Question

Working on the problems on Project Euler to try to learn Clojure. I’m on problem 7.

I’m getting a nullpointer and I can’t work out what on earth I’ve done wrong.

;;By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can
;;see that the 6th prime is 13.
;;
;;What is the 10 001st prime number?

(defn isPrime [x]
    (if-not (> x 2) false (loop [i 2]
            (if (= i x)
                true
                (if (= (mod x i) 0) false (recur (inc i)) )
            )
        )
    )
)


(loop [ x [2]
        y 3   ]
    (if (= (count x) 6)
        (println (last x))
        ( if (isPrime y) ((let [x (conj x y)]) (println (str (last x)))) )
    )
    (if-not (= (count x) 10001) (recur x (inc y)) )
)

The error is with the ( if (isPrime y) ((let [x (conj x y)]) (println (str (last x)))) ), according to java.

Here’s the traceback..

gcoles@gcoles-pc:~/euler$ clj 7.clj 
2
Exception in thread "main" java.lang.NullPointerException
    at user$eval2.invoke(7.clj:21)
    at clojure.lang.Compiler.eval(Compiler.java:6511)
    at clojure.lang.Compiler.load(Compiler.java:6952)
    at clojure.lang.Compiler.loadFile(Compiler.java:6912)
    at clojure.main$load_script.invoke(main.clj:283)
    at clojure.main$script_opt.invoke(main.clj:343)
    at clojure.main$main.doInvoke(main.clj:427)
    at clojure.lang.RestFn.invoke(RestFn.java:408)
    at clojure.lang.Var.invoke(Var.java:415)
    at clojure.lang.AFn.applyToHelper(AFn.java:161)
    at clojure.lang.Var.applyTo(Var.java:532)
    at clojure.main.main(main.java:37)

The nullpointer actually doesn’t occur until the second time around the loop (Seen by putting a print after that let), so this confuses me a bit..

Anyone got any ideas on this?
Thanks.

Answer 1

The let is of the following form:

(let [binding values]
  (body))

This will return the result of evaluating body. In the code above, your let form looks like this:

(let [x (conj x y)])

Notice how there’s no body. Because of this, the let form returns nil. Since it’s nil, the code you’re really envoking looks like this:

(nil (println (str (last x))))

Because of this, you’re effectively trying calling nil as a function. This causes your null pointer exception. If, for example, you make your code:

(let [x (conj x y)] (println (str (last x))))

It will print out the last value of x. However, it’s worth noting that this new x (the result of (conj x y)) is scoped only to the let statement. If you actually want to use it elsewhere, you’ll have to, for example, recur with it.