(index xrel ks)
Returns a map of the distinct values of ks in the xrel mapped to a set of the maps in xrel with the corresponding values of ks.
(use '[clojure.set :only (index)])
;; Suppose you have a set of descriptions of the weights of animals:
user=> (def weights #{ {:name 'betsy :weight 1000}
{:name 'jake :weight 756}
{:name 'shyq :weight 1000} })
;; You want the names of all the animals that weight 1000. One way to do
;; that uses `index`. First, you can group the set elements (the maps) so
;; that those with the same weights are in the same group.
user=> (def by-weight (index weights [:weight]))
#'user/by-weight
;; index returns a map. The keys are maps themselves, where {:weight 756} and {:weight 1000} are taken from the maps in the weights set. The values in the map returned by index are sets that contain map entries from the above weights set.
user=> by-weight
{{:weight 756} #{{:name jake, :weight 756}},
{:weight 1000} #{{:name shyq, :weight 1000}
{:name betsy, :weight 1000}}}
;; To better visualize the by-weight map that is returned by index, you can query it using get, using {:weight 756} as the key. This will return all the maps (animals) that contain a weight of 756. In this case, there is only one result, which is a set containing a single map.
user=> (get by-weight {:weight 756})
#{{:name jake, :weight 756}}
;; To see that there are two animals with a weight of 1000, you can query by-weight with the key {:weight 1000}. This returns a set containing two maps.
user=> (get by-weight {:weight 1000})
#{{:name shyq, :weight 1000} {:name betsy :weight 1000}}
;; You can verify by using count
user=> (count (get by-weight {:weight 1000}))
2
;; To get the names of those two animals we can map a name-extracting function
;; over the set of two maps. Since a keyword in a map is also a function that
;; returns its corresponding value, we can just use `:name` as our function:
user=> (map :name (get by-weight {:weight 1000}))
(shyq betsy)