NullPointerException after simple clojure program

Question

I am working on learning clojure, and have run into a NullPointerException that seems to have nothing to do with my code. The program runs to completion before producing the error. The code:

; solves the collatz conjecture

; return one step in the sequence
(defn collatz-step [n] 
  (if (= (rem n 2) 0)
    (/ n 2)
    (+ 1 (* 3 n))))

; recurse over all numbers
(defn collatz [n] 
  (if (= n 1)
    (println "All done!")
    ((println (format "N = %d" n)) 
     (collatz (collatz-step n)))))

; get input and run it
(println "Enter a positive number:")
(collatz (read-string (read-line)))

Is there something I'm missing?

Arthur Ulfeldt Arthur Ulfeldt · Accepted Answer · 2013-03-27T00:33:57

when this line runs:

((println (format "N = %d" n)) 
 (collatz (collatz-step n)))

the println and colatz will finish leving the form like this:

 (return-value-of-println return-value-of-collatz)

println returns nil yielding:

 (nil return-value-of-collatz)

which is a function call to the function nil resulting in an NPE

take out the extra ()

collatzrecur

NullPointerException after simple clojure program

1 Answers