Chapter 3 Cleaning and wrangling data

3.1 Overview

This chapter is centered around defining tidy data—a data format that is suitable for analysis—and the tools needed to transform raw data into this format. This will be presented in the context of a real-world data science application, providing more practice working through a whole case study.

3.2 Chapter learning objectives

By the end of the chapter, readers will be able to do the following:

  • Define the term “tidy data.”
  • Discuss the advantages of storing data in a tidy data format.
  • Define what vectors, lists, and data frames are in R, and describe how they relate to each other.
  • Describe the common types of data in R and their uses.
  • Recall and use the following functions for their intended data wrangling tasks:
    • across
    • c
    • filter
    • group_by
    • select
    • map
    • mutate
    • pull
    • pivot_longer
    • pivot_wider
    • rowwise
    • separate
    • summarize
  • Recall and use the following operators for their intended data wrangling tasks:
    • ==
    • %in%
    • !
    • &
    • |
    • |> and %>%

3.3 Data frames, vectors, and lists

In Chapters 1 and 2, data frames were the focus: we learned how to import data into R as a data frame, and perform basic operations on data frames in R. In the remainder of this book, this pattern continues. The vast majority of tools we use will require that data are represented as a data frame in R. Therefore, in this section, we will dig more deeply into what data frames are and how they are represented in R. This knowledge will be helpful in effectively utilizing these objects in our data analyses.

3.3.1 What is a data frame?

A data frame is a table-like structure for storing data in R. Data frames are important to learn about because most data that you will encounter in practice can be naturally stored as a table. In order to define data frames precisely, we need to introduce a few technical terms:

  • variable: a characteristic, number, or quantity that can be measured.
  • observation: all of the measurements for a given entity.
  • value: a single measurement of a single variable for a given entity.

Given these definitions, a data frame is a tabular data structure in R that is designed to store observations, variables, and their values. Most commonly, each column in a data frame corresponds to a variable, and each row corresponds to an observation. For example, Figure 3.1 displays a data set of city populations. Here, the variables are “region, year, population”; each of these are properties that can be collected or measured. The first observation is “Toronto, 2016, 2235145”; these are the values that the three variables take for the first entity in the data set. There are 13 entities in the data set in total, corresponding to the 13 rows in Figure 3.1.

A data frame storing data regarding the population of various regions in Canada. In this example data frame, the row that corresponds to the observation for the city of Vancouver is colored yellow, and the column that corresponds to the population variable is colored blue.

Figure 3.1: A data frame storing data regarding the population of various regions in Canada. In this example data frame, the row that corresponds to the observation for the city of Vancouver is colored yellow, and the column that corresponds to the population variable is colored blue.

R stores the columns of a data frame as either lists or vectors. For example, the data frame in Figure 3.2 has three vectors whose names are region, year and population. The next two sections will explain what lists and vectors are.

Data frame with three vectors.

Figure 3.2: Data frame with three vectors.

3.3.2 What is a vector?

In R, vectors are objects that can contain one or more elements. The vector elements are ordered, and they must all be of the same data type; R has several different basic data types, as shown in Table 3.1. Figure 3.3 provides an example of a vector where all of the elements are of character type. You can create vectors in R using the c function (c stands for “concatenate”). For example, to create the vector region as shown in Figure 3.3, you would write:

year <- c("Toronto", "Montreal", "Vancouver", "Calgary", "Ottawa")
year
## [1] "Toronto"   "Montreal"  "Vancouver" "Calgary"   "Ottawa"

Note: Technically, these objects are called “atomic vectors.” In this book we have chosen to call them “vectors,” which is how they are most commonly referred to in the R community. To be totally precise, “vector” is an umbrella term that encompasses both atomic vector and list objects in R. But this creates a confusing situation where the term “vector” could mean “atomic vector” or “the umbrella term for atomic vector and list,” depending on context. Very confusing indeed! So to keep things simple, in this book we always use the term “vector” to refer to “atomic vector.” We encourage readers who are enthusiastic to learn more to read the Vectors chapter of Advanced R (Wickham 2019).

Example of a vector whose type is character.

Figure 3.3: Example of a vector whose type is character.

