文章目錄
相關文章:
Sklearn 支持向量機
Sklearn.svm 中用於分類的 SVM 方法:
-
svm.LinearSVC: Linear Support Vector Classification.
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svm.NuSVC: Nu-Support Vector Classification.
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svm.OneClassSVM: Unsupervised Outlier Detection.
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svm.SVC: C-Support Vector Classification.
Sklearn.svm 中用於迴歸的 SVM 方法:
-
svm.LinearSVR: Linear Support Vector Regression.
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svm.NuSVR: Nu Support Vector Regression.
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svm.SVR: Epsilon-Support Vector Regression.
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svm.l1_min_c: Return the lowest bound for C such that for C in (l1_min_C, infinity) the model is guaranteed not to be empty.
可以通過 model.support_vectors_ 查看支持向量。
SVM 對特徵的縮放非常敏感,如下圖所示,在左圖中,垂直刻度比水平刻度大得多,因此可能的分離超平面接近於水平。在特徵縮放後(如使用 Sklearn 的 StandardScaler)後(右圖),決策邊界看起來好看很多。
常用參數解釋:
-
:懲罰係數,用於近似線性數據中。在近似線性支持向量機中,損失函數由兩部分組成:最大化支持向量間隔的大小以及 進入分類邊界的數據點的懲罰大小。因此當 越大時,對進入邊界的數據懲罰越大,表現爲進入分類邊界的數據越少(分類間隔越小)。 值的確定與問題有關,如醫療模型或垃圾郵件分類問題。
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:損失函數。線性支持向量機中的目標函數可以分爲兩部分,第一部分爲損失函數,第二部分爲正則化項。默認的損失函數爲合頁損失函數(hinge loss function)
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:非線性支持向量機中的核函數。常用的核函數由:線性核(即變爲線性支持向量機)、多項式核、高斯 RBF 核、Sigmoid 核。
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:高斯核中的參數。, 即正態分佈中圖像的橫向寬度,所以 與 呈反比,當 越大時,正態圖越高瘦; 越小時,正態圖越矮胖。在 SVM 中表現如下:
其截面爲:
因此 gamma 越大,越可能過擬合; gamma 越小,越可能欠擬合。
1. 支持向量機分類
1.1 線性 SVM 分類
參數設置:
C: float, optional (default=1.0)
【懲罰參數,默認爲1,C越大間隔越小,間隔中的實例也越少】
loss: string, ‘hinge’ or ‘squared_hinge’ (default=’squared_hinge’)
【loss 參數應設爲 ‘hinge’ ,因爲它不是默認值】
dual bool, (default=True)
【默認 True除非特徵數量比訓練實例還多,否則應設爲 False】
其他參數見官方文檔。
LinearSVC 類會對偏執項進行正則化,所以需要先減去平均值,使訓練集集中。如果使用 StandardScaler 會自動進行這一步。
LinearSVC() 相當於 SVC(kernel=’linear’) ,但這要慢得多。
import numpy as np
from sklearn import datasets
from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.svm import LinearSVC
iris = datasets.load_iris()
X = iris["data"][:, (2, 3)] # petal length, petal width
y = (iris["target"] == 2).astype(np.float64) # Iris-Virginica
svm_clf = Pipeline([
("scaler", StandardScaler()),
("linear_svc", LinearSVC(C=1, loss="hinge", random_state=42)),
])
svm_clf.fit(X, y)
Pipeline(memory=None,
steps=[('scaler',
StandardScaler(copy=True, with_mean=True, with_std=True)),
('linear_svc',
LinearSVC(C=1, class_weight=None, dual=True,
fit_intercept=True, intercept_scaling=1,
loss='hinge', max_iter=1000, multi_class='ovr',
penalty='l2', random_state=42, tol=0.0001,
verbose=0))],
verbose=False)
svm_clf.predict([[5.5, 1.7]])
array([1.])
