Groupby column and find min and max of each group

Question

I have the following dataset,

        Day    Element  Data_Value
6786    01-01   TMAX    112
9333    01-01   TMAX    101
9330    01-01   TMIN    60
11049   01-01   TMIN    0
6834    01-01   TMIN    25
11862   01-01   TMAX    113
1781    01-01   TMAX    115
11042   01-01   TMAX    105
1110    01-01   TMAX    111
651     01-01   TMIN    44
11350   01-01   TMIN    83
1798    01-02   TMAX    70
4975    01-02   TMAX    79
12774   01-02   TMIN    0
3977    01-02   TMIN    60
2485    01-02   TMAX    73
4888    01-02   TMIN    31
11836   01-02   TMIN    26
11368   01-02   TMAX    71
2483    01-02   TMIN    26

I want to group by the Day and then find the overall min of TMIN an the max of TMAX and put these in to a data frame, so I get an output like...

Day    DayMin    DayMax
01-01  0         115
01-02  0         79

I know I need to do,

df.groupby(by='Day')

but I am a stuck with the next step - should create columns to store the TMAX and TMIN values?

Answer 1

You can use a assign + abs, followed by groupby + agg:

df = df.assign(Data_Value=df.Data_Value.abs())\
       .groupby(['Day']).Data_Value.agg([('Min' , 'min'), ('Max', 'max')])\
       .add_prefix('Day')

df 
       DayMin  DayMax
Day                  
01-01       0     115
01-02       0      79

Answer 2

Use

In [5265]: def maxmin(x):
      ...:     mx = x[x.Element == 'TMAX'].Data_Value.max()
      ...:     mn = x[x.Element == 'TMIN'].Data_Value.min()
      ...:     return pd.Series({'DayMin': mn, 'DayMax': mx})
      ...:

In [5266]: df.groupby('Day').apply(maxmin)
Out[5266]:
       DayMax  DayMin
Day
01-01     115       0
01-02      79       0

Also,

In [5268]: df.groupby('Day').apply(maxmin).reset_index()
Out[5268]:
     Day  DayMax  DayMin
0  01-01     115       0
1  01-02      79       0

Or, use query instead of x[x.Element == 'TMAX'] as x.query("Element == 'TMAX'")

Answer 3

Create duplicate columns and find min and max using agg i.e

ndf = df.assign(DayMin = df['Data_Value'].abs(),DayMax=df['Data_Value'].abs()).groupby('Day')\
     .agg({'DayMin':'min','DayMax':'max'})

     DayMax  DayMin
Day                  
01-01     115       0
01-02      79       0

Incase you want both TMIN and TMAX then groupby(['Day','Element'])