```
# NOT RUN {
data(dietox)
dietox12 <- subset(dietox,Time==12)
fun <- function(x){
c(m=mean(x), v=var(x), n=length(x))
}
summaryBy(cbind(Weight, Feed) ~ Evit + Cu, data=dietox12,
FUN=fun)
summaryBy(list(c("Weight", "Feed"), c("Evit", "Cu")), data=dietox12,
FUN=fun)
## Computations on several variables is done using cbind( )
summaryBy(cbind(Weight, Feed) ~ Evit + Cu, data=subset(dietox, Time > 1),
FUN=fun)
## Calculations on transformed data is possible using cbind( ), but
# the transformed variables must be named
summaryBy(cbind(lw=log(Weight), Feed) ~ Evit + Cu, data=dietox12, FUN=mean)
## There are missing values in the 'airquality' data, so we remove these
## before calculating mean and variance with 'na.rm=TRUE'. However the
## length function does not accept any such argument. Hence we get
## around this by defining our own summary function in which length is
## not supplied with this argument while mean and var are:
sumfun <- function(x, ...){
c(m=mean(x, na.rm=TRUE, ...), v=var(x, na.rm=TRUE, ...), l=length(x))
}
summaryBy(cbind(Ozone, Solar.R) ~ Month, data=airquality, FUN=sumfun )
## Using '.' on the right hand side of a formula means to stratify by
## all variables not used elsewhere:
data(warpbreaks)
summaryBy(breaks ~ wool + tension, warpbreaks, FUN=mean)
summaryBy(breaks ~ ., warpbreaks, FUN=mean)
summaryBy(. ~ wool + tension, warpbreaks, FUN=mean)
# }
```

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