Machine Learning with R: R gives you access to the cutting-edge software you need to prepare data for machine learning. No previous knowledge required – this book will take you methodically through every stage of applying machine learning.

There are numerous types of data structures across programming languages, each with strengths and weaknesses specific to particular tasks. Since R is a programming language used widely for statistical data analysis, the data structures it utilizes are designed to make it easy to manipulate data for this type of work. The R data structures used most frequently in machine learning are vectors, factors, lists, arrays, and data frames. Each of these data types is specialized for a specific data management task, which makes it important to understand how they will interact in your R project.

The fundamental R data structure is the vector, which stores an ordered set of values called elements. A vector can contain any number of elements. However, all the elements must be of the same type; for instance, a vector cannot contain both numbers and text.

There are several vector types commonly used in machine learning: integer (numbers without decimals), numeric (numbers with decimals), character (text data), or logical (TRUE or FALSE values). There are also two special values: NULL, which is used to indicate the absence of any value, and NA, which indicates a missing value.

It is tedious to enter large amounts of data manually, but simple vectors can be created by using the combine function c(). The vector can also be given a name using the arrow <- operator, which is R's assignment operator, used in a similar way to the = assignment operator in many other programming languages.

For example, let's construct a set of vectors containing data on three medical patients. We'll...

If you recall from Chapter 1, Introducing Machine Learning, features that represent a characteristic with categories of values are known as nominal. Although it is possible to use a character vector to store nominal data, R provides a data structure known as a factor specifically for this purpose. A factor is a special case of vector that is solely used for representing nominal variables. In the medical dataset we are building, we might use a factor to represent gender, because it uses two categories: MALE and FEMALE.

Why not use character vectors? An advantage of using factors is that they are generally more efficient than character vectors because the category labels are stored only once. Rather than storing MALE, MALE, FEMALE, the computer may store 1, 1, 2. This can save memory. Additionally, certain machine learning algorithms use special routines to handle categorical variables. Coding categorical variables as factors ensures that the model will treat this data appropriately...

One of the challenges faced when working with massive datasets involves gathering, preparing, and otherwise managing data from a variety of sources. This task is facilitated by R's tools for loading data from many common formats.

> save(x, y, z, file = "mydata.RData")

After collecting data and loading it into R data structures, the next step in the machine learning process involves examining the data in detail. It is during this step that you will begin to explore the data's features and examples, and realize the peculiarities that make your data unique. The better you understand your data, the better you will be able to match a machine learning model to your learning problem.

The best way to understand the process of data exploration is by example. In this section, we will explore the usedcars.csv dataset, which contains actual data about used cars recently advertised for sale on a popular U.S. website.

Since the dataset is stored in CSV form, we can use the read.csv() function to load the data into an R data frame:

usedcars...

In this chapter, we learned about the basics of managing data in R. We started by taking an in-depth look at the structures used for storing various types of data. The foundational R data structure is the vector, which is extended and combined into more complex data types such as lists and data frames. The data frame is an R data structure that corresponds to the notion of a dataset, having both features and examples.

We also learned about how to get data into R from a variety of sources. R provides functions for reading from and saving to CSV files; SQL databases can be queried with the RODBC package.

Finally, we applied these skills while exploring a real-world dataset containing data on used car prices. We examined numeric variables using common summary statistics of center and spread, and visualized relationships between prices and odometer readings with a scatterplot. We examined nominal variables using tables. In examining the used car data, we followed an exploratory process...