SICP sqrt NullPointerException

6
votes

I've encountered an unexpected NullPointerException while implementing some beginning SICP code in Clojure. In particular, I want to implement the iterative square root procedure from section 1.1.7.

Here's the code:

(defn square [x] (* x x))

(defn abs [x]
  (cond 
    (< x 0) (- x)
    :else x))

(defn average [x y]
  (/ (+ x y) 2))

(defn sqrt
  ([x] (sqrt 1.0 x))
  ([guess x]
    (letfn [(good-enough? [guess]
              (< (abs (- (square guess) x)) 0.001))
            (improve [guess]
              (average guess (/ x guess)))]
      (if (good-enough? guess)
        guess
        (recur (improve guess) x)))))

This works fine for small enough values, e.g. (sqrt 16). I get a NullPointerException clojure.lang.Numbers.lt (Numbers.java:3693) error for any inputs larger than approximately (square 2718).

Any ideas?

Update with the whole traceback (the previous one is all I get in the repl):

Exception in thread "main" java.lang.NullPointerException at clojure.lang.Numbers.lt(Numbers.java:3693) at sicp_in_clojure.chapter_one$sqrt$good_enough_QMARK___14.invoke(chapter_one.clj:40) at sicp_in_clojure.chapter_one$sqrt.invoke(chapter_one.clj:43) at sicp_in_clojure.chapter_one$sqrt.invoke(chapter_one.clj:37) at sicp_in_clojure.chapter_one$eval19.invoke(chapter_one.clj:48) at clojure.lang.Compiler.eval(Compiler.java:6465) at clojure.lang.Compiler.load(Compiler.java:6902) at clojure.lang.Compiler.loadFile(Compiler.java:6863) at clojure.main$load_script.invoke(main.clj:282) at clojure.main$script_opt.invoke(main.clj:342) at clojure.main$main.doInvoke(main.clj:426) at clojure.lang.RestFn.invoke(RestFn.java:408) at clojure.lang.Var.invoke(Var.java:401) at clojure.lang.AFn.applyToHelper(AFn.java:161) at clojure.lang.Var.applyTo(Var.java:518) at clojure.main.main(main.java:37)

2
clojurenullpointerexceptionsicp
Can you paste the whole traceback? – poolie
I can't reproduce the error, it works fine for me, even for larger numbers. What versions of Clojure / Java are you working with? – Óscar López
I also can't reproduce -- I assumed your square functions is (defn square [x] (* x x)). Copy-pasted the code otherwise. – Paul
Paul: yes, I forgot to post my defn of square; I'll edit that in. – Alan O'Donnell
I'm working with Clojure 1.3.0. java version "1.6.0_26" Java(TM) SE Runtime Environment (build 1.6.0_26-b03-384-10M3425) Java HotSpot(TM) 64-Bit Server VM (build 20.1-b02-384, mixed mode) – Alan O'Donnell

2 Answers

1
votes

Not sure if this is still relevant or not, but I thought it worth trying it out using the LightTable Playground app that lets you see how things are interpreted:

light table demo screenshot of above code

Have you tried with a recent build, say the new Clojure 1.4.0 release?

0
votes

Hey it worked fine for me. I Am using clojure 1.3.0. Following is the terminal output.The code works fine.

[user@myhost ~]$ clj 
Clojure 1.3.0
user=> (defn square [x] (* x x))
#'user/square
(defn abs [x]
  (cond 
    (< x 0) (- x)
    :else x))
#'user/abs
(defn average [x y]
  (/ (+ x y) 2))
#'user/average
(defn sqrt
  ([x] (sqrt 1.0 x))
  ([guess x]
    (letfn [(good-enough? [guess]
              (< (abs (- (square guess) x)) 0.001))
            (improve [guess]
              (average guess (/ x guess)))]
      (if (good-enough? guess)
        guess
        (recur (improve guess) x)))))
#'user/sqrt
user=> (sqrt 16)
4.000000636692939
user=> (sqrt 2718)
52.134441897781194
user=> (sqrt 3000)
54.77225658092904