clojure.set/map-invert

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Type: function Added: Clojure 1.0 Examples: 3 Runnable: 0

([m])

Returns the map with the vals mapped to the keys.

Examples

by Brian Marick on 2010-07-13 . May have evaluation errors. ▶

;; Despite being in clojure.set, this has nothing to do with sets. (map-invert {:a 1, :b 2}) {2 :b, 1 :a} ;; If there are duplicate keys, one is chosen: (map-invert {:a 1, :b 1}) {1 :b} ;; I suspect it'd be unwise to depend on which key survives the clash.

by bruno-oliveira on 2017-01-06 . May have evaluation errors. ▶

;; The inverted map of an empty map is also an empty map. (map-invert {}) {} ;; Using complex values (which serve as keys in the inverted map) is possible. ((map-invert {:a {:c 5}}) {:c 5}) :a

by reborg on 2018-07-20 . May have evaluation errors. ▶

;; simple text obfuscation and back, with map-invert (def scramble-key {\a \t \b \m \c \o \d \l \e \z \f \i \g \b \h \u \i \h \j \n \k \s \l \r \m \a \n \q \o \d \p \e \q \k \r \y \s \f \t \c \u \p \v \w \w \x \x \j \y \g \z \v \space \space}) (defn scramble [text scramble-key] (apply str (map scramble-key text))) (defn unscramble [text scramble-key] (apply str (map (map-invert scramble-key) text))) (scramble "try to read this if you can" scramble-key) ;; "cyg cd yztl cuhf hi gdp otq" (unscramble "cyg cd yztl cuhf hi gdp otq" scramble-key) ;; "try to read this if you can"

Note by optevo

If you have the possibility of duplicate values, in your map, the invert-map function will only preserve one of them. An alternative which doesn't eliminate values is to ensure all values are sets then use the following function:

(defn invert-map-of-sets [m]
   (reduce (fn [a [k v]] (assoc a k (conj (get a k #{}) v))) {} (for [[k s] m v s] [v k]))))

This will work as follows:

;; From
{1 #{:a :b :c} 2 #{:b :c :d}}

;; To
{:c #{1 2}, :b #{1 2}, :a #{1}, :d #{2}}

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