PANDAS dataframe python：想要按组对值进行排序

Question

Link to census data

I have the following link above for a CSV file containing the raw data for which I wish to manipulate.

census_df = df = pd.read_csv('https://raw.githubusercontent.com/Qian-Han/coursera-Applied-Data-Science-with-Python/master/Introduction-to-Data-Science-in-Python/original_data/census.csv')
sortedit = census_df.sort_values(by = ['STNAME','CENSUS2010POP'],ascending=False)

I am trying to order the data in descending order by the column 'CENSUS2010POP'.

I also want to order the data by 'state' alphabetically, hence why I have including the 'STNAME' column in the formula above.

However, I only want to select the 3 highest values for 'CENSUS2010POP' from each state ('STNAME').

Thus, if there are 146 states in total, I should (146 x 3) rows in my new dataframe (and thus in the 'CENSUS2010POP' column).

I would be so grateful if anybody could give me a helping hand?

Answer 1

IIUC, groupby with .nalrgest to create an index filter, chained with sort_values

df2 = df.iloc[df.groupby('STNAME')['CENSUS2010POP']\
                              .nlargest(3).index.get_level_values(1)]\
                              .sort_values(['STNAME','CENSUS2010POP'],ascending=True)

print(df['STNAME'].nunique())

51


print(df2.shape)
(152, 100)

---

print(df2[['STNAME','CENSUS2010POP']])

   STNAME  CENSUS2010POP
49      Alabama         412992
37      Alabama         658466
0       Alabama        4779736
76       Alaska          97581
71       Alaska         291826
...         ...            ...
3137  Wisconsin         947735
3096  Wisconsin        5686986
3182    Wyoming          75450
3180    Wyoming          91738
3169    Wyoming         563626

[152 rows x 2 columns]

Answer 2

try this:

df = census_df.groupby(["STNAME"]).apply(lambda x: x.sort_values(["CENSUS2010POP"], ascending = False)).reset_index(drop=True)

df.groupby('STNAME').head(3)[['STNAME','CENSUS2010POP']]

The first statement returns dataframe sorted by CENSUS2010POP in each STNAME .

The second statement returns the top 3.