gistfile1.lisp

(ns seven.core
  (:use clojure.math.combinatorics))

(defn rpn-eval
  [e]
  (loop [[arg1 arg2 op & nxt] e]
    (if (nil? arg2)
      arg1
      (let [result (try (eval (list op arg1 arg2))
                        (catch Exception e nil))]
        (if (nil? result)
          nil
          (recur (cons result nxt)))))))

(defn build-exprs
  []
  (let [numbers [7 7 7 7 7 1]
        ops '[- * - /]]
   (mapcat (fn [currentops]
             (permutations (concat numbers currentops)))
           (selections ops (dec (count numbers))))))

(defn solve
  []
  (let [exprs (build-exprs)]
    ;; (doseq [x exprs]
    ;;   (printf "%s -> %s\n" (list x) (rpn-eval x)))
    (first (filter (fn [expr] (= (rpn-eval expr) 100)) exprs))))

(defn -main
  []
  (let [sol (solve)]
    (printf "solution is: ")
    (prn sol)))