```
# NOT RUN {
DT = data.table(x=c(20,10,10,30,30,20), y=c("a", "a", "a", "b", "b", "b"), z=1:6)
rowid(DT$x) # 1,1,2,1,2,2
rowidv(DT, cols="x") # same as above
rowid(DT$x, prefix="group") # prefixed with 'group'
rowid(DT$x, DT$y) # 1,1,2,1,2,1
rowidv(DT, cols=c("x","y")) # same as above
DT[, .(N=seq_len(.N)), by=.(x,y)]$N # same as above
# convenient usage with dcast
dcast(DT, x ~ rowid(x, prefix="group"), value.var="z")
# x group1 group2
# 1: 10 2 3
# 2: 20 1 6
# 3: 30 4 5
# }
```

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