[R] Remove Rows Based on Factor

[Sparks, John James]

Dear R Helpers,

I did a search for deleting rows based on conditions but wasn't able to
find an example that addressed the error that I am getting.  I am hoping
that this is a simple syntax phenomenon that somebody else knows off the
top of their head.  My apologies for not providing a reproducible example
but I think that the information given will allow someone to give me a
hint.

I want to delete the rows of the data frame ZZ where Index is earlier that
Jan 1 of 2007.  That Index column is a factor.  When I tired a couple of
different methods, I got the error shown below.  Can anybody tell me what
I am doing wrong?  I would really appreciate it.

--John Sparks

> str(ZZ)
'data.frame':   1584 obs. of  7 variables:
 $ Index   : Factor w/ 1583 levels "2006-04-07","2006-04-10",..: 1 2 3 4 5
6 7 8 9 10 ...
 $ Open    : num  17.5 17.6 16.8 17.2 17 ...
 $ High    : num  18.2 17.6 17.2 17.2 17.1 ...
 $ Low     : num  17.3 16.8 16.8 16.8 16.6 ...
 $ Close   : num  17.5 16.8 17.1 16.8 16.7 ...
 $ Volume  : num  23834500 2916000 1453700 991400 967400 ...
 $ Adjusted: num  16.8 16.2 16.4 16.2 16 ...
> test<-ZZ[ZZ$Index>"2007-01-01",]
Warning message:
In Ops.factor(ZZ$Index, "2007-01-01") : > not meaningful for factors
>
> test<-subset(ZZ,Index>2007-01-01)
Warning message:
In Ops.factor(Index, 2007 - 1 - 1) : > not meaningful for factors