[R] extracting components of a list

[Robin Hankin]

Hi

how do I extract those components of a list that satisfy a certain 
requirement?  If

jj <- list(list(a=1,b=4:7),list(a=5,b=3:6),list(a=10,b=4:5))


I want just the components of jj that have b[1] ==4 which in this case 
would be the first and
third of jj, viz    list (jj[[1]],jj[[3]]).

How to do this efficiently?

My only idea was to loop through jj, and set unwanted components to 
NULL, but
FAQ 7.1 warns against this.




--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743