How to vectorize an operation that uses previous values?

Question

I want to do something like this:

df['indicator'] = df.at[x-1] + df.at[x-2]

or

df['indicator'] = df.at[x-1] > df.at[x-2]

I guess edge cases would be taken care of automatically, e.g. skip the first few rows.

Answer 1

This line should give you what you need. The first two rows for your indicator column will be automatically filled with 'NaN'.

df['indicator'] = df.at.shift(1) + df.at.shift(2)

For example, if we had the following dataframe:

a = pd.DataFrame({'date':['2017-06-01','2017-06-02','2017-06-03',
                         '2017-06-04','2017-06-05','2017-06-06'],
                 'count'    :[10,15,17,5,3,7]})


          date     at
0   2017-06-01     10
1   2017-06-02     15
2   2017-06-03     17
3   2017-06-04      5
4   2017-06-05      3
5   2017-06-06      7

Then running this line will give the below result:

df['indicator'] = df.at.shift(1) + df.at.shift(2)

          date  at   indicator
0   2017-06-01  10         NaN
1   2017-06-02  15         NaN
2   2017-06-03  17        25.0
3   2017-06-04   5        32.0
4   2017-06-05   3        22.0
5   2017-06-06   7         8.0