Dear R listers,

I am trying to create a new variable that uniquely identifies groups of
observations in a dataset. So far I couldn't figure out how to do this in
R. In Stata I would simply type:
egen newvar = group(dim1, dim2, dim3)


Please, find below a quick example to show what I am dealing with:
I have a dataset with 4 variables:
var <- runif(50)   ## a variable that I want to group
dim1 <- factor(rep(1:3, length.out= 50), labels = c("x","y","z") ) ## 3
variables that should form the groups
dim2 <- rep(1:2, length.out= 50)
dim3 <- rep(1:5, length.out= 50)

data <- data.frame(var, dim1, dim2, dim3)

I am trying to build a fifth one (let's say: group_id) to uniquely identify
groups of observations as defined by dim1, dim2 and dim3, i.e. 30  groups.

can you please help me figuring out how to do it?
thanks in advance,
f.

-- 
Francesco Sarracino, Ph.D.
https://sites.google.com/site/fsarracino/

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