Replace values in data frame based on other data frame in R

Question

In the below example, userids is my reference data frame and userdata is the data frame where the replacements should take place.

> userids <- data.frame(USER=c('Ann','Jim','Lee','Bob'),ID=c(1,2,3,4))
> userids
  USER ID
1  Ann  1
2  Jim  2
3  Lee  3
4  Bob  4

> userdata <- data.frame(INFO=c('foo','bar','foo','bar'), ID=c('Bob','Jim','Ann','Lee'),AGE=c('43','33','53','26'), FRIENDID=c('Ann',NA,'Lee','Jim'))
> userdata
  INFO  ID AGE FRIENDID
1  foo Bob  43      Ann
2  bar Jim  33       NA
3  foo Ann  53      Lee
4  bar Lee  26      Jim

How do I replace ID and FRIENDID in userdata with the ID corresponding to USER in userids?

The desired output:

  INFO  ID AGE FRIENDID
1  foo   4  43        1
2  bar   2  33       NA
3  foo   1  53        3
4  bar   3  26        2

Answer 1

Use match:

userdata$ID <- userids$ID[match(userdata$ID, userids$USER)]
userdata$FRIENDID <- userids$ID[match(userdata$FRIENDID, userids$USER)]

Answer 2

This is a possibility:

library(qdap)
userdata$FRIENDID <- lookup(userdata$FRIENDID, userids)
userdata$ID <- lookup(userdata$ID, userids)

or to win the one line prize:

userdata[, c(2, 4)] <- lapply(userdata[, c(2, 4)], lookup, key.match=userids)