Added a stddev() function to the running_stats object.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403411

dlib/statistics/statistics.h

@@ -132,6 +132,19 @@ namespace dlib
                 return 0;
         }
 
+        T stddev (
+        ) const
+        {
+            // make sure requires clause is not broken
+            DLIB_ASSERT(current_n() > 1,
+                "\tT running_stats::stddev"
+                << "\n\tyou have to add some numbers to this object first"
+                << "\n\tthis: " << this
+                );
+
+            return std::sqrt(variance());
+        }
+
         T scale (
             const T& val
         ) const

dlib/statistics/statistics_abstract.h

@@ -118,6 +118,16 @@ namespace dlib
                   object so far.
         !*/
 
+        T stddev (
+        ) const;
+        /*!
+            requires
+                - current_n() > 1
+            ensures
+                - returns the standard deviation of all the values presented to this
+                  object so far.
+        !*/
+
         T max (
         ) const;
         /*!