與 Logistic 迴歸分類器不同的是,SVM 分類器不會輸出每個類別的概率。
1.2 非線性 SVM 分類
雖然在許多情況下,線性 SVM 分類器是有效的,並且通常出人意料的好,但是,有很多數據集是非線性可分的。因此需要非線性支持向量機將數據變成線性可分的,如下圖所示,利用多項式對數據進行變換:
要使用 Sklearn 實現這個想法,有兩種方法:第一種是首先使用多項式變換並對特徵進行縮放,接着就可以返回線性 linear_svc 分類器了;第二種是直接使用 SVC 分類器並選定多項式內核。
我們首先來看第一種,使用衛星數據來進行測試一下:
from sklearn.datasets import make_moons
import matplotlib.pyplot as plt
X, y = make_moons(n_samples=100, noise=0.15, random_state=42)
def plot_dataset(X, y, axes):
plt.plot(X[:, 0][y==0], X[:, 1][y==0], "bs")
plt.plot(X[:, 0][y==1], X[:, 1][y==1], "g^")
plt.axis(axes)
plt.grid(True, which='both')
plt.xlabel(r"$x_1$", fontsize=20)
plt.ylabel(r"$x_2$", fontsize=20, rotation=0)
plot_dataset(X, y, [-1.5, 2.5, -1, 1.5])
plt.show()
from sklearn.datasets import make_moons
from sklearn.pipeline import Pipeline
from sklearn.preprocessing import PolynomialFeatures
polynomial_svm_clf = Pipeline([
("poly_features", PolynomialFeatures(degree=3)),
("scaler", StandardScaler()),
("svm_clf", LinearSVC(C=10, loss="hinge", random_state=42))
])
polynomial_svm_clf.fit(X, y)
Pipeline(memory=None,
steps=[('poly_features',
PolynomialFeatures(degree=3, include_bias=True,
interaction_only=False, order='C')),
('scaler',
StandardScaler(copy=True, with_mean=True, with_std=True)),
('svm_clf',
LinearSVC(C=10, class_weight=None, dual=True,
fit_intercept=True, intercept_scaling=1,
loss='hinge', max_iter=1000, multi_class='ovr',
penalty='l2', random_state=42, tol=0.0001,
verbose=0))],
verbose=False)
def plot_predictions(clf, axes):
x0s = np.linspace(axes[0], axes[1], 100)
x1s = np.linspace(axes[2], axes[3], 100)
x0, x1 = np.meshgrid(x0s, x1s)
X = np.c_[x0.ravel(), x1.ravel()]
y_pred = clf.predict(X).reshape(x0.shape)
y_decision = clf.decision_function(X).reshape(x0.shape)
plt.contourf(x0, x1, y_pred, cmap=plt.cm.brg, alpha=0.2)
plt.contourf(x0, x1, y_decision, cmap=plt.cm.brg, alpha=0.1)
plot_predictions(polynomial_svm_clf, [-1.5, 2.5, -1, 1.5])
plot_dataset(X, y, [-1.5, 2.5, -1, 1.5])
plt.show()
另外一種方法是使用 SVC 函數實現。
參數設置:
C: float, optional (default=1.0)
Penalty parameter C of the error term.
kernel: string, optional (default=’rbf’)
Specifies the kernel type to be used in the algorithm. It must be one of ‘linear’, ‘poly’, ‘rbf’, ‘sigmoid’, ‘precomputed’ or a callable. If none is given, ‘rbf’ will be used. If a callable is given it is used to pre-compute the kernel matrix from data matrices; that matrix should be an array of shape (n_samples, n_samples).
degree: int, optional (default=3)
Degree of the polynomial kernel function (‘poly’). Ignored by all other kernels.
gamma: {‘scale’, ‘auto’} or float, optional (default=’scale’)
Kernel coefficient for ‘rbf’, ‘poly’ and ‘sigmoid’.
if gamma='scale' (default) is passed then it uses 1 / (n_features * X.var()) as value of gamma,
if ‘auto’, uses 1 / n_features. Changed in version 0.22: The default value of gamma changed from ‘auto’ to ‘scale’.
coef0: float, optional (default=0.0)
Independent term in kernel function. It is only significant in ‘poly’ and ‘sigmoid’.
