A function to format a number with commas

commatize

(defn commatize [n]
  (if (nil? n)
    ""
    (let [s (str n)]
      (apply str
        (reverse (drop-last
                   (interleave
                     (reverse s)
                     (take (count s) (flatten (repeat [nil nil \,]))))))))))

Or:

(defn commatize [n]
  (if (nil? n)
    ""
    (apply str (reverse (flatten (interpose \, (partition-all 3 (reverse (str n)))))))))

I find this way a bit more readable:

(defn commatize [n]
  (if n 
    (->> n 
         str 
         reverse 
         (partition-all 3) 
         (interpose \,) 
         flatten 
         reverse 
         (apply str)) 
  ""))

Or just:

(defn commatize [n] (if n (-> (java.text.DecimalFormat.) (.format n)) ""))