Changed the definition of the is_binary_classification_problem() function

so that it is a little more reasonable and also easier to understand.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403042

dlib/svm/svm.h

@@ -103,6 +103,8 @@ namespace dlib
         const U& x_labels
     )
     {
+        bool seen_neg_class = false;
+        bool seen_pos_class = false;
         if (x.nc() != 1 || x_labels.nc() != 1) return false; 
         if (x.nr() != x_labels.nr()) return false;
         if (x.nr() <= 1) return false;
@@ -110,9 +112,14 @@ namespace dlib
         {
             if (x_labels(r) != -1 && x_labels(r) != 1)
                 return false;
+
+            if (x_labels(r) == 1)
+                seen_pos_class = true;
+            if (x_labels(r) == -1)
+                seen_neg_class = true;
         }
 
-        return true;
+        return seen_pos_class && seen_neg_class;
     }
 
     template <

dlib/svm/svm_abstract.h

@@ -73,10 +73,12 @@ namespace dlib
             - U == a matrix or something convertible to a matrix via vector_to_matrix()
         ensures
             - returns true if all of the following are true and false otherwise:
-                - x.nc()        == 1 (i.e. x is a column vector)
-                - x_labels.nc() == 1 (i.e. x_labels is a column vector)
-                - x.nr() == x_labels.nr() 
-                - x.nr() > 1
+                - is_col_vector(x) == true
+                - is_col_vector(x_labels) == true
+                - x.size() == x_labels.size() 
+                - x.size() > 1
+                - there exists at least one sample from both the +1 and -1 classes.
+                  (i.e. all samples can't have the same label)
                 - for all valid i:
                     - x_labels(i) == -1 or +1
     !*/