Table 3.1: Basic data types in R
Data type Abbreviation Description Example
character chr letters or numbers surrounded by quotes “1” , “Hello world!”
double dbl numbers with decimals values 1.2333
integer int numbers that do not contain decimals 1L, 20L (where “L” tells R to store as an integer)
logical lgl either true or false TRUE, FALSE
factor fct used to represent data with a limited number of values (usually categories) a color variable with levels red, green and orange

It is important in R to make sure you represent your data with the correct type. Many of the tidyverse functions we use in this book treat the various data types differently. You should use integers and double types (which both fall under the “numeric” umbrella type) to represent numbers and perform arithmetic. Doubles are more common than integers in R, though; for instance, a double data type is the default when you create a vector of numbers using c(), and when you read in whole numbers via read_csv. Characters are used to represent data that should be thought of as “text,” such as words, names, paths, URLs, and more. Factors help us encode variables that represent categories; a factor variable takes one of a discrete set of values known as levels (one for each category). The levels can be ordered or unordered. Even though factors can sometimes look like characters, they are not used to represent text, words, names, and paths in the way that characters are; in fact, R internally stores factors using integers! There are other basic data types in R, such as raw and complex, but we do not use these in this textbook.

3.3.3 What is a list?

Lists are also objects in R that have multiple, ordered elements. Vectors and lists differ by the requirement of element type consistency. All elements within a single vector must be of the same type (e.g., all elements are characters), whereas elements within a single list can be of different types (e.g., characters, integers, logicals, and even other lists).

A vector versus a list.

Figure 3.4: A vector versus a list.

3.3.4 What does this have to do with data frames?

A data frame is really a special kind of list that follows two rules:

  1. Each element itself must either be a vector or a list.
  2. Each element (vector or list) must have the same length.

Not all columns in a data frame need to be of the same type. Figure 3.5 shows a data frame where the columns are vectors of different types. But remember: because the columns in this example are vectors, the elements must be the same data type within each column. On the other hand, if our data frame had list columns, there would be no such requirement. It is generally much more common to use vector columns, though, as the values for a single variable are usually all of the same type.

Data frame and vector types.

Figure 3.5: Data frame and vector types.

The functions from the tidyverse package that we use often give us a special class of data frame called a tibble. Tibbles have some additional features and benefits over the built-in data frame object. These include the ability to add useful attributes (such as grouping, which we will discuss later) and more predictable type preservation when subsetting. Because a tibble is just a data frame with some added features, we will collectively refer to both built-in R data frames and tibbles as data frames in this book.

Note: You can use the function class on a data object to assess whether a data frame is a built-in R data frame or a tibble. If the data object is a data frame, class will return "data.frame". If the data object is a tibble it will return "tbl_df" "tbl" "data.frame". You can easily convert built-in R data frames to tibbles using the tidyverse as_tibble function. For example we can check the class of the Canadian languages data set, can_lang, we worked with in the previous chapters and we see it is a tibble.

class(can_lang)
## [1] "spec_tbl_df" "tbl_df"      "tbl"         "data.frame"

Vectors, data frames and lists are basic types of data structure in R, which are core to most data analyses. We summarize them in Table 3.2. There are several other data structures in the R programming language (e.g., matrices), but these are beyond the scope of this book.

Table 3.2: Basic data structures in R
Data Structure Description
vector An ordered collection of one, or more, values of the same data type.
list An ordered collection of one, or more, values of possibly different data types.
data frame A list of either vectors or lists of the same length, with column names. We typically use a data frame to represent a data set.

3.4 Tidy data

There are many ways a tabular data set can be organized. This chapter will focus on introducing the tidy data format of organization and how to make your raw (and likely messy) data tidy. A tidy data frame satisfies the following three criteria (Wickham 2014):

  • each row is a single observation,
  • each column is a single variable, and
  • each value is a single cell (i.e., its entry in the data frame is not shared with another value).

Figure 3.6 demonstrates a tidy data set that satisfies these three criteria.

Tidy data satisfies three criteria.

Figure 3.6: Tidy data satisfies three criteria.

