Created
November 26, 2013 09:01
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class SVMTrainer(object): | |
def __init__(self, kernel, c): | |
self._kernel = kernel | |
self._c = c | |
def train(self, X, y): | |
"""Given the training features X with labels y, returns a SVM | |
predictor representing the trained SVM. | |
""" | |
lagrange_multipliers = self._compute_multipliers(X, y) | |
return self._construct_predictor(X, y, lagrange_multipliers) | |
def _gram_matrix(self, X): | |
n_samples, n_features = X.shape | |
K = np.zeros((n_samples, n_samples)) | |
# TODO(tulloch) - vectorize | |
for i, x_i in enumerate(X): | |
for j, x_j in enumerate(X): | |
K[i, j] = self._kernel(x_i, x_j) | |
return K | |
def _construct_predictor(self, X, y, lagrange_multipliers): | |
support_vector_indices = \ | |
lagrange_multipliers > MIN_SUPPORT_VECTOR_MULTIPLIER | |
support_multipliers = lagrange_multipliers[support_vector_indices] | |
support_vectors = X[support_vector_indices] | |
support_vector_labels = y[support_vector_indices] | |
# http://www.cs.cmu.edu/~guestrin/Class/10701-S07/Slides/kernels.pdf | |
# bias = y_k - \sum z_i y_i K(x_k, x_i) | |
# Thus we can just predict an example with bias of zero, and | |
# compute error. | |
bias = np.mean( | |
[y_k - SVMPredictor( | |
kernel=self._kernel, | |
bias=0.0, | |
weights=support_multipliers, | |
support_vectors=support_vectors, | |
support_vector_labels=support_vector_labels).predict(x_k) | |
for (y_k, x_k) in zip(support_vector_labels, support_vectors)]) | |
return SVMPredictor( | |
kernel=self._kernel, | |
bias=bias, | |
weights=support_multipliers, | |
support_vectors=support_vectors, | |
support_vector_labels=support_vector_labels) | |
def _compute_multipliers(self, X, y): | |
n_samples, n_features = X.shape | |
K = self._gram_matrix(X) | |
# Solves | |
# min 1/2 x^T P x + q^T x | |
# s.t. | |
# Gx \coneleq h | |
# Ax = b | |
P = cvxopt.matrix(np.outer(y, y) * K) | |
q = cvxopt.matrix(-1 * np.ones(n_samples)) | |
# -a_i \leq 0 | |
# TODO(tulloch) - modify G, h so that we have a soft-margin classifier | |
G_std = cvxopt.matrix(np.diag(np.ones(n_samples) * -1)) | |
h_std = cvxopt.matrix(np.zeros(n_samples)) | |
# a_i \leq c | |
G_slack = cvxopt.matrix(np.diag(np.ones(n_samples))) | |
h_slack = cvxopt.matrix(np.ones(n_samples) * self._c) | |
G = cvxopt.matrix(np.vstack((G_std, G_slack))) | |
h = cvxopt.matrix(np.vstack((h_std, h_slack))) | |
A = cvxopt.matrix(y, (1, n_samples)) | |
b = cvxopt.matrix(0.0) | |
solution = cvxopt.solvers.qp(P, q, G, h, A, b) | |
# Lagrange multipliers | |
return np.ravel(solution['x']) |
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