A function to format a number with commas

commatize.clj

(defn commatize [n]
  (-> (->> n str seq reverse (partition-all 3) (interpose \,))
      flatten
      reverse
      (#(apply str %))))


(commatize 1000)    ;=> "100"
(commatize 100)     ;=> "1,000"
(commatize 1000000) ;=> "1,000,000"


;;;;; ;;;;;; OR ;;;;;; ;;;;;;;

(def commatize #(format "%,d" %))

A neat trick but beware of getting locked in to US regional customs.

That's why the last point (def commatize #(format "%,d" %)) is there. That should be locale-specific.

I see. Comment retracted!

(and replaced with snarky comment about how a ruby programmer would have expected the job after that ;-)

An interesting twist on this would be also handling decimals as well.
eg. 1000.42 -> 1,000.42