There are many good reasons for making sure your data are tidy as a first step in your analysis. The most important is that it is a single, consistent format that nearly every function in the tidyverse recognizes. No matter what the variables and observations in your data represent, as long as the data frame is tidy, you can manipulate it, plot it, and analyze it using the same tools. If your data is not tidy, you will have to write special bespoke code in your analysis that will not only be error-prone, but hard for others to understand. Beyond making your analysis more accessible to others and less error-prone, tidy data is also typically easy for humans to interpret. Given these benefits, it is well worth spending the time to get your data into a tidy format upfront. Fortunately, there are many well-designed tidyverse data cleaning/wrangling tools to help you easily tidy your data. Let’s explore them below!

Note: Is there only one shape for tidy data for a given data set? Not necessarily! It depends on the statistical question you are asking and what the variables are for that question. For tidy data, each variable should be its own column. So, just as it’s essential to match your statistical question with the appropriate data analysis tool, it’s important to match your statistical question with the appropriate variables and ensure they are represented as individual columns to make the data tidy.

3.4.1 Tidying up: going from wide to long using pivot_longer

One task that is commonly performed to get data into a tidy format is to combine values that are stored in separate columns, but are really part of the same variable, into one. Data is often stored this way because this format is sometimes more intuitive for human readability and understanding, and humans create data sets. In Figure 3.7, the table on the left is in an untidy, “wide” format because the year values (2006, 2011, 2016) are stored as column names. And as a consequence, the values for population for the various cities over these years are also split across several columns.

For humans, this table is easy to read, which is why you will often find data stored in this wide format. However, this format is difficult to work with when performing data visualization or statistical analysis using R. For example, if we wanted to find the latest year it would be challenging because the year values are stored as column names instead of as values in a single column. So before we could apply a function to find the latest year (for example, by using max), we would have to first extract the column names to get them as a vector and then apply a function to extract the latest year. The problem only gets worse if you would like to find the value for the population for a given region for the latest year. Both of these tasks are greatly simplified once the data is tidied.

Another problem with data in this format is that we don’t know what the numbers under each year actually represent. Do those numbers represent population size? Land area? It’s not clear. To solve both of these problems, we can reshape this data set to a tidy data format by creating a column called “year” and a column called “population.” This transformation—which makes the data “longer”—is shown as the right table in Figure 3.7.

Pivoting data from a wide to long data format.

Figure 3.7: Pivoting data from a wide to long data format.

We can achieve this effect in R using the pivot_longer function from the tidyverse package. The pivot_longer function combines columns, and is usually used during tidying data when we need to make the data frame longer and narrower. To learn how to use pivot_longer, we will work through an example with the region_lang_top5_cities_wide.csv data set. This data set contains the counts of how many Canadians cited each language as their mother tongue for five major Canadian cities (Toronto, Montréal, Vancouver, Calgary and Edmonton) from the 2016 Canadian census. To get started, we will load the tidyverse package and use read_csv to load the (untidy) data.

library(tidyverse)
lang_wide <- read_csv("data/region_lang_top5_cities_wide.csv")
lang_wide
## # A tibble: 214 × 7
##    category           language       Toronto Montréal Vancouver Calgary Edmonton
##    <chr>              <chr>            <dbl>    <dbl>     <dbl>   <dbl>    <dbl>
##  1 Aboriginal langua… Aboriginal la…      80       30        70      20       25
##  2 Non-Official & No… Afrikaans          985       90      1435     960      575
##  3 Non-Official & No… Afro-Asiatic …     360      240        45      45       65
##  4 Non-Official & No… Akan (Twi)        8485     1015       400     705      885
##  5 Non-Official & No… Albanian         13260     2450      1090    1365      770
##  6 Aboriginal langua… Algonquian la…       5        5         0       0        0
##  7 Aboriginal langua… Algonquin            5       30         5       5        0
##  8 Non-Official & No… American Sign…     470       50       265     100      180
##  9 Non-Official & No… Amharic           7460      665      1140    4075     2515
## 10 Non-Official & No… Arabic           85175   151955     14320   18965    17525
## # … with 204 more rows

