# Non-Linear Classification in R

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Statistical programming languages such as R have become staples in the realm of data analysis, given their capabilities for statistical modeling, machine learning, and data visualization. Classification is one of the most common tasks in machine learning, where we categorize data into predefined classes or labels. When the relationship between the predictor variables and the output variable is not linear but rather more complex, we employ non-linear classification techniques. This article will dive into an overview of non-linear classification in R, touching on methods such as decision trees, random forests, and support vector machines.

### Understanding Non-Linear Classification

To understand non-linear classification, it’s first important to understand its counterpart – linear classification. In linear classification, we use a linear equation to segregate the different classes in the feature space. The simplest example of linear classification is logistic regression, where a linear decision boundary separates the classes.

However, real-world data is often messy and complex. The relationship between predictors and the output variable may not be simple and straight-line based. Instead, these relationships can be highly complex, requiring curves, multiple lines, or multi-dimensional surfaces to properly segregate the classes. Non-linear classification models are capable of handling such complexities.

### Decision Trees

Decision trees are one of the most intuitive and popular non-linear classification methods. A decision tree uses a tree-like model of decisions and their possible outcomes. It breaks down a dataset into smaller and smaller subsets based on distinct decisions (i.e., questions about the features of the data), forming a tree with decision nodes and leaf nodes.

The rpart package in R can be used for creating decision trees.

# install the package
install.packages('rpart')

# load the package
library(rpart)

# fit the model
fit <- rpart(Class ~ ., data = your_data, method = "class")

# print the model
printcp(fit)

# plot tree
plot(fit, uniform = TRUE, main = "Classification Tree")
text(fit, use.n = TRUE, all = TRUE, cex = .8)

In this code, we’re using the rpart() function from the rpart package to build a classification tree.

### Random Forests

While decision trees are intuitive and easy to interpret, they tend to overfit the training data and have high variance. To overcome this, we can use a Random Forest, an ensemble learning method that operates by constructing multiple decision trees and outputting the class that is the mode of the classes output by individual trees.

Random forests are implemented in R with the randomForest package.

# install the package
install.packages('randomForest')

# load the package
library(randomForest)

# fit the model
fit <- randomForest(Class ~ ., data = your_data, ntree = 500, mtry = 2)

# print the model
print(fit)

# Importance of each predictor
importance(fit)

In this code, we’re using the randomForest() function from the randomForest package to build a random forest classifier.

### Support Vector Machines

Support Vector Machines (SVM) are another powerful method for non-linear classification. SVMs can efficiently perform linear classification, but they can also perform non-linear classification by implicitly mapping inputs into high-dimensional feature spaces.

The e1071 package in R implements the SVM.

# install the package
install.packages('e1071')

# load the package
library(e1071)

# fit the model
fit <- svm(Class ~ ., data = your_data, kernel = "radial")

# print the model
print(fit)

In this code, we’re using the svm() function from the e1071 package to build a SVM classifier. The kernel argument is used to specify the type of SVM. Here we use a radial basis function kernel for non-linear classification.

### Conclusion

Non-linear classification techniques play a crucial role in machine learning and data analysis. In real-world scenarios, data often does not follow a linear trend and requires flexible models that can capture complex patterns. R, with its extensive set of packages and in-built functions, is a powerful tool for implementing these non-linear classification techniques. Whether it be decision trees, random forests, or support vector machines, each method offers its unique capabilities in capturing non-linear patterns and should be chosen based on the specific task at hand. It’s important to understand these techniques and know how to implement them, as they are indispensable tools in the toolbox of every data scientist.