[R] identical()

[array chip]

Hi, I am really puzzled by this. hope someone can help me

I have a 2 small data frames "a" and "b" derived from a larger data frames. They 
look exactly the same to me, but identical() always returns FALSE.

> a
         a b
2 10011048 L
4 10011048 R
6 10011049 L
8 10011049 R
> b
         a b
1 10011048 L
3 10011048 R
5 10011049 L
7 10011049 R

> identical(a,b)
[1] FALSE

some information about the attributes of the 2 data frames:

> class(a)
[1] "data.frame"
> class(b)
[1] "data.frame"
> class(a$a)
[1] "integer"
> class(a$b)
[1] "character"
> class(b$a)
[1] "integer"
> class(b$b)
[1] "character"


However, if I generate these 2 data frame from scratches, identical() would 
returns TRUE

>x<-as.data.frame(cbind(a=c(10011048,10011048,10011049,10011049),b=c('L','R','L','R')))
>)
>y<-as.data.frame(cbind(a=c(10011048,10011048,10011049,10011049),b=c('L','R','L','R')))
>)

> identical(x,y)
[1] TRUE

Looks like a & b objects takes some invisible residual information from the 
larger data frame where they were derived, which is not the same between them. 
But what is it?

Thanks

John