clojure - why this code is wrong ? How does 'recur' work? -

i don't know why code below wrong:

(defn factorial [n]   (loop [n n          acc 1]     (if (zero? n)       acc       (recur (* acc n)(dec n)))))   (= 1 (factorial 1)) 

how recur work?

the arguments recur wrong way round.

• n should become (dec n)
• acc should become (* acc n)

so should

(recur (dec n) (* acc n)) 

---

we can recast given algorithm see what's going on inside it.

if represent pair of arguments vector, function generates next pair is

(fn [[n acc]] [(* acc n) (dec n)]) 

we can generate endless sequence of possible pairs given noby applying iterate function above, starting [no 1].

(fn [no]   (iterate (fn [[n acc]] [(* acc n) (dec n)]) [no 1])) 

applying 1 generates

([1 1] [1 0] [0 0] [0 -1] ...) 

we stop @ element 2, first initial 0, returning other 0.

if put arguments right way round, can proper factorial thus:

(defn factorial [no]   ((comp second first)          (drop-while            (comp not zero? first)            (iterate (fn [[n acc]] [(dec n) (* acc n)]) [no 1])))) 

this returns second element of first pair in sequence 0 first (duh!).

hopelessly overcomplicated normal use, work?

=> (map factorial (range 6)) (1 1 2 6 24 120) 

yes.