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
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