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Commit 03ec260c authored by Davis King's avatar Davis King
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examples/svm_ex.cpp
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......@@ -30,16 +30,16 @@ int main()
// The svm functions use column vectors to contain a lot of the data on which they
// operate. So the first thing we do here is declare a convenient typedef.
// This typedef declares a matrix with 2 rows and 1 column. It will be the
// object that contains each of our 2 dimensional samples. (Note that if you wanted
// more than 2 features in this vector you can simply change the 2 to something else.
// Or if you don't know how many features you want until runtime then you can put a 0
// here and use the matrix.set_size() member function)
// This typedef declares a matrix with 2 rows and 1 column. It will be the object that
// contains each of our 2 dimensional samples. (Note that if you wanted more than 2
// features in this vector you can simply change the 2 to something else. Or if you
// don't know how many features you want until runtime then you can put a 0 here and
// use the matrix.set_size() member function)
typedef matrix<double, 2, 1> sample_type;
// This is a typedef for the type of kernel we are going to use in this example.
// In this case I have selected the radial basis kernel that can operate on our
// 2D sample_type objects
// This is a typedef for the type of kernel we are going to use in this example. In
// this case I have selected the radial basis kernel that can operate on our 2D
// sample_type objects
typedef radial_basis_kernel<sample_type> kernel_type;
......@@ -47,9 +47,9 @@ int main()
std::vector<sample_type> samples;
std::vector<double> labels;
// Now lets put some data into our samples and labels objects. We do this
// by looping over a bunch of points and labeling them according to their
// distance from the origin.
// Now lets put some data into our samples and labels objects. We do this by looping
// over a bunch of points and labeling them according to their distance from the
// origin.
for (int r = -20; r <= 20; ++r)
{
for (int c = -20; c <= 20; ++c)
......@@ -69,11 +69,11 @@ int main()
}
// Here we normalize all the samples by subtracting their mean and dividing by their standard deviation.
// This is generally a good idea since it often heads off numerical stability problems and also
// prevents one large feature from smothering others. Doing this doesn't matter much in this example
// so I'm just doing this here so you can see an easy way to accomplish this with
// the library.
// Here we normalize all the samples by subtracting their mean and dividing by their
// standard deviation. This is generally a good idea since it often heads off
// numerical stability problems and also prevents one large feature from smothering
// others. Doing this doesn't matter much in this example so I'm just doing this here
// so you can see an easy way to accomplish this with the library.
vector_normalizer<sample_type> normalizer;
// let the normalizer learn the mean and standard deviation of the samples
normalizer.train(samples);
......@@ -82,15 +82,16 @@ int main()
samples[i] = normalizer(samples[i]);
// Now that we have some data we want to train on it. However, there are two parameters to the
// training. These are the nu and gamma parameters. Our choice for these parameters will
// influence how good the resulting decision function is. To test how good a particular choice
// of these parameters is we can use the cross_validate_trainer() function to perform n-fold cross
// validation on our training data. However, there is a problem with the way we have sampled
// our distribution above. The problem is that there is a definite ordering to the samples.
// That is, the first half of the samples look like they are from a different distribution
// than the second half. This would screw up the cross validation process but we can
// fix it by randomizing the order of the samples with the following function call.
// Now that we have some data we want to train on it. However, there are two
// parameters to the training. These are the nu and gamma parameters. Our choice for
// these parameters will influence how good the resulting decision function is. To
// test how good a particular choice of these parameters is we can use the
// cross_validate_trainer() function to perform n-fold cross validation on our training
// data. However, there is a problem with the way we have sampled our distribution
// above. The problem is that there is a definite ordering to the samples. That is,
// the first half of the samples look like they are from a different distribution than
// the second half. This would screw up the cross validation process but we can fix it
// by randomizing the order of the samples with the following function call.
randomize_samples(samples, labels);
......@@ -115,29 +116,31 @@ int main()
trainer.set_nu(nu);
cout << "gamma: " << gamma << " nu: " << nu;
// Print out the cross validation accuracy for 3-fold cross validation using the current gamma and nu.
// cross_validate_trainer() returns a row vector. The first element of the vector is the fraction
// of +1 training examples correctly classified and the second number is the fraction of -1 training
// Print out the cross validation accuracy for 3-fold cross validation using
// the current gamma and nu. cross_validate_trainer() returns a row vector.
