[R] Selecting n observation

[David Winsemius]

On Oct 11, 2012, at 12:48 PM, bibek sharma wrote:

> 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), ]))

Why wouldn't the solution be something along the lines of:

do.call("rbind",
       by(df, INDICES=df$ID, FUN=function(DF) tail(DF, 2) ))


>  ID week outcome
> 1  1    6      42
> 4  4   12      85
> 9  9   12      84
> 
> 


David Winsemius, MD
Alameda, CA, USA