Counting occurences (solution to the exercise)

unsorted — cgrand, 28 April 2009 @ 18 h 00 min
What does (merge-with + {:a 12} {:b 4} {:a 3 :b 7}) return?
{:b 11, :a 15} when there are several values for a key, these values are merged (two at a time) using the specified function — here they are summed.
Can you count occurrences of each value in a collection using merge-with?
(apply merge-with + (map (fn [x] {x 1}) coll)) or, using for: (apply merge-with + (for [x coll] {x 1})).
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