# Implementing SVM from scratch?

I am trying to implement the rbf kernel for SVM from scratch as practice for my coming interviews. I attempted to use cvxopt to solve the optimization problem. However, when I compute the accuracy and compare it to the actual SVM library on sklearn, there is an extremely large discrepancy. I have attempted to isolate the problem but I cannot seem to fix it. Any help would be greatly appreciated. Posted below is the code. If anyone could please let me know what I am doing wrong or help suggest an alternative approach I would greatly appreciate it.

import numpy as np
import cvxopt

def rbf_kernel(gamma, **kwargs):
def f(x1, x2):
distance = np.linalg.norm(x1 - x2) ** 2
return np.exp(-gamma * distance)
return f

class SupportVectorMachine(object):
def __init__(self, C=1, kernel=rbf_kernel, power=4, gamma=None, coef=4):
self.C = C
self.kernel = kernel
self.power = power
self.gamma = gamma
self.coef = coef
self.lagr_multipliers = None
self.support_vectors = None
self.support_vector_labels = None
self.intercept = None

def fit(self, X, y):

n_samples, n_features = np.shape(X)

# Set gamma to 1/n_features by default
if not self.gamma:
self.gamma = 1 / n_features

# Initialize kernel method with parameters
self.kernel = self.kernel(
power=self.power,
gamma=self.gamma,
coef=self.coef)

# Calculate kernel matrix
kernel_matrix = np.zeros((n_samples, n_samples))
for i in range(n_samples):
for j in range(n_samples):
kernel_matrix[i, j] = self.kernel(X[i], X[j])

# Define the quadratic optimization problem
P = cvxopt.matrix(np.outer(y, y) * kernel_matrix, tc='d')
q = cvxopt.matrix(np.ones(n_samples) * -1)
A = cvxopt.matrix(y, (1, n_samples), tc='d')
b = cvxopt.matrix(0, tc='d')

if not self.C: #if its empty
G = cvxopt.matrix(np.identity(n_samples) * -1)
h = cvxopt.matrix(np.zeros(n_samples))
else:
G_max = np.identity(n_samples) * -1
G_min = np.identity(n_samples)
G = cvxopt.matrix(np.vstack((G_max, G_min)))
h_max = cvxopt.matrix(np.zeros(n_samples))
h_min = cvxopt.matrix(np.ones(n_samples) * self.C)
h = cvxopt.matrix(np.vstack((h_max, h_min)))

# Solve the quadratic optimization problem using cvxopt
minimization = cvxopt.solvers.qp(P, q, G, h, A, b)

# Lagrange multipliers
lagr_mult = np.ravel(minimization['x'])

# Extract support vectors
# Get indexes of non-zero lagr. multipiers
idx = lagr_mult > 1e-11
# Get the corresponding lagr. multipliers
self.lagr_multipliers = lagr_mult[idx]
# Get the samples that will act as support vectors
self.support_vectors = X[idx]
# Get the corresponding labels
self.support_vector_labels = y[idx]

# Calculate intercept with first support vector
self.intercept = self.support_vector_labels
for i in range(len(self.lagr_multipliers)):
self.intercept -= self.lagr_multipliers[i] * self.support_vector_labels[
i] * self.kernel(self.support_vectors[i], self.support_vectors)

def predict(self, X):
y_pred = []
# Iterate through list of samples and make predictions
for sample in X:
prediction = 0
# Determine the label of the sample by the support vectors
for i in range(len(self.lagr_multipliers)):
prediction += self.lagr_multipliers[i] * self.support_vector_labels[
i] * self.kernel(self.support_vectors[i], sample)
prediction += self.intercept
y_pred.append(np.sign(prediction))
return np.array(y_pred)

def main():
print ("-- SVM Classifier --")

# previous error
#X = normalize(data.data)
#y = data.target

# correct version
X = normalize(data.data[data.target != 0])
y = data.target[data.target != 0]
y[y == 1] = -1
y[y == 2] = 1

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.4)
clf = SupportVectorMachine(kernel=rbf_kernel, gamma = 1)
clf.fit(X_train, y_train)
y_pred = clf.predict(X_test)
accuracy = accuracy_score(y_test, y_pred)
print ("Accuracy (scratch):", accuracy)

clf_sklearn = SVC(gamma = 'auto')
clf_sklearn.fit(X_train, y_train)
y_pred2 = clf_sklearn.predict(X_test)
accuracy = accuracy_score(y_test, y_pred2)
print ("Accuracy :", accuracy)

if __name__ == "__main__":
main()


RESULTS: Accuracy (from scratch): 0.31666666666666665 Accuracy (using SVM Library) : 1.0

Note, I did not add the libraries to save space

• Could you please post the hyperparameters after you fitted the model, both from sklearn and your own model? – Pedro Henrique Monforte Mar 25 '19 at 18:51