[R] combining columns into a "combination index" of the same length

[Emmanuel Levy]

Hi,

The answer to this is probably straightforward, I have a dataframe and I'd
like to build an index of column combinations, e.g.

col1 col2  --> col3 (the index I need)
A     1           1
A     1           1
A     2           2
B     1           3
B     2           4
B     2           4


At the moment I use:
col3 <- apply(mat[,sel.col], 1, paste0)

But I wonder if another approach could be faster?

Thanks,

Emmanuel

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