R: Manipulating a data frame with contents from a different data frame

Question

Say I have a data frame with the contents:

Trial Person Time
1     John   1.2
2     John   1.3
3     John   1.1
1     Bill   2.3
2     Bill   2.5
3     Bill   2.7

and another data frame with the contents:

Person Offset
John   0.5
Bill   1.0

and I want to modify the original frame based on the appropriate value from the second. I could do this easily in any other language or in SQL, and I'm sure I could manage using for loops and what, but with everything else I see in R, I'm guessing it has special syntax to do this as a one-liner. So, if so, how? And if not, could you show how it could be done using loops. I haven't actually got around to learning looping in R yet since it has amazing things to simply extract and manipulate whatever values.

For reference, the output would:

Trial Person Time
1     John   0.7
2     John   0.8
3     John   0.6
1     Bill   1.3
2     Bill   1.5
3     Bill   1.7 

Answer 1

There are many possibilities. Here is a simple one using merge() and a simple column-wise subtraction in the enlarged data.frame:

R> DF1 <- data.frame(trial=rep(1:3,2), \
                     Person=rep(c("John","Bill"), each=3), \
                     Time=c(1.2,1.3,1.1,2.3,2.5,2.7))
R> DF2 <- data.frame(Person=c("John","Bill"), Offset=c(0.5,1.0))
R> DF <- merge(DF1, DF2)
R> DF
  Person trial Time Offset
1   Bill     1  2.3    1.0
2   Bill     2  2.5    1.0
3   Bill     3  2.7    1.0
4   John     1  1.2    0.5
5   John     2  1.3    0.5
6   John     3  1.1    0.5
R> DF$NewTime <- DF$Time - DF$Offset
R> DF
  Person trial Time Offset NewTime
1   Bill     1  2.3    1.0     1.3
2   Bill     2  2.5    1.0     1.5
3   Bill     3  2.7    1.0     1.7
4   John     1  1.2    0.5     0.7
5   John     2  1.3    0.5     0.8
6   John     3  1.1    0.5     0.6
R> 

Answer 2

One liner:

transform(merge(d1,d2), Time=Time - Offset, Offset=NULL)