```
DT <- data.table(a=rep(1:3, each=2), b=1:6)
DT2 <- transform(DT, c = a^2)
DT[, c:=a^2]
identical(DT,DT2)
DT2 <- within(DT, {
b <- rev(b)
c <- a*2
rm(a)
})
DT[,`:=`(b = rev(b),
c = a*2,
a = NULL)]
identical(DT,DT2)
DT$d = ave(DT$b, DT$c, FUN=max) # copies entire DT, even if it is 10GB in RAM
DT = DT[, transform(.SD, d=max(b)), by="c"] # same, but even worse as .SD is copied for each group
DT[, d:=max(b), by="c"] # same result, but much faster, shorter and scales
# Multiple update by group. Convenient, fast, scales and easy to read.
DT[, `:=`(minb = min(b),
meanb = mean(b),
bplusd = sum(b+d)), by=c%/%5]
DT
```

Run the code above in your browser using DataLab