Clojure: Converting an Array/Set into a Hash Map
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Join For FreeWhen I was implementing the Elo Rating algorithm a few weeks ago one thing I needed to do was come up with a base ranking for each team.
I started out with a set of teams that looked like this:
(def teams #{ "Man Utd" "Man City" "Arsenal" "Chelsea"})
and I wanted to transform that into a map from the team to their ranking e.g.
Man Utd -> {:points 1200} Man City -> {:points 1200} Arsenal -> {:points 1200} Chelsea -> {:points 1200}
I had read the documentation of array-map, a function which can be used to transform a collection of pairs into a map, and it seemed like it might do the trick.
I started out by building an array of pairs using mapcat:
> (mapcat (fn [x] [x {:points 1200}]) teams) ("Chelsea" {:points 1200} "Man City" {:points 1200} "Arsenal" {:points 1200} "Man Utd" {:points 1200})
array-map constructs a map from pairs of values e.g.
> (array-map "Chelsea" {:points 1200} "Man City" {:points 1200} "Arsenal" {:points 1200} "Man Utd" {:points 1200}) ("Chelsea" {:points 1200} "Man City" {:points 1200} "Arsenal" {:points 1200} "Man Utd" {:points 1200})
Since we have a collection of pairs rather than individual pairs we need to use the apply function as well:
> (apply array-map ["Chelsea" {:points 1200} "Man City" {:points 1200} "Arsenal" {:points 1200} "Man Utd" {:points 1200}]) {"Chelsea" {:points 1200}, "Man City" {:points 1200}, "Arsenal" {:points 1200}, "Man Utd" {:points 1200}}
And if we put it all together we end up with the following:
> (apply array-map (mapcat (fn [x] [x {:points 1200}]) teams)) {"Man Utd" {:points 1200}, "Man City" {:points 1200}, "Arsenal" {:points 1200}, "Chelsea" {:points 1200}}
It works but the function we pass to mapcat feels a bit clunky. Since we just need to create a collection of team/ranking pairs we can use the vector and repeat functions to build that up instead:
> (mapcat vector teams (repeat {:points 1200})) ("Chelsea" {:points 1200} "Man City" {:points 1200} "Arsenal" {:points 1200} "Man Utd" {:points 1200})
And if we put the apply array-map code back in we still get the desired result:
> (apply array-map (mapcat vector teams (repeat {:points 1200}))) {"Chelsea" {:points 1200}, "Man City" {:points 1200}, "Arsenal" {:points 1200}, "Man Utd" {:points 1200}}
Alternatively we could use assoc like this:
> (apply assoc {} (mapcat vector teams (repeat {:points 1200}))) {"Man Utd" {:points 1200}, "Arsenal" {:points 1200}, "Man City" {:points 1200}, "Chelsea" {:points 1200}}
I also came across the into function which seemed useful but took in a collection of vectors:
> (into {} [["Chelsea" {:points 1200}] ["Man City" {:points 1200}] ["Arsenal" {:points 1200}] ["Man Utd" {:points 1200}] ])
We therefore need to change the code to use map instead of mapcat:
> (into {} (map vector teams (repeat {:points 1200}))) {"Chelsea" {:points 1200}, "Man City" {:points 1200}, "Arsenal" {:points 1200}, "Man Utd" {:points 1200}}
However, my favourite version so far uses the zipmap function like so:
> (zipmap teams (repeat {:points 1200})) {"Man Utd" {:points 1200}, "Arsenal" {:points 1200}, "Man City" {:points 1200}, "Chelsea" {:points 1200}}
I’m sure there are other ways to do this as well so if you know any let me know in the comments.
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