What is wrong with the untidy format above? The table on the left in Figure 3.8 represents the data in the “wide” (messy) format. From a data analysis perspective, this format is not ideal because the values of the variable region (Toronto, Montréal, Vancouver, Calgary and Edmonton) are stored as column names. Thus they are not easily accessible to the data analysis functions we will apply to our data set. Additionally, the mother tongue variable values are spread across multiple columns, which will prevent us from doing any desired visualization or statistical tasks until we combine them into one column. For instance, suppose we want to know the languages with the highest number of Canadians reporting it as their mother tongue among all five regions. This question would be tough to answer with the data in its current format. We could find the answer with the data in this format, though it would be much easier to answer if we tidy our data first. If mother tongue were instead stored as one column, as shown in the tidy data on the right in Figure 3.8, we could simply use one line of code (max(mother_tongue)) to get the maximum value.

Going from wide to long with the pivot_longer function.

Figure 3.8: Going from wide to long with the pivot_longer function.

Figure 3.9 details the arguments that we need to specify in the pivot_longer function to accomplish this data transformation.

Syntax for the pivot_longer function.

Figure 3.9: Syntax for the pivot_longer function.

We use pivot_longer to combine the Toronto, Montréal, Vancouver, Calgary, and Edmonton columns into a single column called region, and create a column called mother_tongue that contains the count of how many Canadians report each language as their mother tongue for each metropolitan area. We use a colon : between Toronto and Edmonton to tell R to select all the columns between Toronto and Edmonton:

lang_mother_tidy <- pivot_longer(lang_wide,
  cols = Toronto:Edmonton,
  names_to = "region",
  values_to = "mother_tongue"
)

lang_mother_tidy
## # A tibble: 1,070 × 4
##    category                                language        region  mother_tongue
##    <chr>                                   <chr>           <chr>           <dbl>
##  1 Aboriginal languages                    Aboriginal lan… Toronto            80
##  2 Aboriginal languages                    Aboriginal lan… Montré…            30
##  3 Aboriginal languages                    Aboriginal lan… Vancou…            70
##  4 Aboriginal languages                    Aboriginal lan… Calgary            20
##  5 Aboriginal languages                    Aboriginal lan… Edmont…            25
##  6 Non-Official & Non-Aboriginal languages Afrikaans       Toronto           985
##  7 Non-Official & Non-Aboriginal languages Afrikaans       Montré…            90
##  8 Non-Official & Non-Aboriginal languages Afrikaans       Vancou…          1435
##  9 Non-Official & Non-Aboriginal languages Afrikaans       Calgary           960
## 10 Non-Official & Non-Aboriginal languages Afrikaans       Edmont…           575
## # … with 1,060 more rows

Note: In the code above, the call to the pivot_longer function is split across several lines. This is allowed in certain cases; for example, when calling a function as above, as long as the line ends with a comma , R knows to keep reading on the next line. Splitting long lines like this across multiple lines is encouraged as it helps significantly with code readability. Generally speaking, you should limit each line of code to about 80 characters.

The data above is now tidy because all three criteria for tidy data have now been met:

  1. All the variables (category, language, region and mother_tongue) are now their own columns in the data frame.
  2. Each observation, i.e., each category, language, region, and count of Canadians where that language is the mother tongue, are in a single row.
  3. Each value is a single cell, i.e., its row, column position in the data frame is not shared with another value.

3.4.2 Tidying up: going from long to wide using pivot_wider

Suppose we have observations spread across multiple rows rather than in a single row. For example, in Figure 3.10, the table on the left is in an untidy, long format because the count column contains three variables (population, commuter, and incorporated count) and information about each observation (here, population, commuter, and incorporated counts for a region) is split across three rows. Remember: one of the criteria for tidy data is that each observation must be in a single row.

Using data in this format—where two or more variables are mixed together in a single column—makes it harder to apply many usual tidyverse functions. For example, finding the maximum number of commuters would require an additional step of filtering for the commuter values before the maximum can be computed. In comparison, if the data were tidy, all we would have to do is compute the maximum value for the commuter column. To reshape this untidy data set to a tidy (and in this case, wider) format, we need to create columns called “population,” “commuters,” and “incorporated.” This is illustrated in the right table of Figure 3.10.

Going from long to wide data.

Figure 3.10: Going from long to wide data.

