NullPointerException using Clojure - clojure

Question

I'm new to clojure, attempting to write a function (all-bit-seqs n) that generates all bit strings of length n as a list. So if I were to call (all-bit-seqs 2), it would output ((0 0) (0 1) (1 0) (1 1)) in any order. However, I am getting a NullPointerException when I call the helper function rest-bit-seqs, and I can't figure out why. My code is as follows, any help would be appreciated.
(defn not-bit [x]
(* -1 (- x 1))
)
(defn inc-bit-seq [x]
(cond
(= 0 (not-bit (first x))) (cons 0 (inc-bit-seq (rest x)))
:else (cons 1 (rest x))
)
)
(defn pow [x, y]
(cond
(not= y 0) (* x (pow x (- y 1)))
:else 1
)
)
(defn rest-bit-seqs [n, x, lst]
(cond
(not= x (pow 2 n)) (cons lst (rest-bit-seqs n (+ 1 x) (inc-bit-seq lst)))
:else '()
)
)
(defn zero-seq [n]
(cond
(= n 0) '()
:else (cons 0 (zero-seq (- n 1)))
)
)
(defn all-bit-seqs [n]
(rest-bit-seqs n 0 (zero-seq n))
)

Answer 1

that is because you don't handle the case of an empty sequence here:
(defn inc-bit-seq [x]
(cond
(= 0 (not-bit (first x))) (cons 0 (inc-bit-seq (rest x)))
:else (cons 1 (rest x))
)
)
so at one point you pass (first x) (which is nil for an empty seq) to not-bit.
the following fix solves this:
(defn inc-bit-seq [x]
(when (seq x)
(cond
(= 0 (not-bit (first x))) (cons 0 (inc-bit-seq (rest x)))
:else (cons 1 (rest x))
))
)
in repl:
user> (all-bit-seqs 2)
((0 0) (1 0) (0 1) (1 1))
the other things that are totally weird here, is your choice of cond instead of simple if, and uncommon formatting. I would consider rewriting the code this way:
(defn not-bit [x]
(* -1 (- x 1)))
(defn inc-bit-seq [x]
(when (seq x)
(if (zero? (not-bit (first x)))
(cons 0 (inc-bit-seq (rest x)))
(cons 1 (rest x)))))
(defn pow [x, y]
(if-not (zero? y)
(* x (pow x (- y 1)))
1))
(defn rest-bit-seqs [n, x, lst]
(when-not (== x (pow 2 n))
(cons lst (rest-bit-seqs n (+ 1 x) (inc-bit-seq lst)))))
(defn zero-seq [n]
(when-not (zero? n)
(cons 0 (zero-seq (- n 1)))))
(defn all-bit-seqs [n]
(rest-bit-seqs n 0 (zero-seq n)))
(it's about style, haven't looked at overall code correctness)

Answer 2

Your error is here:
(= 0 (not-bit (first x)))
If x is empty, (first x) will return nil, so this will happen:
(= 0 (not-bit nil))
(= 0 (* -1 (- nil 1))
When you try to evaluate (- nil 1), you'll get a NullPointerException.
The quick-and-dirty way to fix the problem is to get rid of not-bit and replace that condition with
(= 1 (first x))
However, there are much shorter/simpler ways to solve this problem. Here's one way:
(defn inc-bit-seq [[head & tail]]
(if (zero? head)
(cons 1 tail)
(cons 0 (inc-bit-seq tail))))
(defn all-bit-seqs [n]
(take (bit-shift-left 1 n)
(iterate inc-bit-seq (repeat n 0))))
Another way would be to use strings:
(defn pad-first [n x xs]
(concat (repeat (- n (count xs)) x) xs))
(defn all-bit-seqs [n]
(map (comp (partial pad-first n 0)
(partial map #(Character/getNumericValue %))
#(Long/toBinaryString %))
(range (bit-shift-left 1 n))))