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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