如何根据另一列中的连续两行添加 dataframe 列

Question

I have column (A) in this pandas dataframe

A	B
1	1
1	2
2	3
5	7

Column (B) is created using the following formula:

col B (2)= col A(1)+ col A(2)

How do I create column (B) from column A?

Answer 1

Use shift() to get the rows one row down and add it back to column a .

df['b'] = df['a'] + df['a'].shift(fill_value=0)

Answer 2

You can do

df["B"] = df['A'] + df['A'].shift(1, fill_value= 0.)