Clojure: Updating keys in a map
I’ve been playing with Clojure over the last few weeks and as a result I’ve been using a lot of maps to represent the data.
For example if we have the following map of teams to Glicko ratings and ratings deviations:
(def teams { "Man. United" {:points 1500 :rd 350}
"Man. City" {:points 1450 :rd 300} })We might want to increase Man. United’s points score by one for which we could use the http://clojuredocs.org/clojure_core/1.2.0/clojure.core/update-in function:
> (update-in teams ["Man. United" :points] inc)
{"Man. United" {:points 1501, :rd 350}, "Man. City" {:points 1450, :rd 300}}The 2nd argument to update-in is a nested associative structure i.e. a sequence of keys into the map in this instance.
If we wanted to reset Man. United’s points score we could use http://clojuredocs.org/clojure_core/clojure.core/assoc-in:
> (assoc-in teams ["Man. United" :points] 1)
{"Man. United" {:points 1, :rd 350}, "Man. City" {:points 1450, :rd 300}}If we want to update multiple keys at once then we can chain them using the -> (thread first) macro:
(-> teams
(assoc-in ["Man. United" :points] 1600)
(assoc-in ["Man. United" :rd] 200))
{"Man. United" {:points 1600, :rd 200}, "Man. City" {:points 1450, :rd 300}}If instead of replacing just one part of the value we want to replace the whole entry we could use http://clojuredocs.org/clojure_core/clojure.core/assoc instead:
> (assoc teams "Man. United" {:points 1600 :rd 300})
{"Man. United" {:points 1600, :rd 300}, "Man. City" {:points 1450, :rd 300}}assoc can also be used to add a new key/value to the map. e.g.
> (assoc teams "Arsenal" {:points 1500 :rd 330})
{"Man. United" {:points 1500, :rd 350}, "Arsenal" {:points 1500, :rd 330}, "Man. City" {:points 1450, :rd 300}}http://clojuredocs.org/clojure_core/clojure.core/dissoc plays the opposite role and returns a new map without the specified keys:
> (dissoc teams "Man. United" "Man. City")
{}And those are all the map based functions I’ve played around with so far...
About the author
I'm currently working on short form content at ClickHouse. I publish short 5 minute videos showing how to solve data problems on YouTube @LearnDataWithMark. I previously worked on graph analytics at Neo4j, where I also co-authored the O'Reilly Graph Algorithms Book with Amy Hodler.