R help - Oct 2012 - Selecting n observation

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

On 2012-10-11 12:48, 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), ]))
>    ID week outcome
> 1  1    6      42
> 4  4   12      85
> 9  9   12      84
With the plyr package:

   library(plyr)
   ddply(df, .(ID), function(x) tail(x, 2))

or, slightly simpler:

   ddply(df, .(ID), tail, 2)

Peter Ehlers

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