Implement monte carlo in ruby (#361)

* Implement monte carlo in ruby
* Fix percent error

Author: jasmaa

contents/monte_carlo_integration/code/ruby/monte_carlo.rb

@@ -0,0 +1,32 @@
+def in_circle(x, y, radius=1)
+  # Check if coords are in circle via Pythagorean Thm
+  return (x*x + y*y) < radius*radius
+end
+
+def monte_carlo(n_samples, radius=1)
+  # estimate pi via monte carlo sampling
+  in_circle_count = 0.0
+  
+  for _ in 0...n_samples
+    # randomly choose coords within square
+	x = rand()*radius
+	y = rand()*radius
+	if in_circle(x, y, radius)
+	  in_circle_count += 1
+	end
+  end
+  
+  # circle area is pi*r^2 and rect area is 4r^2
+  # ratio between the two is then pi/4 so multiply by 4 to get pi
+  return 4 * (in_circle_count / n_samples)
+  
+end
+
+
+# Main
+pi_estimate = monte_carlo(100000)
+percent_error = 100 * (pi_estimate - Math::PI).abs / Math::PI
+
+puts "The estimate of pi is: #{pi_estimate.round(3)}"
+puts "The percent error is: #{percent_error.round(3)}"
+

contents/monte_carlo_integration/monte_carlo_integration.md

@@ -67,6 +67,8 @@ each point is tested to see whether it's in the circle or not:
 [import:23-23, lang:"csharp"](code/csharp/Circle.cs)
 {% sample lang="nim" %}
 [import:6-7, lang:"nim"](code/nim/monte_carlo.nim)
+{% sample lang="ruby" %}
+[import:1-4, lang:"ruby"](code/ruby/monte_carlo.rb)
 {% endmethod %}
 
 If it's in the circle, we increase an internal count by one, and in the end,
@@ -133,6 +135,8 @@ Feel free to submit your version via pull request, and thanks for reading!
 [import, lang:"csharp"](code/csharp/Program.cs)
 {% sample lang="nim" %}
 [import, lang:"nim"](code/nim/monte_carlo.nim)
+{% sample lang="ruby" %}
+[import, lang:"ruby"](code/ruby/monte_carlo.rb)
 {% endmethod %}