Cumsum as a new column in an existing Pandas data

Question

I have a pandas dataframe defined as:

A   B   SUM_C       1   1   10      1   2   20    

I would like to do a cumulative sum of SUM_C and add it as a new column to the same dataframe. In other words, my end goal is to have a dataframe that looks like below:

A   B   SUM_C   CUMSUM_C        1   1   10      10      1   2   20      30    

Using cumsum in pandas on group() shows the possibility of generating a new dataframe where column name SUM_C is replaced with cumulative sum. However, my ask is to add the cumulative sum as a new column to the existing dataframe.

Thank you

Answer 1

Just apply cumsum on the pandas.Series df['SUM_C'] and assign it to a new column:

df['CUMSUM_C'] = df['SUM_C'].cumsum() 

Result:

df Out[34]:     A  B  SUM_C  CUMSUM_C 0  1  1     10       10 1  1  2     20       30