Assign oridinal number to unique data in a pandas dataframe column

I have a dataframe like this:

ID  Time
0   100
1   100
2   200
3   200
4   500
5   500
6   890
...

I want to add a column of ordinal number based on column ‘Time’ in ascending order, like this:

ID  Time  Ordinal
0   100      1
1   100      1
2   200      2
3   200      2
4   500      3
5   500      3
6   890      4
...

Is there any elegant way to do this?

>Solution :

You could use pd.dataframe.rank:

>>> df["Ordinal"] = df["Time"].rank(method="dense")  # Optionally, add .astype(int) to cast to int
>>> df
   ID  Time  Ordinal
0   0   100      1.0
1   1   100      1.0
2   2   200      2.0
3   3   200      2.0
4   4   500      3.0
5   5   500      3.0
6   6   890      4.0