Introduction to Data Frames

In this module, we'll begin working with data frame objects, which are the primary data storage type used in R. In many ways, data frames are similar to a two-dimensional row/column layout that you should be familiar with from spreadsheet programs like Microsoft Excel. This module will cover various ways of creating Data frames, and describe how they are related to other data types in R.

Helpful links:

Creating Data Frames

As stated above, data frames are really just lists with some additional properties that make them well suited for working with data. More specifically, data frames are lists in which each element is a vector of the same length. To relate this to the familiar row/column layout of a 2-dimensional table, each vector in your data frame is a column of your table. For example, consider the following table of weights and heights:

You can think of this as a list of two vectors: height and weight. It makes sense that the vectors are columns, not rows, because a row of data may contain elements of different types (and vectors are collections of elements all of the same type).

While this is not how you will typically work with data, it's important to understand that you can construct a data frame by combining multiple vectors. To accomplish this, you can use the data.frame function, which accepts vectors as arguments, and creates a column for each vector. For example:

# Vector of heights
height <- 58:62

# Vector of weights
weight <- c(115, 117, 120, 123, 126)

# Combine the vectors into a data.frame
my.data <- data.frame(height, weight)

Because data frame elements are lists, we can access the values from my.data using the same syntax as above:

# Using the same weights/heights as above:
my.data <- data.frame(height, weight)

# Retrieve weights
my.weights <- my.data$weight

# Retrieve heights
my.heights <- my.data[['height']]

Describing Dataframes

Here are a few ways in which you can start to explore you data frame elements:

Function	Result
`nrow(my.data.frame)`	Number of rows in the data frame
`ncol(my.data.frame)`	Number of columns in the data frame
`dim(my.data.frame)`	Dimensions (rows, columns) in the data frame
`colnames(my.data.frame)`	Column names of the data frame
`rownames(my.data.frame)`	Row names of the data frame
`head(my.data.frame)`	Prints the top subset of rows of the data frame
`tail(my.data.frame)`	Prints the last subset of rows of the data frame
`View(my.data.frame)`	Opens up the RStudio data frame viewer (only in RStudio)

Accessing Data in Data Frames

There are a variety of approaches for retrieving desired information from within data frames. While we'll begin to use other R packages for data frame manipulation, it's crucial to understand the built in approaches for accessing specific cells within a data frame.

The section above already covered methods for retrieving data frame columns using the double-bracket (data[['col.name']]) and dollar sign (data$col.name) methods. Below is a table of additional methods for retrieving individual cells, or rows from a data frame.

Syntax	Example	Result
`data[row.num, col.num]`	`data[2,3]`	Retrieve element in the second row, third column
`data[row.num, col.name]`	`data[2,'height']`	Retrieve second element in the `height` column
`data[row.name, col.name]`	`data['steve','height']`	Retrieve the `height` of row named `steve`
`data[row.num, ]`	`data[2,]`	Retrieve all columns and only the second row
`data[row.nums, ]`	`data[2:4,]`	Retrieve all columns and the second through fourth rows
`data[, col.name]`	`data[,'height']`	Retrieve all rows and only the `height` column

For practice retrieving information and manipulating data frames, see exercise-1 and exercise-3.

Reading CSV Data

While R is equipped to ingest a variety of different forms of data, in this module we'll focus on reading in tabular data in comma separated value (csv) format. Most spreadsheet programs like Microsoft Excel, Numbers, or Google Sheets are simply interfaces for formatting and interacting with data that is saved in a format like .csv files. These programs easily import and export .csv files, however .csv files are unable to save the formatting done in those programs (the files only store the data, separated by commas).

Reading in .csv data is simple using the read.csv function:

# Read your data into a data frame `my.data`
my.data <- read.csv('file_name.csv')

By default, read.csv will return a data frame of the dataset. This is almost ready to use out of the box, but first we need to understand how the function treats factor variables.

Factor Variables

Factor variables are variables that consist of a finite set of categories (i.e., they are categorical variables). Imagine that you had a vector of shirt sizes which could only take on the values small, medium, or large. If you were working with a large dataset, it would end up taking up a lot of memory to store the character strings for each one of those variables. However, you could create a factor variable that stores integers instead of strings, and keeps track of the relationship between the integers and their labels. For example:

# Start with a character vector of shirt sizes
shirt.sizes <- c('small', 'medium', 'small', 'large', 'medium', 'large')

# Convert to a vector of factor data
shirt.sizes.factor <- as.factor(shirt.sizes)

When you print out the shirt.sizes.factor variable, R still (intelligently) prints out the labels that you are presumably interested in. It also indicates the levels, which are the only possible values that elements can take on.

As you can see, if you attempt to assign a new value, R will assign it NA and produce an error message:

All of this is to say, when you use the read.csv function, it will interpret strings as factors by default. This is good for memory and various statistical analyses, but it will likely confuse your data processing. As a results, make sure to specify the following argument in your read.csv function:

# Read in data file, making sure character variables are not factors
my.data <- read.csv('file_name.csv', stringsAsFactors = FALSE)

For practice reading and working with data, see exercise-3 and exercise-4.

mariahtao / m10-dataframes