Update a Value in One Column Based on Criteria in Other Columns

Question

If my data frame (df) looks like this:

Name        State
John Smith  MI
John Smith  WI
Jeff Smith  WI

I want to rename the John Smith from

Answer 1

As the OP has mentioned that he has "a very big data frame", it might be advantageous to use a binary search

library(data.table)
setDT(DF)[.("John Smith",  "WI"), on = .(Name=V1, State=V2), 
          Name := paste0(Name, 1)][]

          Name State
1:  John Smith    MI
2: John Smith1    WI
3:  Jeff Smith    WI

instead of a vector scan

setDT(df)[State == "WI" & Name == "John Smith", Name := paste0(Name, "1")]

In both variations the data object is updated by reference, i.e., without copying the whole object which save time and memory.