R help - Dec 2002 - Newbie - referencing and vectorization questions

Dear List,
 
    I am working on learning R and have come up with a few questions that I
haven't been able to answer with books from the local library. I am
attempting to find the mean and variance of subsets of a ~140k item vector(lets
say variable X). Using groups of 5 variables from a pool of 7, I limit the
values of X by only selecting those who remain in the set after sorting on
interpercentile ranges of the 5 other variables. There are 21 groups of 5
sorting variables, and if I were to use interquartile ranges, 5^4 possible
combinations for each group.  So, I am trying to learn to write fast code, while
learning R.
 
My questions are these:
 
     Is there a way to select a vector from a dataframe(containing values of
sorting variables) using a character string located in another dataframe
(containing possible groups of variables) that has been referenced with a
numeric index?
 
     If there is, is there a way to do it using a vectorized process, rather
than an iterative with several for loops or something?
 
 
Thanks for your consideration,
 
Paul