Added sign() for matrix objects.

dlib/matrix/matrix_math_functions.h

@@ -79,6 +79,15 @@ namespace dlib
             return val*val;
         }
 
+        template <typename type>
+        inline type sign (const type& val)
+        {
+            if (val >= 0)
+                return +1;
+            else
+                return -1;
+        }
+
         template <typename type>
         type cubed (const type& val)
         {
@@ -153,6 +162,7 @@ namespace dlib
     DLIB_DEFINE_FUNCTION_M(op_round_zeros2, round_zeros, impl::round_zeros, 7);
     DLIB_DEFINE_FUNCTION_M(op_cubed, cubed, impl::cubed, 7);
     DLIB_DEFINE_FUNCTION_M(op_squared, squared, impl::squared, 6);
+    DLIB_DEFINE_FUNCTION_M(op_sign, sign, impl::sign, 6);
     DLIB_DEFINE_FUNCTION_MS(op_pow1, pow, impl::pow1, 7);
     DLIB_DEFINE_FUNCTION_SM(op_pow2, pow, impl::pow2, 7);
     DLIB_DEFINE_FUNCTION_M(op_reciprocal, reciprocal, impl::reciprocal, 6);

dlib/matrix/matrix_math_functions_abstract.h

@@ -136,6 +136,24 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 //                               Miscellaneous
 // ----------------------------------------------------------------------------------------
+// ----------------------------------------------------------------------------------------
+
+    const matrix_exp sign (
+        const matrix_exp& m
+    );
+    /*!
+        ensures
+            - returns a matrix that tells the sign of each element in m. In particular:
+              returns a matrix R such that:
+                - R::type == the same type that was in m
+                - R has the same dimensions as m
+                - for all valid r and c:
+                    - if (m(r,c) >= 0) then
+                        - R(r,c) == +1
+                    - else
+                        - R(r,c) == -1
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     const matrix_exp sigmoid (

dlib/test/matrix3.cpp

@@ -997,6 +997,21 @@ namespace
             DLIB_TEST(sum_rows(a) == c);
 
         }
+
+        {
+            matrix<int> m(3,4), s(3,4);
+            m = -2, 1, 5, -5,
+                5, 5, 5, 5,
+                9, 0, -4, -2;
+
+            s = -1, 1, 1, -1,
+                 1, 1, 1, 1, 
+                 1, 1, -1, -1;
+
+            DLIB_TEST(sign(m) == s);
+            DLIB_TEST(sign(matrix_cast<double>(m)) == matrix_cast<double>(s));
+        }
+
     }