[R] by-group processing

[William Dunlap]

Max,

Since the dataset is sorted by ID, with ties broken by N, the following
should do it and do it quickly.  It grabs the rows just before ID
changes.

> with(data, data[ c(ID[-1] != ID[-length(ID)], TRUE),, drop=FALSE])
      ID Type N
7  45900    I 7
10 49270    E 3
24 46550    I 7

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Max Webber
> Sent: Wednesday, May 06, 2009 3:09 PM
> To: r-help at r-project.org
> Subject: [R] by-group processing
> 
> Given a dataframe like
> 
>   > data
>         ID Type N
>   1  45900    A 1
>   2  45900    B 2
>   3  45900    C 3
>   4  45900    D 4
>   5  45900    E 5
>   6  45900    F 6
>   7  45900    I 7
>   8  49270    A 1
>   9  49270    B 2
>   10 49270    E 3
>   18 46550    A 1
>   19 46550    B 2
>   20 46550    C 3
>   21 46550    D 4
>   22 46550    E 5
>   23 46550    F 6
>   24 46550    I 7
>   >
> 
> containing an identifier (ID), a variable type code (Type), and
> a running count of the number of records per ID (N), how can I
> return a dataframe of only those records with the maximum value
> of N for each ID? For instance,
> 
>   > data
>         ID Type N
>   7  45900    I 7
>   10 49270    E 3
>   24 46550    I 7
> 
> I know that I can use
> 
>    > tapply ( data $ N , data $ ID , max )
>    45900 46550 49270
>        7     7     3
>    >
> 
> to get the values of the maximum N for each ID, but how is it
> that I can find the index of these values to subsequently use to
> subscript data?
> 
> 
> --
> maxine-webber
> 
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