How can I randomly change the values of some rows in a pandas DataFrame?

Question

I have a pandas Dataframe like below:

UserId    ProductId    Quantity
1         1            6
1         4            1
1         7            3
2         4            2
3         2            7
3         1            2

Now, I want to randomly select the 20% of rows of this DataFrame, using df.sample(n), and change the value of the Quantity column of these rows to zero. I would also like to keep the indexes of the altered rows. So the resulting DataFrame would be:

UserId    ProductId    Quantity
1         1            6
1         4            1
1         7            3
2         4            0
3         2            7
3         1            0

and I would like to keep in a list that the rows 3 and 5 have been altered. How can I achieve that?

Answer 1

By using update

dfupdate=df.sample(2)
dfupdate.Quantity=0
df.update(dfupdate)
update_list = dfupdate.index.tolist() # from  cᴏʟᴅsᴘᴇᴇᴅ  :)
df
Out[44]: 
   UserId  ProductId  Quantity
0     1.0        1.0       6.0
1     1.0        4.0       0.0
2     1.0        7.0       3.0
3     2.0        4.0       0.0
4     3.0        2.0       7.0
5     3.0        1.0       2.0

Answer 2

Using loc to change the data i.e

change = df.sample(2).index
df.loc[change,'Quantity'] = 0

Output:

  UserId  ProductId  Quantity
0       1          1         0
1       1          4         1
2       1          7         3
3       2          4         0
4       3          2         7
5       3          1         2

change.tolist() : [3, 0]