Question

I’ve been playing with Clojure lately and I can’t get this algorithm to work:

(defn reverse-number [number reversed]
   (if (= number 0)
     reversed
     (reverse-number (/ number 10) 
                     (+ (rem number 10) (* reversed 10)))))

This is how I should call it (reverse-number 123 0) and the result I expect is: 321.
When I run this, the REPL just hangs.

Can someone explain me, please, what is happening, what I did wrong and how to get this function working?

Note: I know I can use string functions to reverse a number. Actually, I already did this, but I’m not interested in this solution. All I want is to make the leap to functional languages. That’s why I try multiple approaches.

Using string functions:

(defn reverse-number [n]
  (Integer. (clojure.string/reverse (str n))))

(reverse-number 123)  ; --> 321

I don’t like this version since it feels like cheating by using the string version of reverse.

Answer 1

You should use quot instead of /.

/ in clojure will give you a fraction so number will never be 0 (unless it’s 0 from the beginning), while quot will give you “integer division”.

Examples:

user=> (/ 123 10)                  
123/10
user=> (quot 123 10)
12