[datatable-help] Is there a way to merge-assign on something other than the key?

[Frank Erickson]

Hi,

I really like the DT1[DT2,z:=...] idiom. Unfortunately, the value of a
merge() on other columns is a new data.table, so modifying DT1, like
merge(DT1,DT2,by=...)[,z:=...], is not possible. Or is there actually a way
to do this that I am missing?

If this syntax -- DT1[DT2,z:=...,by=c(key(DT1),x)] -- behaved differently,
allowing the "by" to determine which columns were merged on, that would
solve my issue, I guess.

By the way, when you use by=c(key(DT),x), you get a speedup from DT's being
keyed, right?

*Some background* (rest of email): I've coded a value function iteration
rather inefficiently and am looking into a few different directions for
improving it. Efficiency matters because the result will enter a likelihood
I need to maximize. (Value function iteration is solving a dynamic
programming problem with discrete periods and a finite horizon from the
horizon/final period backwards.)

I was alternating between two data tables and doing things like
DT1[DT2[y==t],z:=...] and DT2[DT1[y==t-1],q:=...], changing the keys on
both tables before each merge-assign. If secondary keys were
implemented<http://r-forge.r-project.org/tracker/index.php?func=detail&aid=1007&group_id=240&atid=978>,
I'd have just gone with that. (Tom's secondary key
method<http://lists.r-forge.r-project.org/pipermail/datatable-help/2010-May/000028.html>mentioned
in the last link only works for subsetting, not merge-assigning,
as far as I can tell.)

I think I'm getting a big slow-down because I'm rekeying four times per
iteration (two tables x two merge-assigns) and because I'm rekeying the
entire table, when I'm only assigning to a subset. That second problem is
easier to fix, I guess.

Now I am considering making a single DT with
key=intersect(key(DT1),key(DT2)) and using that instead, if I can figure
out a way to do what I need to with it.

Thanks,

Frank
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20130815/10c656bc/attachment.html>