add monte carlo in clojure (#192)

* Add Monte Carlo in Clojure

Author: earthfail

chapters/monte_carlo/code/clojure/monte_carlo.clj

@@ -0,0 +1,32 @@
+(ns monte-carlo.core)
+
+(defn in-circle? [pv r]
+  "take a vector representing point and radius return true if the
+  point is inside the circle"
+  (< (->>
+      pv
+      (map #(* % %))
+      (reduce +))
+     (* r r)))
+(defn rand-point [r]
+  "return a random point from (0,0) inclusive to (r,r) exclusive"
+  (repeatedly 2 #(rand r)))
+(defn monte-carlo [n r]
+  "take the number of random points and radius return an estimate to
+pi"
+  (*' 4 (/ n)
+      (loop [i n count 0]
+        (if (zero? i)
+          count
+          (recur (dec i)
+                 (if (in-circle? (rand-point r) r)
+                   (inc count)
+                   count))))))
+(defn -main []
+  (let [constant-pi Math/PI
+        computed-pi (monte-carlo 10000000 2) ;; this may take some time on lower end machines
+        difference (Math/abs (- constant-pi computed-pi))
+        error (* 100 (/ difference constant-pi))]
+    (println "world's PI: " constant-pi
+             ",our PI: "    (double computed-pi)
+             ",error: " error)))

chapters/monte_carlo/monte_carlo.md

@@ -39,6 +39,8 @@ each point is tested to see whether it's in the circle or not:
 {% method %}
 {% sample lang="jl" %}
 [import:2-7, lang:"julia"](code/julia/monte_carlo.jl)
+{% sample lang="clj" %}
+[import:3-10, lang:"clojure"](code/clojure/monte_carlo.clj)
 {% sample lang="c" %}
 [import:7-9, lang:"c_cpp"](code/c/monte_carlo.c)
 {% sample lang="js" %}
@@ -85,6 +87,9 @@ Feel free to submit your version via pull request, and thanks for reading!
 {% sample lang="jl" %}
 ### Julia
 [import, lang:"julia"](code/julia/monte_carlo.jl)
+{% sample lang="clj" %}
+### Clojure
+[import, lang:"clojure"](code/clojure/monte_carlo.clj)
 {% sample lang="c" %}
 ### C
 [import, lang:"c_cpp"](code/c/monte_carlo.c)