[R] by-group processing

[Max Webber]

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