[R] Faster alternative to by?

[Jacques VESLOT]

table(mapped$col2)
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Jacques VESLOT

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michael watson (IAH-C) a écrit :
> Hi
> 
> I have a data.frame, two columns, 12304 rows.  Both columns are factors.
> I want to do an equivalent of an SQL "group by" statement, and count the
> number of rows in the data frame for each unique value of the second
> column.
> 
> I have:
> 
> countl <- by(mapped, mapped$col2, nrow)
> 
> Now, mapped$col2 has 10588 levels, so this statement takes a really long
> time to run.  Is there a more efficient way of doing this in R?
> 
> Thanks
> 
> Mick
> 
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