euler-prob36.clj

(defn palindrome? [string]
  "Check whether a given string is a palindrome."
  (loop [string string
         start 0
         end  (dec (count string))]
    (cond
     ;;compare string's start and string's end
     (not (= (get string start) (get string end))) false
     ;; return true after checking middle char
     (<= end start) true
     ;; loop inner characters
     :else (recur string (inc start) (dec end)))))

(defn binarystr [num]
  "Convert a base10 number to base2 number string."
  (Integer/toString num 2))

(defn double-palindromic? [num]
  "Check whether a number is palindrome for both bases (base2/base10)."
  (and (palindrome? (str num))
       (palindrome? (binarystr num))))

(defn find-sum-double-palindromics [upto]
  "Find the sum of all palindromic numbers below `upto'."
  (reduce + (for [x (range 1 upto) :when (double-palindromic? x)] x)))