How to calculate mean values grouped on another column in Pandas

Question

For the following dataframe:

StationID  HoursAhead    BiasTemp  
SS0279           0          10
SS0279           1          20
KEOPS            0          0
KEOPS            1          5
BB               0          5
BB               1          5

I’d like to get something like:

StationID  BiasTemp  
SS0279     15
KEOPS      2.5
BB         5

I know I can script something like this to get the desired result:

def transform_DF(old_df,col):
    list_stations = list(set(old_df['StationID'].values.tolist()))
    header = list(old_df.columns.values)
    header.remove(col)
    header_new = header
    new_df = pandas.DataFrame(columns = header_new)
    for i,station in enumerate(list_stations):
        general_results = old_df[(old_df['StationID'] == station)].describe()
        new_row = []
        for column in header_new:
            if column in ['StationID']: 
                new_row.append(station)
                continue
            new_row.append(general_results[column]['mean'])
        new_df.loc[i] = new_row
    return new_df

But I wonder if there is something more straightforward in pandas.

Asked By: Rafa

||

Source

Answer 1

This is what groupby is for:

In [117]:
df.groupby('StationID')['BiasTemp'].mean()

Out[117]:
StationID
BB         5.0
KEOPS      2.5
SS0279    15.0
Name: BiasTemp, dtype: float64

Here we groupby the ‘StationID’ column, we then access the ‘BiasTemp’ column and call mean on it

There is a section in the docs on this functionality.

Answered By: EdChum

Answer 2

You could groupby on StationID and then take mean() on BiasTemp. To output Dataframe, use as_index=False

In [4]: df.groupby('StationID', as_index=False)['BiasTemp'].mean()
Out[4]:
  StationID  BiasTemp
0        BB       5.0
1     KEOPS       2.5
2    SS0279      15.0

Without as_index=False, it returns a Series instead

In [5]: df.groupby('StationID')['BiasTemp'].mean()
Out[5]:
StationID
BB            5.0
KEOPS         2.5
SS0279       15.0
Name: BiasTemp, dtype: float64

Read more about groupby in this pydata tutorial.

Answered By: Zero

Answer 3

can be done as follows:

df.groupby('StationID').mean()

Answered By: svp

How to calculate mean values grouped on another column in Pandas

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