Question

While working on the Clojure Koans, I had to calculate the factorial of a number iterativly, I did find the solution, but I have a question about the difference between 2 solutions, one that works and one that doens’t, although I don’t understand why:

The one that works:

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

The one that desn’t:

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

Note that the only difference is the returned value of the If block if the condition is met.

Answer 1

The second factorial function always returns 1. The code is built to use an accumulator variable (acc), and the first code block gets it right by returning this accumulator variable.

A factorial function can be written to return 1, though, if an accumulator variable is not used. Since this method does not utilize loop / recur, it can cause a stack overflow easily: try (fact 5000).

(defn factorial [x]
  (if (<= x 1)
      1
      (* x (factorial (- x 1)))))

(source)