Filter nil values from Clojure map?

Question

What is the best way to filter nil values from a Clojure map {}?

{ :a :x :b nil :c :z }
;;=>  { :a :x, :c :z }

Answer 1

The nicest way of doing this is actually(into {} (keep second {:a :x :b nil}))

Answer 2

Probably not the best solution, but here's one that uses list comprehension:

(into {} 
  (for [[k v] {:a 1 :b nil :c :z} :when (not (nil? v))]
    [k v]))

Answer 3

I would use:

(into {} (filter (comp some? val) {:a :x, :b nil, :c :z}))

=> {:a :x, :c :z}

Doing the some? check explicitly is important because if you just do (into {} (filter val {...})) then you will erroneously remove values that are boolean false.

Answer 4

I use following code:

(into {} (filter val {:a 1, :b 2, :c nil}))
;;=> {:a 1, :b 2}

NOTE: this will remove false values as well as nils

Answer 5

(into {} (keep (fn [e] (if (val e) e)) {:a :x :b nil :c :z}))
;;=> {:a :x, :c :z}

or a little shorter:

(into {} (keep #(if (val %) %) {:a :x :b nil :c :z}))

In fact, your filter suggestion is much better and shorter, so I would just use that:

(into {} (filter val {:a :x :b nil :c :z}))