[R] keep the row indexes/names when do aggregate

[Adaikalavan Ramasamy]

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

zhihuali wrote:
> 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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