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I'm going through chapter 5 (SVM) in Geron's book "Hands on Machine Learning" (ipynb link).

The task is to classify the Iris Dataset. The code:

import numpy as np
import matplotlib.pyplot as plt
from sklearn import datasets
from sklearn.linear_model import SGDClassifier
from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.svm import LinearSVC, SVC

iris = datasets.load_iris()

# The petal len + width:
X = iris["data"][:, (2, 3)]
y = iris["target"].astype(np.float64)

svm_clf = Pipeline([
    ("scaler", StandardScaler()),
    # ("linear_svc", SVC(1, "linear"))
    ("linear_svc", LinearSVC(C=1, loss="hinge"))
    # ("linear_svc", SGDClassifier(alpha=0.001, loss="hinge"))
])

svm_clf.fit(X, y)
x0, x1 = np.meshgrid(
    np.linspace(0, 8, 200).reshape(-1, 1),
    np.linspace(0, 3, 200).reshape(-1, 1)
)

X_new = np.c_[x0.ravel(), x1.ravel()]
y_predict = svm_clf.predict(X_new)
zz = y_predict.reshape(x0.shape)

plt.contourf(x0, x1, zz)

plt.plot(X[y == 2, 0], X[y == 2, 1], "g^", label="Iris-Virginica")
plt.plot(X[y == 1, 0], X[y == 1, 1], "bs", label="Iris-Versicolor")
plt.plot(X[y == 0, 0], X[y == 0, 1], "yo", label="Iris-Setosa")
plt.show()

Produces:

enter image description here

My question: I expected the yellow and blue dots to be separated better. I thought the SVM classifier would put the boundary right in the middle between the two clusters.

When I use the SVC classifier instead of LinearSVC (commented in the above code). I get what I expected:

enter image description here

So: How come the boundary is so bad in the LinearSVC classifier?

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1 Answer 1

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You should unscale the coefficients of the decision boundary if you want to plot it. check this: https://github.com/ageron/handson-ml/issues/250

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