R help - Sep 2010 - "sequeeze" a data frame

Hi, suppose I have a data frame as below:

dat<-cbind(expand.grid(id=c(1,2,3),time=c(0,3,6),mode=c('R','L'),rep=(1:3)),y=rnorm(54))


I kind of want to "squeeze" the data frame into a new one with
averaged "y" over
"rep" for the same id, time and mode. taking average is easy with
tapply:

tapply(dat$y, list(dat$id, dat$time, dat$mode), mean)

But I want the result to be in the same format as "dat". Certainly, we
always
can transform the result (array) into the data frame using a lot of codes. But 
is there a simple way to do this?

Thanks

John

On Sep 9, 2010, at 5:47 PM, array chip wrote:
> Hi, suppose I have a data frame as below:
>
> dat<- 
> cbind 
> (expand 
> .grid 
>
(id=c(1,2,3),time=c(0,3,6),mode=c('R','L'),rep=(1:3)),y=rnorm(54))
>
>
> I kind of want to "squeeze" the data frame into a new one with  
> averaged "y" over
> "rep" for the same id, time and mode. taking average is easy with
> tapply:
>
> tapply(dat$y, list(dat$id, dat$time, dat$mode), mean)
Try:

dat$avg.y <- ave(dat$y, dat$id, dat$time, dat$mode, FUN=mean)

?ave

The syntax of ave is different than "tapply" or "by". The
grouping
factors are not presented as a list and the FUN argument comes after  
the ,..., so it needs to be named if different than the default of  
"mean".
>
> But I want the result to be in the same format as "dat".
Certainly,
> we always
> can transform the result (array) into the data frame using a lot of  
> codes. But
> is there a simple way to do this?
>
> Thanks
David Winsemius, MD
West Hartford, CT