[R] Grouping data in a data frame: is there an efficient way to do it?

[David Winsemius]

table is reasonably fast. I have more than 4 X 10^6 records and a 2D  
table takes very little time:

  nUA <- with (TRdta, table(URwbc, URrbc)) # both URwbc and URrbc are  
factors
  nUA

This does the same thing and took about 5 seconds just now:

xtabs( ~ URwbc + URrbc, data=TRdta)

On Sep 2, 2009, at 6:39 PM, Leo Alekseyev wrote:

> I have a data frame with about 10^6 rows; I want to group the data
> according to entries in one of the columns and do something with it.
> For instance, suppose I want to count up the number of elements in
> each group.  I tried something like aggregate(my.df$my.field,
> list(my.df$my.field), length) but it seems to be very slow.  Likewise,
> the split() function was slow (I killed it before it completed).  Is
> there a way to efficiently accomplish this in R?..  I am almost
> tempted to write an external Perl/Python script entering every row
> into a hashtable keyed by my.field and iterating over the keys...
> Might this be faster?..



David Winsemius, MD
Heritage Laboratories
West Hartford, CT