Clojure: Combine hash-maps only if key is in both maps?

Question

I have two hash maps and am trying to merge them together while only keeping the keys found in both maps.
Ex:

{a true, b true, c true, d true, e true}
merged with {c true, d true, e true, f true}
would yield {c true, d true, e true}

I am pretty new to Clojure and just can't seem to figure out how to do this. Thanks

How are you merging the values? You can use select-keys to select the keys you need from the second map. — Lee
– Lee, Commented Feb 8, 2018 at 18:00
Thats what I need help with. I want to merge them as I showed in the code but I can't figure out how to do so — Rattus
– Rattus, Commented Feb 8, 2018 at 18:01
So you just want to discard the first map? Or do you want to combine the corresponding values in some way? It's not clear how a => true in the first map and a => true in the second combine to result in a => true in the result. — Lee
– Lee, Commented Feb 8, 2018 at 18:42

fl00r · Accepted Answer · 2018-02-08 18:05:39Z

7

There is a select-keys function in the standard library

(let [a {:a true :b true :c true :d true :e true}
      b {:c true :d true :e true :f true}
      b-keys (keys b)]
  (select-keys a b-keys))
#=> {:c true, :d true, :e true}

answered Feb 8, 2018 at 18:05

fl00r

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1 Comment

Rattus Over a year ago

Thanks that was exactly what I was looking for

tap · Accepted Answer · 2018-02-09 00:48:15Z

1

Apart from select-keys, min-key is also useful. This function can be used with an arbitrary number of map

(defn min-merge [& ms]
  (let [min-keys (keys (apply min-key count ms))]
    (select-keys (apply merge ms) min-keys)))

Try

user=> (min-merge {:a 1 :b 2} {:a 3} {:a 4 :b 5 :c 6})
{:a 4}

answered Feb 9, 2018 at 0:48

tap

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