【控制模型受高階多項式還是低階多項式影響的程度】
其他參數設置見官方文檔。
尋找正確的超參數值的常用方法是網絡搜索。先進行一次粗略的網絡搜索,然後在最好的值附近展開一輪更精細的網絡搜索,這樣通常會快一些。
1.2.1 多項式內核
使用 SVC(kernel=“poly”, degree=3) 進行非線性多項式內核的 SVM 分類:
from sklearn.svm import SVC
from sklearn.datasets import make_moons
import matplotlib.pyplot as plt
X, y = make_moons(n_samples=100, noise=0.15, random_state=42)
poly_kernel_svm_clf = Pipeline([
("scaler", StandardScaler()),
("svm_clf", SVC(kernel="poly", degree=3, coef0=1, C=5))
])
poly_kernel_svm_clf.fit(X, y)
Pipeline(memory=None,
steps=[('scaler',
StandardScaler(copy=True, with_mean=True, with_std=True)),
('svm_clf',
SVC(C=5, cache_size=200, class_weight=None, coef0=1,
decision_function_shape='ovr', degree=3,
gamma='auto_deprecated', kernel='poly', max_iter=-1,
probability=False, random_state=None, shrinking=True,
tol=0.001, verbose=False))],
verbose=False)
poly100_kernel_svm_clf = Pipeline([
("scaler", StandardScaler()),
("svm_clf", SVC(kernel="poly", degree=10, coef0=100, C=5))
])
poly100_kernel_svm_clf.fit(X, y)
Pipeline(memory=None,
steps=[('scaler',
StandardScaler(copy=True, with_mean=True, with_std=True)),
('svm_clf',
SVC(C=5, cache_size=200, class_weight=None, coef0=100,
decision_function_shape='ovr', degree=10,
gamma='auto_deprecated', kernel='poly', max_iter=-1,
probability=False, random_state=None, shrinking=True,
tol=0.001, verbose=False))],
verbose=False)
plt.figure(figsize=(11, 4))
plt.subplot(121)
plot_predictions(poly_kernel_svm_clf, [-1.5, 2.5, -1, 1.5])
plot_dataset(X, y, [-1.5, 2.5, -1, 1.5])
plt.title(r"$d=3, r=1, C=5$", fontsize=18)
plt.subplot(122)
plot_predictions(poly100_kernel_svm_clf, [-1.5, 2.5, -1, 1.5])
plot_dataset(X, y, [-1.5, 2.5, -1, 1.5])
plt.title(r"$d=10, r=100, C=5$", fontsize=18)
plt.show()
1.2.2 高斯 RBF 內核
使用 SVC(kernel=‘rbf’, gamma=5, C=0.001) 對非線性數據進行分類:
from sklearn.datasets import make_moons
import matplotlib.pyplot as plt
X, y = make_moons(n_samples=100, noise=0.15, random_state=42)
rbf_kernel_svm_clf = Pipeline([
("scaler", StandardScaler()),
("svm_clf", SVC(kernel="rbf", gamma=5, C=0.001))
])
rbf_kernel_svm_clf.fit(X, y)
Pipeline(memory=None,
steps=[('scaler',
StandardScaler(copy=True, with_mean=True, with_std=True)),
('svm_clf',
SVC(C=0.001, cache_size=200, class_weight=None, coef0=0.0,
decision_function_shape='ovr', degree=3, gamma=5,
kernel='rbf', max_iter=-1, probability=False,
random_state=None, shrinking=True, tol=0.001,
verbose=False))],
verbose=False)
實現簡單的網絡搜索:
from sklearn.svm import SVC
gamma1, gamma2 = 0.1, 5
C1, C2 = 0.001, 1000
hyperparams = (gamma1, C1), (gamma1, C2), (gamma2, C1), (gamma2, C2)
svm_clfs = []
for gamma, C in hyperparams:
rbf_kernel_svm_clf = Pipeline([
("scaler", StandardScaler()),
("svm_clf", SVC(kernel="rbf", gamma=gamma, C=C))
])
rbf_kernel_svm_clf.fit(X, y)
svm_clfs.append(rbf_kernel_svm_clf)
plt.figure(figsize=(11, 7))
for i, svm_clf in enumerate(svm_clfs):
plt.subplot(221 + i)
plot_predictions(svm_clf, [-1.5, 2.5, -1, 1.5])
plot_dataset(X, y, [-1.5, 2.5, -1, 1.5])
gamma, C = hyperparams[i]
plt.title(r"$\gamma = {}, C = {}$".format(gamma, C), fontsize=16)
plt.show()
2. 支持向量機迴歸
SVM 算法非常全面:它不僅支持線性和非線性分類,而且還支持線性和非線性迴歸。訣竅在於將目標反轉一下:不再是嘗試擬合最大分離間隔,SVM 迴歸要做的是讓儘可能多的實例位於間隔中間,同時限制間隔違例。間隔的寬度受超參數 控制。
2.1 線性 SVM 迴歸
sklearn.svm.LinearSVR (訓練數據需要先縮放並集中)
參數設置:
epsilon: float, optional (default=0.0)
Epsilon parameter in the epsilon-insensitive loss function. Note that the value of this parameter depends on the scale of the target variable y. If unsure, set epsilon=0.
