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+// The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
+/*
+
+    This is an example showing how to use sparse feature vectors with
+    the dlib C++ library's machine learning tools.
+
+    This example creates a simple binary classification problem and shows
+    you how to train a support vector machine on that data.
+
+    The data used in this example will be 100 dimensional data and will
+    come from a simple linearly separable distribution.  
+*/
+
+
+#include <iostream>
+#include <ctime>
+#include <vector>
+#include <dlib/svm.h>
+
+using namespace std;
+using namespace dlib;
+
+
+int main()
+{
+    // In this example program we will be dealing with feature vectors that are sparse (i.e. most
+    // of the values in each vector are zero).  So rather than using a dlib::matrix we can use
+    // one of the containers from the STL to represent our sample vectors.  In particular, we 
+    // can use the std::map to represent sparse vectors.  (Note that you don't have to use std::map.
+    // Any STL container of std::pair objects that is sorted can be used.  So for example, you could 
+    // use a std::vector<std::pair<unsigned long,double> > here so long as you took care to sort every vector)
+    typedef std::map<unsigned long,double> sample_type;
+
+
+    // This is a typedef for the type of kernel we are going to use in this example.
+    // Since our data is linearly separable I picked the linear kernel.  Note that if you
+    // are using a sparse vector representation like std::map then you have to use a kernel
+    // meant to be used with that kind of data type.  
+    typedef sparse_linear_kernel<sample_type> kernel_type;
+
+
+    // Here we create an instance of the pegasos svm trainer object we will be using.
+    svm_pegasos<kernel_type> trainer;
+    // Here we setup a parameter to this object.  See the dlib documentation for a 
+    // description of what this parameter does. 
+    trainer.set_lambda(0.00001);
+
+    // Let's also use the svm trainer specially optimized for the linear_kernel and
+    // sparse_linear_kernel.
+    svm_c_linear_trainer<kernel_type> linear_trainer;
+    // This trainer solves the "C" formulation of the SVM.  See the documentation for
+    // details.
+    linear_trainer.set_c(10);
+
+    std::vector<sample_type> samples;
+    std::vector<double> labels;
+
+    // make an instance of a sample vector so we can use it below
+    sample_type sample;
+
+
+    // Now let's go into a loop and randomly generate 10000 samples.
+    srand(time(0));
+    double label = +1;
+    for (int i = 0; i < 10000; ++i)
+    {
+        // flip this flag
+        label *= -1;
+
+        sample.clear();
+
+        // now make a random sparse sample with at most 10 non-zero elements
+        for (int j = 0; j < 10; ++j)
+        {
+            int idx = std::rand()%100;
+            double value = static_cast<double>(std::rand())/RAND_MAX;
+
+            sample[idx] = label*value;
+        }
+
+        // let the svm_pegasos learn about this sample.  
+        trainer.train(sample,label);
+
+        // Also save the samples we are generating so we can let the svm_c_linear_trainer
+        // learn from them below.  
+        samples.push_back(sample);
+        labels.push_back(label);
+    }
+
+    // In addition to the rule we learned with the pegasos trainer, let's also use our
+    // linear_trainer to learn a decision rule.
+    decision_function<kernel_type> df = linear_trainer.train(samples, labels);
+
+    // Now we have trained our SVMs.  Let's test them out a bit.  
+    // Each of these statements prints the output of the SVMs given a particular sample.  
+    // Each SVM outputs a number > 0 if a sample is predicted to be in the +1 class and < 0 
+    // if a sample is predicted to be in the -1 class.
+
+
+    sample.clear();
+    sample[4] = 0.3;
+    sample[10] = 0.9;
+    cout << "This is a +1 example, its SVM output is: " << trainer(sample) << endl;
+    cout << "df: " << df(sample) << endl;
+
+    sample.clear();
+    sample[83] = -0.3;
+    sample[26] = -0.9;
+    sample[58] = -0.7;
+    cout << "This is a -1 example, its SVM output is: " << trainer(sample) << endl;
+    cout << "df: " << df(sample) << endl;
+
+    sample.clear();
+    sample[0] = -0.2;
+    sample[9] = -0.8;
+    cout << "This is a -1 example, its SVM output is: " << trainer(sample) << endl;
+    cout << "df: " << df(sample) << endl;
+
+}
+