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Mastering Machine Learning with scikit-learn

Mastering Machine Learning with scikit-learn

By : Gavin Hackeling
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Mastering Machine Learning with scikit-learn

Mastering Machine Learning with scikit-learn

5 (2)
By: Gavin Hackeling

Overview of this book

Machine learning is the buzzword bringing computer science and statistics together to build smart and efficient models. Using powerful algorithms and techniques offered by machine learning you can automate any analytical model. This book examines a variety of machine learning models including popular machine learning algorithms such as k-nearest neighbors, logistic regression, naive Bayes, k-means, decision trees, and artificial neural networks. It discusses data preprocessing, hyperparameter optimization, and ensemble methods. You will build systems that classify documents, recognize images, detect ads, and more. You will learn to use scikit-learn’s API to extract features from categorical variables, text and images; evaluate model performance, and develop an intuition for how to improve your model’s performance. By the end of this book, you will master all required concepts of scikit-learn to build efficient models at work to carry out advanced tasks with the practical approach.
Table of Contents (15 chapters)
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9
From Decision Trees to Random Forests and Other Ensemble Methods

Extracting features from categorical variables


Many problems have explanatory variables that are categorical or nominal. A categorical variable can take one of a fixed set of values. For example, an application that predicts the salary for a job might use categorical variables such as the city in which the position is located. Categorical variables are commonly encoded using one-of-k encoding, or one-hotencoding, in which the explanatory variable is represented using one binary feature for each of its possible values.

For example, let's assume our model has a city variable that can take one of three values: New York, San Francisco, or Chapel Hill. One-hot encoding represents the variable using one binary feature for each of the three possible cities. scikit-learn's DictVectorizer class is a transformer that can be used to one-hot encode categorical features:

# In[1]:
from sklearn.feature_extraction import DictVectorizer
onehot_encoder = DictVectorizer()
X = [
    {'city': 'New York'},
   ...

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