Creating a new column based on the difference of another column-CodePudding

I have a dataframe as below:

d = {'col1': ["A", "A", "A", "B", "B", "B"], 'col2': [2015, 2016, 2017, 2015, 2016, 2017], 'col3': [10, 20, 25, 10, 12, 14]}

I would like to get the difference of col3 by col1 and col2. Such that, col1 represent companies, while col2 years and col3 stock price. I am trying to get the stock price difference each year. Such that, the output should be:

d2= {'col4': ['nan', 10, 5, 'nan', 2, 4]}

Thank you for your suggestions in advance.

Note: we cannot re-index the dataframe, we have col5, col6, col7... etc.. many other columns.

CodePudding user response：

groupby diff:

df.groupby('col1').col3.transform('diff')

0     NaN
1    10.0
2     5.0
3     NaN
4     2.0
5     2.0
Name: col3, dtype: float64