clojure precision counter

Question

How do I determine the count of digits after the decimal point.

[ 1.6712 2.053 3.52 ]
;;1.6712 => 4
;;2.053 => 3
;;3.52 => 2

Answer 1

Keep in mind: What Every Computer Scientist Should Know About Floating-Point Arithmetic

So a trivial/mathematical attempt could be.

But this will not work for every double!

(defn more-than-5 [number]
   (let [n (* number 10E4)]
     (pos? (- n (int n)))))

This approach should work:

(defn primefactors
  ([n]
   (primefactors n 2 '()))
  ([n candidate acc]
   (cond (<= n 1) (reverse acc)
     (zero? (rem n candidate)) (recur (/ n candidate) candidate (cons candidate acc))
     :else (recur n (inc candidate) acc))))

primefactors function from https://stackoverflow.com/a/9556744

(defn length-of-period [n]
  (if (integer? n)
      [0 0]
      (let [groups (->> n
                        rationalize
                        denominator
                        primefactors
                        (group-by #(zero? (mod 10 %))))
            b1 (apply * (get groups true))
            b2 (apply * (get groups false))]
       [(count (take-while
                  #(pos? (mod % b1))
                  (take 20 (iterate #(bigint (* 10 %)) 1))))
        (count (take-while
                  #(pos? (mod % b2))
                  (take 20 (iterate #(bigint (- (* 10 %) 1)) 1))))])))

The result is a vector [length-of-preperiod length-of-period] Example:

(length-of-period 0.123456) => [6 0]
(length-of-period 1/3) => [0 1]
;; 1/3 = 0.3333...
(length-of-period 7/12) => [2 1]
;; 7/12 = 0.583333....

So in case of doubles you can filter your numbers with #(> 6 (first (length-of-period %)))

Answer 2

From: Number of decimal digits in a double

A double is not always an exact representation. You can only say how many decimal places you would have if you converted it to a String.