对同一数据帧的不同部分进行操作

Question

Assume there is a dataframe:

    kind  value
0      1      1
1      1      2
2      1      3
3      1      4
4      1      5
5      2      6
6      2      7
7      2      8
8      2      9
9      2      10

We can do something with a filtered part of a dataframe:

df.loc[df['kind']==1, 'value'] = df.loc[df['kind']==1, 'value'] * 2

How to perform a calculation involving two or more parts of the same dataframe, assuming their size is equal? Something like this:

df.loc[df['kind']==1, 'value'] = 
    df.loc[df['kind']==1, 'value'] * df.loc[df['kind']==2, 'value']

(this doesn't work)

Answer 1

Try this:

In [107]: df.loc[df['kind']==1, 'value'] *= df.loc[df['kind']==2, 'value'].values

In [108]: df
Out[108]:
   kind  value
0     1      6
1     1     14
2     1     24
3     1     36
4     1     50
5     2      6
6     2      7
7     2      8
8     2      9
9     2     10

Answer 2

Use:

m = df['kind']==1
df.loc[m, 'value'] = df.loc[m, 'value'].values * df.loc[df['kind']==2, 'value'].values
print (df)
   kind  value
0     1      6
1     1     14
2     1     24
3     1     36
4     1     50
5     2      6
6     2      7
7     2      8
8     2      9
9     2     10