Fixed set_prior() so it works with sparse vectors in addition to dense vectors.

dlib/svm/svm_rank_trainer.h

@@ -37,13 +37,15 @@ namespace dlib
             const std::vector<ranking_pair<sample_type> >& samples_,
             const bool be_verbose_,
             const scalar_type eps_,
-            const unsigned long max_iter
+            const unsigned long max_iter,
+            const unsigned long dims_
         ) :
             samples(samples_),
             C(C_),
             be_verbose(be_verbose_),
             eps(eps_),
-            max_iterations(max_iter)
+            max_iterations(max_iter),
+            dims(dims_)
         {
         }
 
@@ -56,7 +58,7 @@ namespace dlib
         virtual long get_num_dimensions (
         ) const 
         {
-            return max_index_plus_one(samples);
+            return dims;
         }
 
         virtual bool optimization_status (
@@ -173,6 +175,7 @@ namespace dlib
         const bool be_verbose;
         const scalar_type eps;
         const unsigned long max_iterations;
+        const unsigned long dims;
     };
 
 // ----------------------------------------------------------------------------------------
@@ -187,11 +190,12 @@ namespace dlib
         const std::vector<ranking_pair<sample_type> >& samples,
         const bool be_verbose,
         const scalar_type eps,
-        const unsigned long max_iterations
+        const unsigned long max_iterations,
+        const unsigned long dims
     )
     {
         return oca_problem_ranking_svm<matrix_type, sample_type>(
-            C, samples, be_verbose, eps, max_iterations);
+            C, samples, be_verbose, eps, max_iterations, dims);
     }
 
 // ----------------------------------------------------------------------------------------
@@ -346,7 +350,7 @@ namespace dlib
                 << "\n\t this: " << this
                 );
 
-            prior = prior_.basis_vectors(0);
+            prior = sparse_to_dense(prior_.basis_vectors(0));
             learn_nonnegative_weights = false;
             last_weight_1 = false;
         }
@@ -421,13 +425,29 @@ namespace dlib
                         << "\n\t prior.size(): " << prior.size() 
                     );
                 }
-                solver( make_oca_problem_ranking_svm<w_type>(C, samples, verbose, eps, max_iterations), 
-                    w, 
-                    prior);
+                const unsigned long dims = std::max(num_dims, (unsigned long)prior.size());
+                // In the case of sparse sample vectors, it is possible that the input
+                // vector dimensionality is larger than the prior vector dimensionality.
+                // We need to check for this case and pad prior with zeros if it is the
+                // case.
+                if ((unsigned long)prior.size() < dims)
+                {
+                    matrix<scalar_type,0,1> prior_temp = join_cols(prior, zeros_matrix<scalar_type>(dims-prior.size(),1));
+                    solver( make_oca_problem_ranking_svm<w_type>(C, samples, verbose, eps, max_iterations, dims), 
+                        w, 
+                        prior_temp);
+                }
+                else
+                {
+                    solver( make_oca_problem_ranking_svm<w_type>(C, samples, verbose, eps, max_iterations, dims), 
+                        w, 
+                        prior);
+                }
+
             }
             else
             {
-                solver( make_oca_problem_ranking_svm<w_type>(C, samples, verbose, eps, max_iterations), 
+                solver( make_oca_problem_ranking_svm<w_type>(C, samples, verbose, eps, max_iterations, num_dims), 
                     w, 
                     num_nonnegative,
                     force_weight_1_idx);

dlib/test/ranking.cpp

@@ -6,6 +6,7 @@
 #include <string>
 #include <cstdlib>
 #include <ctime>
+#include <map>
 
 #include "tester.h"
 
@@ -107,6 +108,41 @@ namespace
         DLIB_TEST(df.basis_vectors(0)(2) > 0);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    void run_prior_sparse_test()
+    {
+        print_spinner();
+        typedef std::map<unsigned long,double> sample_type;
+        typedef sparse_linear_kernel<sample_type> kernel_type;
+
+        svm_rank_trainer<kernel_type> trainer;
+
+        ranking_pair<sample_type> data;
+
+        sample_type samp;
+        samp[0] = 1; data.relevant.push_back(samp); samp.clear();
+        samp[1] = 1; data.nonrelevant.push_back(samp); samp.clear();
+
+        trainer.set_c(10);
+        decision_function<kernel_type> df = trainer.train(data);
+
+        trainer.set_prior(df);
+
+        data.relevant.clear();
+        data.nonrelevant.clear();
+        samp[2] = 1; data.relevant.push_back(samp); samp.clear();
+        samp[1] = 1; data.nonrelevant.push_back(samp); samp.clear();
+
+        df = trainer.train(data);
+
+        matrix<double,0,1> w = sparse_to_dense(df.basis_vectors(0));
+        dlog << LINFO << trans(w);
+        DLIB_TEST(w(0) > 0.1);
+        DLIB_TEST(w(1) < -0.1);
+        DLIB_TEST(w(2) > 0.1);
+    }
+
 // ----------------------------------------------------------------------------------------
 
     void dotest1()
@@ -390,6 +426,7 @@ namespace
             test_svmrank_weight_force_dense<true>();
             test_svmrank_weight_force_dense<false>();
             run_prior_test();
+            run_prior_sparse_test();
 
         }
     } a;