To tidy this type of data in R, we can use the pivot_wider function. The pivot_wider function generally increases the number of columns (widens) and decreases the number of rows in a data set. To learn how to use pivot_wider, we will work through an example with the region_lang_top5_cities_long.csv data set. This data set contains the number of Canadians reporting the primary language at home and work for five major cities (Toronto, Montréal, Vancouver, Calgary and Edmonton).

lang_long <- read_csv("data/region_lang_top5_cities_long.csv")
lang_long
## # A tibble: 2,140 × 5
##    region    category             language                     type         count
##    <chr>     <chr>                <chr>                        <chr>        <dbl>
##  1 Montréal  Aboriginal languages Aboriginal languages, n.o.s. most_at_home    15
##  2 Montréal  Aboriginal languages Aboriginal languages, n.o.s. most_at_work     0
##  3 Toronto   Aboriginal languages Aboriginal languages, n.o.s. most_at_home    50
##  4 Toronto   Aboriginal languages Aboriginal languages, n.o.s. most_at_work     0
##  5 Calgary   Aboriginal languages Aboriginal languages, n.o.s. most_at_home     5
##  6 Calgary   Aboriginal languages Aboriginal languages, n.o.s. most_at_work     0
##  7 Edmonton  Aboriginal languages Aboriginal languages, n.o.s. most_at_home    10
##  8 Edmonton  Aboriginal languages Aboriginal languages, n.o.s. most_at_work     0
##  9 Vancouver Aboriginal languages Aboriginal languages, n.o.s. most_at_home    15
## 10 Vancouver Aboriginal languages Aboriginal languages, n.o.s. most_at_work     0
## # … with 2,130 more rows

What makes the data set shown above untidy? In this example, each observation is a language in a region. However, each observation is split across multiple rows: one where the count for most_at_home is recorded, and the other where the count for most_at_work is recorded. Suppose the goal with this data was to visualize the relationship between the number of Canadians reporting their primary language at home and work. Doing that would be difficult with this data in its current form, since these two variables are stored in the same column. Figure 3.11 shows how this data will be tidied using the pivot_wider function.

Going from long to wide with the pivot_wider function.

Figure 3.11: Going from long to wide with the pivot_wider function.

Figure 3.12 details the arguments that we need to specify in the pivot_wider function.

Syntax for the pivot_wider function.

Figure 3.12: Syntax for the pivot_wider function.

We will apply the function as detailed in Figure 3.12.

lang_home_tidy <- pivot_wider(lang_long,
  names_from = type,
  values_from = count
)
lang_home_tidy
## # A tibble: 1,070 × 5
##    region    category                language          most_at_home most_at_work
##    <chr>     <chr>                   <chr>                    <dbl>        <dbl>
##  1 Montréal  Aboriginal languages    Aboriginal langu…           15            0
##  2 Toronto   Aboriginal languages    Aboriginal langu…           50            0
##  3 Calgary   Aboriginal languages    Aboriginal langu…            5            0
##  4 Edmonton  Aboriginal languages    Aboriginal langu…           10            0
##  5 Vancouver Aboriginal languages    Aboriginal langu…           15            0
##  6 Montréal  Non-Official & Non-Abo… Afrikaans                   10            0
##  7 Toronto   Non-Official & Non-Abo… Afrikaans                  265            0
##  8 Calgary   Non-Official & Non-Abo… Afrikaans                  505           15
##  9 Edmonton  Non-Official & Non-Abo… Afrikaans                  300            0
## 10 Vancouver Non-Official & Non-Abo… Afrikaans                  520           10
## # … with 1,060 more rows

The data above is now tidy! We can go through the three criteria again to check that this data is a tidy data set.

  1. All the statistical variables are their own columns in the data frame (i.e., most_at_home, and most_at_work have been separated into their own columns in the data frame).
  2. Each observation, (i.e., each language in a region) is in a single row.
  3. Each value is a single cell (i.e., its row, column position in the data frame is not shared with another value).

You might notice that we have the same number of columns in the tidy data set as we did in the messy one. Therefore pivot_wider didn’t really “widen” the data, as the name suggests. This is just because the original type column only had two categories in it. If it had more than two, pivot_wider would have created more columns, and we would see the data set “widen.”

