Invert bits of a number in clojurescript

Question

Doing a challenge to print the number that you would get if you convert a given number to binary, invert its bits, then convert back to decimal.

I decided to give it a run in clojurescript (which I don't really know) and while all the below makes sense to me, it has to be possible to do it better and without dropping to js functions as often as I felt I had to.

​

(def invert-number [n] (+ 1 (* -1 n)))
(defn number-complement [num]
 (let [numstr (.toString num 2)
       bits (map (comp invert-number js/parseInt) numstr)
       complement-bits (reduce str bits)]
      js/parseInt complement-bits 2)))

Answer 1

First, fixing a few of the errors that I noted in the comments and altering the formatting a bit, I have:

(defn invert-number [n]
  (+ 1 (* -1 n)))

(defn number-complement [num]
 (let [numstr (.toString num 2)
       bits (map (comp invert-number js/parseInt) numstr)
       complement-bits (reduce str bits)]

   (js/parseInt complement-bits 2)))

(println (number-complement 10)
         (number-complement 12)
         (number-complement 15)
         (number-complement 993061001)
         (number-complement 123))

; 5 3 0 80680822 4

I prefer to have defn function definitions on a separate line, and have at least one empty line between function definitions.

---

invert-number can be fixed up a bit. - can actually be used as an unary operator to do a negation, and inc is arguably more idiomatic than + 1 unless you think you may need to add to the equation later. I changed it to:

(defn invert-number2 [n]
  (inc (- n)))

Since the entire purpose of the function though seems to be just toggling between 0 and 1, I'd just write it as a more explicit toggle:

(defn invert-number2 [n]
  (if (zero? n) 1 0))

I feel like that conveys the purpose much clearer. I'd expect it to perform similarly too.

---

You can get rid of the first call to js/parseInt by just using int:

(map (comp invert-number2 int) numstr)

Honestly, I don't know why this works. int here is not acting like it does in Clojure (which is what I'm familiar with; I don't actually know Cljs). I'm guessing this is due to some weirdness on Javascript's end. If I plug that into a Cljs transpiler, I get some weird clues:

(int "192837465")

becomes

("192837465" | (0));

Because... Javascript happened?

The second js/parseInt is a little harder to deal with though because of the radix argument. I'd just stick with what you have.

---

(reduce str bits) can be changed to (apply str bits). Many variadic functions automatically manually reduce over their arguments (like str and +), so you you can alternatively apply the list directly to the function. (apply str makes a little more sense to me, but that's likely because I've written that many times before.

---

In the end, I ended up with:

(defn invert-number2 [n]
  (if (zero? n) 1 0))

(defn number-complement2 [num]
 (let [numstr (.toString num 2)
       bits (map (comp invert-number2 int) numstr)

       complement-bits (apply str bits)]

   (js/parseInt complement-bits 2)))

(println (number-complement2 10)
         (number-complement2 12)
         (number-complement2 15)
         (number-complement2 993061001)
         (number-complement2 123))

; 5 3 0 80680822 4