this is an example of mutating a column but only for certain rows, if a condition is TRUE
looking at the data, the idea is as follows:
for each person in the data, I need to fill the column called to_fill. However, I want to do so only
1 - where to_fill has more than one unique value by col_a and col_c
(basically, for a given individual grouped by col_a and col_c, to_fill is not constant) BUT
2 - only if it is empty where col_b is "S"
3 - and do so by groups formed by col_a and col_c
after running the code, new_loc2 is the required solution;
values of to_fill where col_b equal "S" are replaced by the values where col_b equals "W"