// The first element of the vector is the fraction of +1 training examples
// correctly classified and the second number is the fraction of -1 training
// examples correctly classified.
cout << " cross validation accuracy: " << cross_validate_trainer(trainer, samples, labels, 3);
}
}
// From looking at the output of the above loop it turns out that a good value for
// nu and gamma for this problem is 0.15625 for both. So that is what we will use.
// From looking at the output of the above loop it turns out that a good value for nu
// and gamma for this problem is 0.15625 for both. So that is what we will use.
// Now we train on the full set of data and obtain the resulting decision function. We use the
// value of 0.15625 for nu and gamma. The decision function will return values >= 0 for samples it predicts
// are in the +1 class and numbers < 0 for samples it predicts to be in the -1 class.
// Now we train on the full set of data and obtain the resulting decision function. We
// use the value of 0.15625 for nu and gamma. The decision function will return values
// >= 0 for samples it predicts are in the +1 class and numbers < 0 for samples it
// predicts to be in the -1 class.
trainer.set_kernel(kernel_type(0.15625));
trainer.set_nu(0.15625);
typedef decision_function<kernel_type> dec_funct_type;
typedef normalized_function<dec_funct_type> funct_type;
// Here we are making an instance of the normalized_function object. This object provides a convenient
// way to store the vector normalization information along with the decision function we are
// going to learn.
// Here we are making an instance of the normalized_function object. This object
// provides a convenient way to store the vector normalization information along with
// the decision function we are going to learn.
funct_type learned_function;
learned_function.normalizer = normalizer; // save normalization information
learned_function.function = trainer.train(samples, labels); // perform the actual SVM training and save the results
......@@ -200,8 +203,9 @@ int main()
// Another thing that is worth knowing is that just about everything in dlib is serializable.
// So for example, you can save the learned_pfunct object to disk and recall it later like so:
// Another thing that is worth knowing is that just about everything in dlib is
// serializable. So for example, you can save the learned_pfunct object to disk and
// recall it later like so:
ofstream fout("saved_function.dat",ios::binary);
serialize(learned_pfunct,fout);
fout.close();
......@@ -210,27 +214,27 @@ int main()
ifstream fin("saved_function.dat",ios::binary);
deserialize(learned_pfunct, fin);
// Note that there is also an example program that comes with dlib called the file_to_code_ex.cpp
// example. It is a simple program that takes a file and outputs a piece of C++ code
// that is able to fully reproduce the file's contents in the form of a std::string object.
// So you can use that along with the std::istringstream to save learned decision functions
// inside your actual C++ code files if you want.
// Note that there is also an example program that comes with dlib called the
// file_to_code_ex.cpp example. It is a simple program that takes a file and outputs a
// piece of C++ code that is able to fully reproduce the file's contents in the form of
// a std::string object. So you can use that along with the std::istringstream to save
// learned decision functions inside your actual C++ code files if you want.
// Lastly, note that the decision functions we trained above involved well over 200
// basis vectors. Support vector machines in general tend to find decision functions
// that involve a lot of basis vectors. This is significant because the more
// basis vectors in a decision function, the longer it takes to classify new examples.
// So dlib provides the ability to find an approximation to the normal output of a
// trainer using fewer basis vectors.
// Here we determine the cross validation accuracy when we approximate the output
// using only 10 basis vectors. To do this we use the reduced2() function. It
// takes a trainer object and the number of basis vectors to use and returns
// a new trainer object that applies the necessary post processing during the creation
// of decision function objects.
// that involve a lot of basis vectors. This is significant because the more basis
// vectors in a decision function, the longer it takes to classify new examples. So
// dlib provides the ability to find an approximation to the normal output of a trainer
// using fewer basis vectors.
// Here we determine the cross validation accuracy when we approximate the output using
// only 10 basis vectors. To do this we use the reduced2() function. It takes a
// trainer object and the number of basis vectors to use and returns a new trainer
// object that applies the necessary post processing during the creation of decision
// function objects.
cout << "\ncross validation accuracy with only 10 support vectors: "
<< cross_validate_trainer(reduced2(trainer,10), samples, labels, 3);
......@@ -238,9 +242,8 @@ int main()
cout << "cross validation accuracy with all the original support vectors: "
<< cross_validate_trainer(trainer, samples, labels, 3);
// When you run this program you should see that, for this problem, you can reduce
// the number of basis vectors down to 10 without hurting the cross validation
// accuracy.
// When you run this program you should see that, for this problem, you can reduce the
// number of basis vectors down to 10 without hurting the cross validation accuracy.
// To get the reduced decision function out we would just do this:
......
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