【間隔寬度】
tol: float, optional (default=1e-4)
Tolerance for stopping criteria.
C: float, optional (default=1.0)
Penalty parameter C of the error term. The penalty is a squared l2 penalty. The bigger this parameter, the less regularization is used.
loss: string, optional (default=’epsilon_insensitive’)
Specifies the loss function. The epsilon-insensitive loss (standard SVR) is the L1 loss, while the squared epsilon-insensitive loss (‘squared_epsilon_insensitive’) is the L2 loss.
dual: bool, (default=True)
Select the algorithm to either solve the dual or primal optimization problem. Prefer dual=False when n_samples > n_features.
from sklearn.svm import LinearSVR
linear_svm_reg = Pipeline([
("scaler", StandardScaler()),
("svm_reg", LinearSVR(epsilon=1.5))
])
linear_svm_reg.fit(X, y)
下圖顯示了用隨機線性數據訓練的兩個線性 SVM迴歸模型,一個間隔較大( ),一個間隔較小( )(訓練數據需要先縮放並集中)。
繪圖代碼:
np.random.seed(42)
m = 50
X = 2 * np.random.rand(m, 1)
y = (4 + 3 * X + np.random.randn(m, 1)).ravel()
from sklearn.svm import LinearSVR
svm_reg = LinearSVR(epsilon=1.5, random_state=42)
svm_reg.fit(X, y)
LinearSVR(C=1.0, dual=True, epsilon=1.5, fit_intercept=True,
intercept_scaling=1.0, loss='epsilon_insensitive', max_iter=1000,
random_state=42, tol=0.0001, verbose=0)
svm_reg1 = LinearSVR(epsilon=1.5, random_state=42)
svm_reg2 = LinearSVR(epsilon=0.5, random_state=42)
svm_reg1.fit(X, y)
svm_reg2.fit(X, y)
def find_support_vectors(svm_reg, X, y):
y_pred = svm_reg.predict(X)
off_margin = (np.abs(y - y_pred) >= svm_reg.epsilon)
return np.argwhere(off_margin)
svm_reg1.support_ = find_support_vectors(svm_reg1, X, y)
svm_reg2.support_ = find_support_vectors(svm_reg2, X, y)
eps_x1 = 1
eps_y_pred = svm_reg1.predict([[eps_x1]])
def plot_svm_regression(svm_reg, X, y, axes):
x1s = np.linspace(axes[0], axes[1], 100).reshape(100, 1)
y_pred = svm_reg.predict(x1s)
plt.plot(x1s, y_pred, "k-", linewidth=2, label=r"$\hat{y}$")
plt.plot(x1s, y_pred + svm_reg.epsilon, "k--")
plt.plot(x1s, y_pred - svm_reg.epsilon, "k--")
plt.scatter(X[svm_reg.support_], y[svm_reg.support_], s=180, facecolors='#FFAAAA')
plt.plot(X, y, "bo")
plt.xlabel(r"$x_1$", fontsize=18)
plt.legend(loc="upper left", fontsize=18)
plt.axis(axes)
plt.figure(figsize=(9, 4))
plt.subplot(121)
plot_svm_regression(svm_reg1, X, y, [0, 2, 3, 11])
plt.title(r"$\epsilon = {}$".format(svm_reg1.epsilon), fontsize=18)
plt.ylabel(r"$y$", fontsize=18, rotation=0)
#plt.plot([eps_x1, eps_x1], [eps_y_pred, eps_y_pred - svm_reg1.epsilon], "k-", linewidth=2)
plt.annotate(
'', xy=(eps_x1, eps_y_pred), xycoords='data',
xytext=(eps_x1, eps_y_pred - svm_reg1.epsilon),
textcoords='data', arrowprops={'arrowstyle': '<->', 'linewidth': 1.5}
)
plt.text(0.91, 5.6, r"$\epsilon$", fontsize=20)
plt.subplot(122)
plot_svm_regression(svm_reg2, X, y, [0, 2, 3, 11])
plt.title(r"$\epsilon = {}$".format(svm_reg2.epsilon), fontsize=18)
plt.show()
2.2 非線性 SVM 迴歸
參數設置:
kernel: string, optional (default=’rbf’)
Specifies the kernel type to be used in the algorithm. It must be one of ‘linear’, ‘poly’, ‘rbf’, ‘sigmoid’, ‘precomputed’ or a callable. If none is given, ‘rbf’ will be used. If a callable is given it is used to precompute the kernel matrix.
