# Passive Aggressive Classifier: Concepts & Examples

The passive aggressive classifier is a machine learning algorithm that is used for classification tasks. This algorithm is a modification of the standard Perceptron algorithm. The passive aggressive classifier was first proposed in 2006 by Crammer et al. as a way to improve the performance of the Perceptron algorithm on linearly separable data sets. In this blog, we will learn about the basic concepts and principles behind the passive aggressive classifier, as well as some examples of its use in real-world applications.

## What is the passive aggressive classifier and how does it work?

The passive aggressive classifier algorithm falls under the category of online learning algorithms, can handle large datasets, and updates its model based on each new instance it encounters.  The passive aggressive algorithm is an online learning algorithm, which means that it can update its weights as new data comes in. The passive aggressive classifier has a parameter, namely, the regularization parameter, C that allows for a tradeoff between the size of the margin and the number of misclassifications. In each iteration, the passive aggressive classifier looks at a new instance, assesses whether it has been correctly classified or not, and then updates its weights accordingly. If the instance is correctly classified, there is no change in weight. However, if it is misclassified, the passive aggressive algorithm adjusts its weights in order to better classify future instances based on this misclassified instance. The degree to which the Passive Aggressive algorithm adjusts its weights is dependent on the regularization parameter C and how confident it is in the classification of that particular instance.

As with other supervised learning algorithms, the passive aggressive classifier works by taking a set of training data and dividing it into two groups: a training set and a test set. The passive aggressive classifier then uses the training set to learn how to correctly classify objects into one of two categories. Once it has learned how to do this, it is then tested on the data in the test set, and its accuracy is measured.

The passive aggressive classifier can be trained using a variety of different loss functions, such as the hinge loss (PA-I) or the squared hinge loss (PA-II). The hinge loss is a linear loss function that is used to minimize the distance between two decision boundaries. This makes it a good choice for situations where you want to classify objects into two categories as accurately as possible. For example, if you are using the passive aggressive classifier to identify cancer cells, you would want to use the hinge loss function so that the boundaries between cancer cells and healthy cells are as distinct as possible. The squared hinge loss is a nonlinear loss function that is used to minimize the distance between two decision boundaries. The squared hinge loss is a variant of the hinge loss that is typically used when the data has a Gaussian distribution. The squared hinge loss is similar to the hinge loss, but it takes into account the variance of the data. This can be helpful when trying to minimize the error in prediction.

## Passive aggressive classifier vs Perceptron

The passive aggressive classifier is very similar to the Perceptron algorithm, with the main difference being that it uses a modified version of Perceptron’s learning rule. Passive aggressive classifiers are similar to the Perceptron in that both of them do not require a learning rate. However, unlike the Perceptron, the learning algorithm of passive aggressive classifier includes a regularization parameter, C. This parameter controls the tradeoff between increasing the margin size and ensuring that the classifier does not misclassify training examples. In addition, Passive aggressive classifiers update their model only when there is a mistake, rather than after every instance like the Perceptron. In real-world applications, Passive aggressive classifiers have been found to perform well in text categorization and collaborative filtering tasks. Overall, Passive aggressive classifiers can be a useful tool in a machine learning practitioner’s toolkit.

## Real-world examples – Passive Aggressive Classifier

The passive aggressive classifier is used to classify data points into two groups. This algorithm can be used in a variety of different settings, including:

• Spam filtering: The passive aggressive classifier can be used to filter spam emails by training the algorithm on a dataset of known spam emails.
• Fraud detection: The passive aggressive classifier can be used to detect fraudulent transactions by training the algorithm on a dataset of known fraud cases.
• Text classification: The passive aggressive classifier can be used to classify text documents into different categories, such as news articles or blog posts.

These are just a few examples of how the passive aggressive algorithm can be used in the real world. There are many other applications for this powerful machine learning algorithm.

## Passive aggressive classifier – Python example

The following is the Python code for training and testing a passive aggressive classifier on the Iris dataset, which contains information on different types of flowers. The Sklearn.linear_model PassiveAggressiveClassifier package is used for the classifier.

import pandas as pd
from sklearn.linear_model import PassiveAggressiveClassifier
from sklearn import datasets
from sklearn.model_selection import train_test_split
from sklearn.preprocessing import StandardScaler
#
#
#
# Create dataframe
#
df = pd.DataFrame({
"sepal_length":iris.data[:, 0],
"sepal_width":iris.data[:, 1],
"petal_length":iris.data[:, 2],
"petal_width": iris.data[:, 3]
})
df["target"] = iris.target
#
# Create train test split
#
X_train, X_test, y_train, y_test = train_test_split(df.iloc[:, 0:4], df.iloc[:, 4], random_state=42, test_size=0.3)
#
# Standardize the training/test data
#
sc = StandardScaler()
X_train_std = sc.fit_transform(X_train)
X_test_std = sc.fit_transform(X_test)
#
# Fit the model
#
clf = PassiveAggressiveClassifier(max_iter=1000, random_state=42)
clf.fit(X_train_std, y_train)
print(clf.score(X_test_std, y_test))


As seen in this code example, the passive aggressive classifier can easily be implemented in Python using Sklearn’s pre-existing packages and functions.

## Conclusion

The passive aggressive classifier is a machine learning algorithm that can be used to classify data points into two groups. This algorithm is based on the Perceptron algorithm, but it includes a regularization parameter, C, that controls the tradeoff between increasing the margin size and ensuring that the classifier does not misclassify training examples. In addition, Passive aggressive classifiers update their model only when there is a mistake, rather than after every instance like the Perceptron. Passive aggressive classifiers have been found to perform well in text categorization and collaborative filtering tasks in real-world applications.