JS to CLJS # 2
I’ll regularly take a JavaScript problem and work it out in ClojureScript. This will hopefully take away some of the Magic™ of functional programming and show how to solve real problems. I promise you won’t have to learn what a monad is.
Problem
Write a function, persistence, that takes in a positive parameter num and returns its multiplicative persistence, which is the number of times you must multiply the digits in num until you reach a single digit.
For example:
persistence(39) === 3 // 3*9 = 27, 2*7 = 14, 1*4=4
// and 4 has only one digit
persistence(999) === 4 // 9*9*9 = 729, 7*2*9 = 126,
// 1*2*6 = 12, 1*2 = 2
persistence(4) === 0 // 4 is already a one-digit numberSolutions
;;#cljs
(defn persistence-recursive [n]
(let [n (str n)]
(if (= 1 (count n))
0
(inc (persistence
(reduce (fn [a1 a2]
(* (js/parseInt a1)
(js/parseInt a2)))
n)))))
(defn persistence-iterative [n]
(loop [acc 0
n (str n)]
(if (= 1 (count n))
acc
(recur (inc acc)
(str
(reduce (fn [a1 a2]
(* (js/parseInt a1)
(js/parseInt a2)))
n))))))Thoughts
persistence-recursive is a linear recursive process while persistence-iterative is a linear iterative process. SICP has a great visual explanation of each.