[R] sequential processing

[Gerard Smits]

Like many others, I am new to R but old to SAS.

Am I correct in understanding that R processes a data frame in a 
sequential ly?  This would imply that large input files could be 
read, without the need to load the entire file into memory.
Related to the manner of reading a frame, I have been looking for the 
equivalent of SAS _n_ (I realize that I can use a variant of which to 
identify an index value) as well as  useful SAS features such as 
first., last., retain, etc.  Any help with this conversion appreciated.

Thanks,

Gerard Smits