degree: int, optional (default=3)
Degree of the polynomial kernel function (‘poly’). Ignored by all other kernels.
gamma: {‘scale’, ‘auto’} or float, optional
(default=’scale’)
Kernel coefficient for ‘rbf’, ‘poly’ and ‘sigmoid’.
if gamma='scale' (default) is passed then it uses 1 / (n_features * X.var()) as value of gamma,
if ‘auto’, uses 1 / n_features.
Changed in version 0.22: The default value of gamma changed from ‘auto’ to ‘scale’.
coef0: float, optional (default=0.0)
Independent term in kernel function. It is only significant in ‘poly’ and ‘sigmoid’.
tol: float, optional (default=1e-3)
Tolerance for stopping criterion.
C: float, optional (default=1.0)
Penalty parameter C of the error term.
epsilon: float, optional (default=0.1)
Epsilon in the epsilon-SVR model. It specifies the epsilon-tube within which no penalty is associated in the training loss function with points predicted within a distance epsilon from the actual value.
【它指定了epsilon-tube,其中訓練損失函數中沒有懲罰與在實際值的距離epsilon內預測的點。】
2.2.1 多項式內核
from sklearn.svm import SVR
svm_poly_reg = SVR(kernel="poly", degree=2, C=100, epsilon=0.1, gamma="auto")
svm_poly_reg.fit(X, y)
下面展示了不同懲罰係數(C)下的 SVM 迴歸:
代碼如下:
np.random.seed(42)
m = 100
X = 2 * np.random.rand(m, 1) - 1
y = (0.2 + 0.1 * X + 0.5 * X**2 + np.random.randn(m, 1)/10).ravel()
設置不同的正則化值(C 值)
from sklearn.svm import SVR
svm_poly_reg1 = SVR(kernel="poly", degree=2, C=100, epsilon=0.1, gamma="auto")
svm_poly_reg2 = SVR(kernel="poly", degree=2, C=0.01, epsilon=0.1, gamma="auto")
svm_poly_reg1.fit(X, y)
svm_poly_reg2.fit(X, y)
SVR(C=0.01, cache_size=200, coef0=0.0, degree=2, epsilon=0.1, gamma='auto',
kernel='poly', max_iter=-1, shrinking=True, tol=0.001, verbose=False)
import matplotlib.pyplot as plt
def plot_svm_regression(svm_reg, X, y, axes):
x1s = np.linspace(axes[0], axes[1], 100).reshape(100, 1)
y_pred = svm_reg.predict(x1s)
plt.plot(x1s, y_pred, "k-", linewidth=2, label=r"$\hat{y}$")
plt.plot(x1s, y_pred + svm_reg.epsilon, "k--")
plt.plot(x1s, y_pred - svm_reg.epsilon, "k--")
plt.scatter(X[svm_reg.support_], y[svm_reg.support_], s=180, facecolors='#FFAAAA')
plt.plot(X, y, "bo")
plt.xlabel(r"$x_1$", fontsize=18)
plt.legend(loc="upper left", fontsize=18)
plt.axis(axes)
plt.figure(figsize=(9, 4))
plt.subplot(121)
plot_svm_regression(svm_poly_reg1, X, y, [-1, 1, 0, 1])
plt.title(r"$degree={}, C={}, \epsilon = {}$".format(svm_poly_reg1.degree, svm_poly_reg1.C, svm_poly_reg1.epsilon), fontsize=18)
plt.ylabel(r"$y$", fontsize=18, rotation=0)
plt.subplot(122)
plot_svm_regression(svm_poly_reg2, X, y, [-1, 1, 0, 1])
plt.title(r"$degree={}, C={}, \epsilon = {}$".format(svm_poly_reg2.degree, svm_poly_reg2.C, svm_poly_reg2.epsilon), fontsize=18)
plt.show()
參考資料
[1] Aurelien Geron, 王靜源, 賈瑋, 邊蕤, 邱俊濤. 機器學習實戰:基於 Scikit-Learn 和 TensorFlow[M]. 北京: 機械工業出版社, 2018: 136-144.