Is there a way to overwrite Nan values in a pandas dataframe with values of the previous row?

Question

I am working with a dataframe called ´tabla_combinada´ that looks like this:

Structure of the dataframe used:

What I am attempting to do is to get rid of the Nan values in the 'End Meter' column and replace it with the value of the same column in the previous row. I tried to implement the following code:

counter=0
for x in tabla_combinada['End Meter']:
    if math.isnan(x):
        x = tabla_combinada['End Meter'][counter-1]
        tabla_combinada['End Meter'][counter-1] = tabla_combinada['Start Meter'][counter]       
    counter = counter + 1

This is not working for me, in the first place I am getting the following warning:

A value is trying to be set on a copy of a slice from a DataFrame.

But what bugs me is that I am obtaining no change in the dataframe at all. I do understand the cause of the warning and I suspect that this is not the optimal approach to solve the problem. I guess there is a proper way to do this with loc, but I couldn't find out how to tell the program to replace the Nan with the value of the previous row. Sorry for the long question and thanks in advance.

Answer 1

All you need to do is this:

tabla_combinada['End Meter'].fillna(method='ffill')

This will propagate non-null values forward