Clojure sort map over value

Question

I'm trying to sort a map over the values.

The input-map looks like:

{:Blabla 1, :foo 1, :bla-bla 1, :Bla 2, :bla/bla 1, :bla 4, :blub 2, :hello 1, :Foo 2}

The output should look like:

{:bla 4 :Bla 2 :blub 2 :Foo 2 :Blabla 1 :bla-bla 1 :bla/bla 1 :foo 1 :hello 1}

I used sorted-map-by like in the documentation here: http://clojuredocs.org/clojure.core/sorted-map-by

(defn sort-keyword-list [texts]
  (let [results (word-counts texts)]
       ;results is now {:Blabla 1, :foo 1, :bla-bla 1, :Bla 2, :bla/bla 1, :bla 4, :blub 2, :hello 1, :Foo 2}
       (into (sorted-map-by (fn [key1 key2]
                                (compare [(get results key2) key2]
                                         [(get results key1) key1])))
             results))
  )

Well I found out that this solution works only if the keywords have no special characters like "/" or "-" inside. Is this a known bug?

So how can I sort a map by values quickly without writing a own and slowly sort-algorithm?

user100464 · Accepted Answer · 2014-10-12 20:55:13Z

2

In my Clojure 1.6.0 REPL, the code in the question already sorts by value:

user=> (into (sorted-map-by (fn [key1 key2]
                     (compare [(get x key2) key2]
                              [(get x key1) key1])))
    x)
{:bla 4, :blub 2, :Foo 2, :Bla 2, :bla/bla 1, :hello 1, :foo 1, :bla-bla 1, :Blabla 1}

If you want entries with the same value to be sorted by key, you need to stringify the keys. Here's why:

user=> x
{:bla-bla 1, :Blabla 1, :bla/bla 1, :hello 1, :bla 4, :foo 1, :Bla 2, :Foo 2, :blub 2}
user=> (sort (keys x))
(:Bla :Blabla :Foo :bla :bla-bla :blub :foo :hello :bla/bla)
user=> (sort (map str (keys x)))
(":Bla" ":Blabla" ":Foo" ":bla" ":bla-bla" ":bla/bla" ":blub" ":foo" ":hello")

edited Oct 12, 2014 at 20:55

answered Oct 12, 2014 at 17:26

user100464

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3 Comments

Kingalione Over a year ago

I want to sort over the values in first order than maybe over the keys.

user100464 Over a year ago

I'm not sure I understand your goal. Your code already produces a map that sorts by value. If you don't want to sort over the keys, you already have working code. If you do want to sort over the keys to disambiguate entries with the same value, you probably want to stringify the keys first.

Kingalione Over a year ago

In my REPL it does not sort the map over the values. The output and input is the same

Yu Shen · Accepted Answer · 2017-07-17 17:57:12Z

2

Here is a solution based on the suggestion by @user100464 with explicit considerations of comparison of keys, when the values are the same.

Note: I choose to sort decreasingly by reversing the order of the arguments to the comparisons: (compare (x k2) (x k1)) and (compare k2 k1).

(defn sort-by-value-then-key [x]
  (into (sorted-map-by (fn [k1, k2]
                         (let [v_c (compare (x k2) (x k1))]
                           (if (= 0 v_c)
                             (compare k2 k1)))))
        x))

One may customize at (compare k2 k1) to implement more elaborate key comparison.

edited Jul 17, 2017 at 17:57

answered Nov 25, 2016 at 4:13

Yu Shen

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