How to make the data greater than or equal to the previous data

Question

I have a dataframe, and I want every value in a specific column to be greater or equal to the previous value.

In the example below, I want it to be applied on column 'd', so the expected result should replace 5 with 7.

So how can I do it?

The actual situation needs to consider efficiency issues, it is best not to use FOR.

Thank you in advance for your reply.

code:

>>> df = pd.DataFrame({'a': [1,3,2,5], 'b': [2, 4,3,4], 'c':[3,5,4,3]})
>>> df
   a  b  c
0  1  2  3
1  3  4  5
2  2  3  4
3  5  4  3

>>> df['d'] = df['a']+df['b']
>>> df
   a  b  c  d
0  1  2  3  3
1  3  4  5  7
2  2  3  4  5
3  5  4  3  9

Answer 1

You can do this using cummax :

>>> df['d'] = df['d'].cummax()
>>> df
   a  b  c  d
0  1  2  3  3
1  3  4  5  7
2  2  3  4  7
3  5  4  3  9