Clojure: Merge Two Maps but Only Keep the Keys of One of Them

Mark Needham

Sep. 25, 13 · Interview

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I’ve been playing around with Clojure maps recently and I wanted to merge two maps of rankings where the rankings in the second map overrode those in the first while only keeping the teams from the first map.

The merge function overrides keys in earlier maps but also adds keys that only appear in later maps. For example, if we merge the following maps:

> (merge {"Man. United" 1500 "Man. City" 1400} {"Man. United" 1550 "Arsenal" 1450})
{"Arsenal" 1450, "Man. United" 1550, "Man. City" 1400}

we get back all 3 teams but I wanted a function which only returned ‘Man. United’ and ‘Man. City’ since those keys appear in the first map and ‘Arsenal’ doesn’t.

I wrote the following function:

(defn merge-rankings [initial-rankings override-rankings]
  (merge initial-rankings
         (into {} (filter #(contains? initial-rankings (key %)) override-rankings))))

If we call that we get the desired result:

> (merge-rankings {"Man. United" 1500 "Man. City" 1400} {"Man. United" 1550 "Arsenal" 1450})
{"Man. United" 1550, "Man. City" 1400}

An alternative version of that function could use select-keys like so:

(defn merge-rankings [initial-rankings override-rankings]
  (select-keys (merge initial-rankings override-rankings) (map key initial-rankings)))

bitemyapp points out in the comments that we can go even further and use the keys function instead of map key, like so:

(defn merge-rankings [initial-rankings override-rankings]
  (select-keys (merge initial-rankings override-rankings) (keys initial-rankings)))

Now let’s generify the function so it would make sense in the context of any maps, not just ranking related ones:

(defn merge-keep-left [left right]
  (select-keys (merge left right) (keys left)))

Merge (version control) Clojure

Published at DZone with permission of Mark Needham, DZone MVB. See the original article here.

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