fix divide by zero on quiz statistics page

Change-Id: I1b3c51c3d39cabb2e86e40ff5535f1b694df6dc4
Reviewed-on: https://gerrit.instructure.com/4299
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

app/models/quiz.rb

@@ -857,7 +857,7 @@ class Quiz < ActiveRecord::Base
         questions_hash[question[:id]] ||= question
       end
     end
-    stats[:submission_score_average] = score_counter.mean.to_f rescue 0 #stats[:submission_count] > 0 ? stats[:submission_score_tally] / stats[:submission_count] : 0
+    stats[:submission_score_average] = score_counter.mean.to_f rescue 0
     stats[:submission_score_high] = score_counter.max
     stats[:submission_score_low] = score_counter.min
     stats[:submission_duration_average] = stats[:submission_count] > 0 ? stats[:submission_duration_tally].to_f / stats[:submission_count].to_f : 0

lib/stats.rb

@@ -130,14 +130,18 @@ module Stats
     end
     
     def mean
-      total.to_f / size
+      size > 0 ? total.to_f / size : 0
     end
     
     def variance
-      all.map {|value|
-        diff = value - mean
-        diff*diff
-      }.sum / size.to_f
+      if size > 0
+        all.map {|value|
+          diff = value - mean
+          diff*diff
+        }.sum / size.to_f
+      else
+        0
+      end
     end
     
     def standard_deviation

spec/lib/stats_spec.rb

@@ -30,11 +30,15 @@ require File.expand_path(File.dirname(__FILE__) + '/../spec_helper.rb')
 #   end
 # end
 
-# include Stats
+include Stats
 
 describe Stats do
-  # context Counter do
-  #   
+  context Counter do
+    it "should not return NaN with no values" do
+      c = Counter.new
+      c.mean.should == 0
+    end
+    
   #   before do
   #     @a = [1,2,4,6,9]
   #     @c = Counter.new(@a)
@@ -132,5 +136,5 @@ describe Stats do
   #     @c.mean.should eql(2.5)
   #     @c.standard_deviation.should be_close( 0.612372435695794, 0.000000001)
   #   end
-  # end
+  end
 end