Here's a quick version in Clojure. I'm sure it can be done cleaner than this.. (def articles [{:title "title1" :author "author1" :tags #{:Java :t2 :t3}}

Here's a quick version in Clojure. I'm sure it can be done cleaner than this.. (def articles [{:title "title1" :author "author1" :tags #{:Java :t2 :t3}} {:title "title2" :author "author1" :tags #{:Jvxa :t2 :t3}} {:title "title3" :author "author3" :tags #{:Java :t3}}]) ;; find the first article in the collection that has the tag “Java”. (first (filter #(contains? (:tags %) :Java) articles)) ;; ==> {:tags #{:t2 :Java :t3}, :title "title1", :author "author1"} ;;get all the elements that match instead of just the first (filter #(contains? (:tags %) :Java) articles) ;; ==> ({:tags #{:t2 :Java :t3}, :title "title1", :author "author1"} {:tags #{:Java :t3}, :title "title3", :author "author3"}) ;;group all the articles based on the author. (group-by :author articles) ;; cheating? ;; ==> {"author1" [{:tags #{:t2 :Java :t3}, :title "title1", :author "author1"} {:tags #{:Jvxa :t2 :t3}, :title "title2", :author "author1"}], "author3" [{:tags #{:Java :t3}, :title "title3", :author "author3"}]} ;;find all the different tags used in the collections (apply clojure.set/union (map :tags articles)) ;; ==> #{:Jvxa :t2 :Java :t3}