Compute rank within subgroups

Is there an operator that will compute the rank of each row according to some attribute value?  If so, is it possible to do so for subgroups within an example set?

e.g.  Compute RANK based on VALUE within each GRP group

GRP ID   VALUE   RANK
1   a    5       1
1   b    10      2
1   c    20      3
2   d    7       1
2   e    50      2
2   f    51      3

Thanks.
Keith