[R] how to find out whether a string is a factor?

[Sam Steingold]

I have two data frames:
> str(ysmd)
'data.frame':	8325 obs. of  6 variables:
 $ X.stock                      : Factor w/ 8325 levels "A","AA","AA-",..: 2702 6547 4118 7664 7587 6350 3341 5640 5107 7589 ...
 $ market.cap                   : num  -1.00 2.97e+10 3.54e+08 3.46e+08 -1.00 ...
 $ X52.week.low                 : num  40.2 22.5 27.5 12.2 20.7 ...
 $ X52.week.high                : num  43.3 38.2 35.1 19.2 32.7 ...
 $ X3.month.average.daily.volume: num  154 7862250 16330 205784 14697 ...
 $ X50.day.moving.average.price : num  41.8 36.3 30.5 15.2 29.9 ...
> str(top1000)
'data.frame':	1000 obs. of  1 variable:
 $ V1: Factor w/ 1000 levels "AA","AAI","AAP",..: 146 96 341 814 382 977 66 1 737 595 ...

I want to split ysmd into two new data frames: ysmd.top1000 and
ysmd.rest so that ysmd.top1000$X.stock only contains factors from
top1000$V1 and ysmd.rest$X.stock contains all the other factors.

I should be able to just write

ysmd.top1000 <- ysmd[ysmd$X.stock "is in" top1000$V1,]
ysmd.rest <- ysmd[ysmd$X.stock "not in" top1000$V1,]

but how so I check whether a string is a member of a factor?

-- 
Sam Steingold (http://sds.podval.org/) on CentOS release 5.6 (Final) X 11.0.60900031
http://mideasttruth.com http://truepeace.org
http://camera.org http://thereligionofpeace.com http://pmw.org.il
Professionalism is being dispassionate about your work.