[R] Selecting a maximum value in same ID

[arun]

Hi,

You could do this either:
DF[unlist(with(DF,tapply(B,list(ID),FUN=function(x) x %in% max(x)))),]
#or
 DF[unlist(with(DF,by(B,ID,FUN=function(x) x%in% max(x)))),]

#or
library(plyr)
 DF[ddply(DF,.(ID),summarize,B %in% max(B))[,2],]
A.K.


Hi, 

I am struggling with this issue and need some helps. The data 
set 'DF' includes persons' IDs and other variables A, B, and C. Each 
Person has multiple values in A, B, and C.  What I am trying to do is 1)
 selecting a maximum value of B within same ID, and 2) making a new data
 set (DF.2) that select rows aligning with the maximum value of B. 

DF and DF.2 are below. I've used functions combining which.max, 
subset, and loop,  but it did not work. If you have ideas, please help. 

> DF 
    ID    A        B	      C 
     1	   12      36      2 
     1	   15      30      2 
     2	   56      11      2 
     2    33      30      2 
     3    83      23      2 
     3    58        7      2 
     4    75        2      2 
     4    82      36      2 
     5    77      35      2 
     5    75      23      2 
     6    73      10      2 
     6    76      35      2 
     7    75      14      2 
     7    21      30      2 
     8    14      11      2 
     8    46      11      2 
     8    75      11      2 
     8    30      36      2 
     9    21      35      2 
     9    75      23      2 


DF.2 
    ID    A        B	      C 
     1	   12      36      2 
     2    33     30      2 
     3    83     23      2 
     4    82      36      2 
     5    77      35      2 
     6    76      35      2 
     7    21      30      2 
     8    30      36      2 
     9    21      35      2 

Thank you in advance, 

Lee