Reference values in the previous row with map or apply

Question

Given a dataframe df, I would like to generate a new variable/column for each row based on the values in the previous row. df is sorted so that the ord

Answer 1

If you just want to do a calculation based on the previous row, you can calculate and then shift:

In [2]: df = pd.DataFrame({'a':[0,1,2], 'b':[0,10,20]})

In [3]: df
Out[3]:
   a   b
0  0   0
1  1  10
2  2  20

# a calculation based on other column
In [4]: df['c'] = df['b'] + 1

# shift the column
In [5]: df['c'] = df['c'].shift()

In [6]: df
Out[6]:
   a   b   c
0  0   0 NaN
1  1  10   1
2  2  20  11

If you want to do a calculation based on multiple rows, you could look at the rolling_apply function (http://pandas.pydata.org/pandas-docs/stable/computation.html#moving-rolling-statistics-moments and http://pandas.pydata.org/pandas-docs/stable/generated/pandas.rolling_apply.html#pandas.rolling_apply)