keep the row indexes/names when do aggregate

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Rather than aggregate, use order and duplicated as in this post:

https://stat.ethz.ch/pipermail/r-help/2008-September/173139.html
Hi, R-users,

If I have a data frame like this:
x<-data.frame(g=c("g1","g2","g1","g1","g2"),v=c(1,7,3,2,8))
  g v
1 g1 1
2 g2 7
3 g1 3
4 g1 2
5 g2 8

It contains two groups, g1 and g2. Now for each group I want the max v:

aggregate(x$v,list(g=x$g),max)
  g x
1 g1 3
2 g2 8

Beautiful. But what if I want to keep the row index of (g1 3) and (g2 8) in the original x?
So I want is:
do something
  g x
 3 g1 3
 5 g2 8

Of course it'd may make much more sense if the row indexes are some row names that I want to keep.

Is there a simple way to do that?

Thanks a lot!

Z

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Not the most elegant solution but here goes.

    df <- data.frame(g=c("g1","g2","g1","g1","g2"),v=c(1,7,3,2,8))

    rownames.which.max <- function(m, col){
                                   w <- which.max( m[ , col] )
                                   return( rownames(m)[w] )
    }

    df.split <- split(df, df$g)
    ws <- sapply( df.split, rownames.which.max, col="v" )

    ws
     g1  g2
    "3" "5"

    df[ws, ]
       g v
    3 g1 3
    5 g2 8

Regards, Adai
Hi, R-users,

If I have a data frame like this:
x<-data.frame(g=c("g1","g2","g1","g1","g2"),v=c(1,7,3,2,8))
   g v
1 g1 1
2 g2 7
3 g1 3
4 g1 2
5 g2 8

It contains two groups, g1 and g2. Now for each group I want the max v:

aggregate(x$v,list(g=x$g),max)
   g x
1 g1 3
2 g2 8

Beautiful. But what if I want to keep the row index of (g1 3) and (g2 8) in the original x? 
So I want is:
do something
   g x
 3 g1 3
 5 g2 8

Of course it'd may make much more sense if the row indexes are some row names that I want to keep.

Is there a simple way to do that?

Thanks a lot!

Z

_________________________________________________________________
[[elided Hotmail spam]]

	[[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.