Pandas 中的 Hash 表映射

Question

I have a large dataset with millions of rows of data. One of the data columns is ID.

I also have another (hash)table that maps the range of indices to a specific group that meets a certain criteria.

What is an efficient way to map the range of indices to include them as an additional column on my dataset in pandas?

As an example, lets say that the dataset looks like this:

In [18]:
print(df_test)

Out [19]:
    ID
0   13
1   14
2   15
3   16
4   17
5   18
6   19
7   20
8   21
9   22
10  23
11  24
12  25
13  26
14  27
15  28
16  29
17  30
18  31
19  32

Now the hash table with the range of indices looks like this:

In [20]:
print(df_hash)

Out [21]:
   ID_first
0         0
1         2
2        10

where the index specifies the group number that I need.

I tried doing something like this:

for index in range(df_hash.size):
    try:
        df_test.loc[df_hash.ID_first[index]:df_hash.ID_first[index + 1], 'Group'] = index
    except:
        df_test.loc[df_hash.ID_first[index]:, 'Group'] = index

Which works well, except that it is really slow as it loops over the length of the hash table dataframe (hundreds of thousands of rows). It produces the following answer (which I want):

In [23]:
print(df_test)

Out [24]:
    ID  Group
0   13    0
1   14    0
2   15    1
3   16    1
4   17    1
5   18    1
6   19    1
7   20    1
8   21    1
9   22    1
10  23    2
11  24    2
12  25    2
13  26    2
14  27    2
15  28    2
16  29    2
17  30    2
18  31    2
19  32    2

Is there a way to do this more efficiently?

Answer 1

You can map the index of df_test using ID_first to the index of df_hash, and then ffill . Need to construct a Series as the pd.Index class doesn't have a ffill method.

df_test['group'] = (pd.Series(df_test.index.map(dict(zip(df_hash.ID_first, df_hash.index))), 
                              index=df_test.index)
                      .ffill(downcast='infer'))

#    ID  group
#0   13      0
#1   14      0
#2   15      1
#...
#9   22      1
#10  23      2
#...
#17  30      2
#18  31      2
#19  32      2

Answer 2

you can do series.isin with series.cumsum

df_test['group'] = df_test['ID'].isin(df_hash['ID_first']).cumsum() #.sub(1)

---

print(df_test)

    ID  group
0    0      1
1    1      1
2    2      2
3    3      2
4    4      2
5    5      2
6    6      2
7    7      2
8    8      2
9    9      2
10  10      3
11  11      3
12  12      3
13  13      3
14  14      3
15  15      3
16  16      3
17  17      3
18  18      3
19  19      3