Changed the assign() function so that it no longer removes zeros from

dense vectors when assigning them to sparse vectors.  Also updated dot()
so that it is much faster when working with "sparse vectors" that are
the product of assigning a dense vector to a sparse vector.

dlib/svm/sparse_vector.h

@@ -182,8 +182,7 @@ namespace dlib
         typedef typename T::value_type item_type;
         for (long i = 0; i < src.size(); ++i)
         {
-            if (src(i) != 0)
-                dest.insert(dest.end(),item_type(i, src(i)));
+            dest.insert(dest.end(),item_type(i, src(i)));
         }
     }
 
@@ -220,9 +219,9 @@ namespace dlib
     namespace impl
     {
         template <typename T, typename U>
-        typename T::value_type::second_type dot (
-        const T& a,
-        const U& b
+        typename T::value_type::second_type general_dot (
+            const T& a,
+            const U& b
         )
         {
             typedef typename T::value_type::second_type scalar_type;
@@ -251,6 +250,58 @@ namespace dlib
 
             return sum;
         }
+
+        template <typename T, typename U>
+        inline typename T::value_type::second_type dot (
+            const T& a,
+            const U& b
+        )
+        {
+            return general_dot(a,b);
+        }
+
+        template <typename T, typename U, typename alloc>
+        U dot (
+            const std::vector<std::pair<T,U>,alloc>& a,
+            const std::vector<std::pair<T,U>,alloc>& b
+        )
+        {
+            // You are getting this error because you are attempting to use sparse sample vectors 
+            // but you aren't using an unsigned integer as your key type in the sparse vectors.
+            COMPILE_TIME_ASSERT(is_unsigned_type<T>::value);
+
+            if (a.size() == 0 || b.size() == 0)
+                return 0;
+
+            // if a is really a dense vector but just represented in a sparse container
+            if (a.back().first == a.size()-1)
+            {
+                double sum = 0;
+                for (unsigned long i = 0; i < b.size(); ++i)
+                {
+                    if (b[i].first >= a.size())
+                        break;
+                    sum += a[b[i].first].second * b[i].second;
+                }
+                return sum;
+            }
+            // if b is really a dense vector but just represented in a sparse container
+            else if (b.back().first == b.size()-1)
+            {
+                double sum = 0;
+                for (unsigned long i = 0; i < a.size(); ++i)
+                {
+                    if (a[i].first >= b.size())
+                        break;
+                    sum += b[a[i].first].second * a[i].second;
+                }
+                return sum;
+            }
+            else
+            {
+                return general_dot(a,b);
+            }
+        }
     }
 
     template <typename T>