Number rows within group in increasing order in a pandas dataframe
Question:
Given the following dataframe:
import pandas as pd
import numpy as np
df=pd.DataFrame({'A':['A','A','A','B','B','B'],
'B':['a','a','b','a','a','a'],
})
df
A B
0 A a
1 A a
2 A b
3 B a
4 B a
5 B a
I’d like to create column ‘C’, which numbers the rows within each group in columns A and B like this:
A B C
0 A a 1
1 A a 2
2 A b 1
3 B a 1
4 B a 2
5 B a 3
I’ve tried this so far:
df['C'] = df.groupby(['A','B'])['B'].transform('rank')
…but it doesn’t work!
Answers:
Use groupby/cumcount
:
In [25]: df['C'] = df.groupby(['A','B']).cumcount()+1; df
Out[25]:
A B C
0 A a 1
1 A a 2
2 A b 1
3 B a 1
4 B a 2
5 B a 3
Use groupby.rank function.
Here the working example.
df = pd.DataFrame({'C1':['a', 'a', 'a', 'b', 'b'], 'C2': [1, 2, 3, 4, 5]})
df
C1 C2
a 1
a 2
a 3
b 4
b 5
df["RANK"] = df.groupby("C1")["C2"].rank(method="first", ascending=True)
df
C1 C2 RANK
a 1 1
a 2 2
a 3 3
b 4 1
b 5 2
OP’s code was missing the appropriate method
to get the correct output.
df['C'] = df.groupby(['A','B'])['B'].transform('rank', method='first')
df
A B C
0 A a 1.0
1 A a 2.0
2 A b 1.0
3 B a 1.0
4 B a 2.0
5 B a 3.0
Given the following dataframe:
import pandas as pd
import numpy as np
df=pd.DataFrame({'A':['A','A','A','B','B','B'],
'B':['a','a','b','a','a','a'],
})
df
A B
0 A a
1 A a
2 A b
3 B a
4 B a
5 B a
I’d like to create column ‘C’, which numbers the rows within each group in columns A and B like this:
A B C
0 A a 1
1 A a 2
2 A b 1
3 B a 1
4 B a 2
5 B a 3
I’ve tried this so far:
df['C'] = df.groupby(['A','B'])['B'].transform('rank')
…but it doesn’t work!
Use groupby/cumcount
:
In [25]: df['C'] = df.groupby(['A','B']).cumcount()+1; df
Out[25]:
A B C
0 A a 1
1 A a 2
2 A b 1
3 B a 1
4 B a 2
5 B a 3
Use groupby.rank function.
Here the working example.
df = pd.DataFrame({'C1':['a', 'a', 'a', 'b', 'b'], 'C2': [1, 2, 3, 4, 5]})
df
C1 C2
a 1
a 2
a 3
b 4
b 5
df["RANK"] = df.groupby("C1")["C2"].rank(method="first", ascending=True)
df
C1 C2 RANK
a 1 1
a 2 2
a 3 3
b 4 1
b 5 2
OP’s code was missing the appropriate method
to get the correct output.
df['C'] = df.groupby(['A','B'])['B'].transform('rank', method='first')
df
A B C
0 A a 1.0
1 A a 2.0
2 A b 1.0
3 B a 1.0
4 B a 2.0
5 B a 3.0