[R] Selecting n observation

[bibek sharma]

Hello R help,
 I have a question similar to what is posted by someone before. my
problem is that Instead of last assessment, I want to choose last two.

I have a data set with several time assessments for each participant.
I want to select the last assessment for each participant. My dataset
looks like this:
ID  week  outcome
1   2   14
1   4   28
1   6   42
4   2   14
4   6   46
4   9   64
4   9   71
4  12   85
9   2   14
9   4   28
9   6   51
9   9   66
9  12   84

Here is one solution for choosing last assessment
do.call("rbind",
        by(df, INDICES=df$ID, FUN=function(DF) DF[which.max(DF$week), ]))
  ID week outcome
1  1    6      42
4  4   12      85
9  9   12      84