a       max   7  1
           min   4  4
   b       max   4  4
           min   7  1

Note in particular that df2 looks like this:

id  type   
1   max   a    3
          b    0
2   max   a    7
          b    4
1   min   a    0
          b    3
2   min   a    4
          b    7

The last column there holds the index values in df that were derived with idxmax & idxmin. So basically all the information you need is in df2. The rest of it is just a matter of merging back with df and making it more readable.

来源：https://stackoverflow.com/questions/40568438/python-pandas-dataframe-find-max-for-each-unique-values-of-an-another-column

标签

python

pandas

dataframe

grouping