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.

Answer 2

You can also use package data.table:

library(data.table)
setDT(df)[State=="WI", Name:=paste0(Name,"1")]

Answer 3

df <- data.frame(Name=c('John Smith', 'John Smith', 'Jeff Smith'),
                 State=c('MI','WI','WI'), stringsAsFactors=F)

df <- within(df, Name[Name == 'John Smith' & State == 'WI'] <- 'John Smith1')

> df
         Name State
1  John Smith    MI
2 John Smith1    WI
3  Jeff Smith    WI

** Edit **

Edited to add that you can put whatever you like in the within expression:

df <- within(df, {
    f <- Name == 'John Smith' & State == 'WI'
    Name[f] <- 'John Smith1'
    State[f] <- 'CA'
}) 

Answer 4

One way:

df[df$Name == "John_Smith" & df$State == "WI", "Name"] <- "John_Smith1"

Another way using the dplyr:

df %>% mutate(Name = ifelse(State == "WI" & Name == "John_Smith", "John_Smith1", Name))

Note: As David Arenburg says, the first column should not be a factor. For this, reading the data set stringsAsFactors = FALSE.