[R] Lookups in R

mfrumin michael at frumin.net
Wed Jul 4 21:01:41 CEST 2007

Hey all; I'm a beginner++ user of R, trying to use it to do some processing
of data sets of over 1M rows, and running into a snafu.  imagine that my
input is a huge table of transactions, each linked to a specif user id.  as
I run through the transactions, I need to update a separate table for the
users, but I am finding that the traditional ways of doing a table lookup
are way too slow to support this kind of operation.


for(i in 1:1000000) {
   userid = transactions$userid[i];
   amt = transactions$amounts[i];
   users[users$id == userid,'amt'] += amt;

I assume this is a linear lookup through the users table (in which there are
10's of thousands of rows), when really what I need is O(constant time), or
at worst O(log(# users)).

is there any way to manage a list of ID's (be they numeric, string, etc) and
have them efficiently mapped to some other table index?

I see the CRAN package for SQLite hashes, but that seems to be going a bit
too far.


Intern, Oyster Card Group, Transport for London
(feel free to email back to this address, I'm posting through NAbble so I
hope it works).
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