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Old 02-25-2018, 10:27 AM
Charles Charles is offline
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Join Date: Feb 2018
Posts: 1
Post *answer* question 13 issue


I am struggling to get the right answer for question 13. I have the algorithm working (in Python). but it's just not good enough to get perfect accuracy (or at least <5%), as every time a few points are not classified correctly. My SVM implementation (copied below) has been working well for other exercises, and I think I added the rbf kernel correctly. Is anyone else having the same issue? I can't figure out what's wrong.

Below is the standalone implementation:
import numpy as np
import math
import cvxopt

class svm():
    ''' Model: support vector machines '''
    ''' Error measure: classification error '''
    ''' Learning algorithm: support vector machines (linearly separable data) '''
    def fit(self, X, y, kernel = 'linear', degree = 2, gamma = 1.):
        ''' returns the alphas '''
        dimension = X.shape[1]
        N = X.shape[0]
        K = np.zeros(shape = (N,N))
        # Computing the inner products (or kernels) for each pair of vectors
        if kernel == 'linear':
            for i in range(N):
                for j in range(N):
                    K[i,j] = np.dot(X[i], X[j].T)
        elif kernel == 'poly':
            for i in range(N):
                for j in range(N):
                    K[i,j] = np.square(1 + np.dot(X[i], X[j].T))
        elif kernel == 'rbf':
            for i in range(N):
                for j in range(N):
                    K[i,j] = np.exp(-gamma * np.linalg.norm(X[i]-X[j]) **2)
        # Generating all the matrices and vectors
        P = cvxopt.matrix(np.outer(y,y) * K, tc='d')
        q = -1. * cvxopt.matrix(np.ones(N), tc='d')
        G = cvxopt.matrix(np.eye(N) * -1, tc='d')
        h = cvxopt.matrix(np.zeros(N), tc='d')
        A = cvxopt.matrix(y, (1,N), tc='d')
        b = cvxopt.matrix(0.0, tc='d')
        solution = cvxopt.solvers.qp(P, q, G, h, A, b)
        a = np.ravel(solution['x'])
        # Create a boolean list of non-zero alphas
        ssv = a > 1e-5
        # Select the corresponding alphas a, support vectors sv and class labels sv_y
        a_small = a[ssv] # alphas
        sv = X[ssv] # support vectors (Xs)
        sv_y = y[ssv] # support vectors (ys)
        # Computing the weights w_svm
        w_svm = np.zeros((1,dimension))
        for each in range(0,len(a_small)):
            w_svm += np.reshape(a_small[each] * sv_y[each] * sv[each], (1,dimension))
        # Computing the intercept b_svm
        b_svm = sv_y[0] - np.dot(w_svm, sv[0].T) 
        # does not matter if divide by sv_y or not
        g = np.sign( np.inner(w_svm,X) + b_svm )

        self.a = a
        self.a_small = a_small
        self.sv = sv
        self.sv_y = sv_y
        self.w = w_svm
        self.b = b_svm
        self.g = g
        return self
    def predict(self, X):
        ''' returns the g as a column vector '''
        self.g = np.sign( np.inner(self.w, X) + self.b )
        return self.g
N = 100
gamma = 1.5
run = 100
Ein = []
sep = []
for r in range(0,run):
    X = np.random.uniform(-1, 1,size = (N,2))
    y = np.sign(X[:,1] - X[:,0] + .25 * np.sin(math.pi * X[:,0]))
    X = np.insert(X, 0, 1, axis=1)
    svm_RBF = svm()
    svm_RBF.fit(X, y, kernel = 'rbf', gamma = 1.5)
    result = svm_RBF.predict(X)

    Ein.append(1 - np.average(np.equal(result, y)))

sep = np.equal(Ein, 0.)
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python, q13, rbf, svm

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