Pandas DENSE RANK

I'm dealing with pandas dataframe and have a frame like this:

Year Value  
2012  10
2013  20
2013  25
2014  30

I want to make an equialent to DENSE_RANK () over (order by year) function. to make an additional column like this:

    Year Value Rank
    2012  10    1
    2013  20    2
    2013  25    2
    2014  30    3

How can it be done in pandas?

Thanks!

Solution 1:

Use pd.Series.rank with method='dense'

df['Rank'] = df.Year.rank(method='dense').astype(int)

df

Solution 2:

The fastest solution is factorize:

df['Rank'] = pd.factorize(df.Year)[0] + 1

Timings:

#len(df)=40k
df = pd.concat([df]*10000).reset_index(drop=True)

In [13]: %timeit df['Rank'] = df.Year.rank(method='dense').astype(int)
1000 loops, best of 3: 1.55 ms per loop

In [14]: %timeit df['Rank1'] = df.Year.astype('category').cat.codes + 1
1000 loops, best of 3: 1.22 ms per loop

In [15]: %timeit df['Rank2'] = pd.factorize(df.Year)[0] + 1
1000 loops, best of 3: 737 µs per loop