R help - Sep 2008 - keep the row indexes/names when do aggregate

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

https://stat.ethz.ch/pipermail/r-help/2008-September/173139.html

On Wed, Sep 24, 2008 at 11:21 AM, zhihuali <lzhtom at hotmail.com>
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
>
>
>
>
>
> _________________________________________________________________
> [[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.
>

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
> 
> 
>  
> 
> 
> _________________________________________________________________
> [[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.