3.4.3 Tidying up: using separate to deal with multiple delimiters

Data are also not considered tidy when multiple values are stored in the same cell. The data set we show below is even messier than the ones we dealt with above: the Toronto, Montréal, Vancouver, Calgary and Edmonton columns contain the number of Canadians reporting their primary language at home and work in one column separated by the delimiter (/). The column names are the values of a variable, and each value does not have its own cell! To turn this messy data into tidy data, we’ll have to fix these issues.

lang_messy <- read_csv("data/region_lang_top5_cities_messy.csv")
lang_messy
## # A tibble: 214 × 7
##    category          language       Toronto  Montréal Vancouver Calgary Edmonton
##    <chr>             <chr>          <chr>    <chr>    <chr>     <chr>   <chr>   
##  1 Aboriginal langu… Aboriginal la… 50/0     15/0     15/0      5/0     10/0    
##  2 Non-Official & N… Afrikaans      265/0    10/0     520/10    505/15  300/0   
##  3 Non-Official & N… Afro-Asiatic … 185/10   65/0     10/0      15/0    20/0    
##  4 Non-Official & N… Akan (Twi)     4045/20  440/0    125/10    330/0   445/0   
##  5 Non-Official & N… Albanian       6380/215 1445/20  530/10    620/25  370/10  
##  6 Aboriginal langu… Algonquian la… 5/0      0/0      0/0       0/0     0/0     
##  7 Aboriginal langu… Algonquin      0/0      10/0     0/0       0/0     0/0     
##  8 Non-Official & N… American Sign… 720/245  70/0     300/140   85/25   190/85  
##  9 Non-Official & N… Amharic        3820/55  315/0    540/10    2730/50 1695/35 
## 10 Non-Official & N… Arabic         45025/1… 72980/1… 8680/275  11010/… 10590/3…
## # … with 204 more rows

First we’ll use pivot_longer to create two columns, region and value, similar to what we did previously. The new region columns will contain the region names, and the new column value will be a temporary holding place for the data that we need to further separate, i.e., the number of Canadians reporting their primary language at home and work.

lang_messy_longer <- pivot_longer(lang_messy,
  cols = Toronto:Edmonton,
  names_to = "region",
  values_to = "value"
)

lang_messy_longer
## # A tibble: 1,070 × 4
##    category                                language               region   value
##    <chr>                                   <chr>                  <chr>    <chr>
##  1 Aboriginal languages                    Aboriginal languages,… Toronto  50/0 
##  2 Aboriginal languages                    Aboriginal languages,… Montréal 15/0 
##  3 Aboriginal languages                    Aboriginal languages,… Vancouv… 15/0 
##  4 Aboriginal languages                    Aboriginal languages,… Calgary  5/0  
##  5 Aboriginal languages                    Aboriginal languages,… Edmonton 10/0 
##  6 Non-Official & Non-Aboriginal languages Afrikaans              Toronto  265/0
##  7 Non-Official & Non-Aboriginal languages Afrikaans              Montréal 10/0 
##  8 Non-Official & Non-Aboriginal languages Afrikaans              Vancouv… 520/…
##  9 Non-Official & Non-Aboriginal languages Afrikaans              Calgary  505/…
## 10 Non-Official & Non-Aboriginal languages Afrikaans              Edmonton 300/0
## # … with 1,060 more rows

Next we’ll use separate to split the value column into two columns. One column will contain only the counts of Canadians that speak each language most at home, and the other will contain the counts of Canadians that speak each language most at work for each region. Figure 3.13 outlines what we need to specify to use separate.

Syntax for the separate function.

Figure 3.13: Syntax for the separate function.

tidy_lang <- separate(lang_messy_longer,
  col = value,
  into = c("most_at_home", "most_at_work"),
  sep = "/"
)

tidy_lang
## # A tibble: 1,070 × 5
##    category                 language           region  most_at_home most_at_work
##    <chr>                    <chr>              <chr>   <chr>        <chr>       
##  1 Aboriginal languages     Aboriginal langua… Toronto 50           0           
##  2 Aboriginal languages     Aboriginal langua… Montré… 15           0           
##  3 Aboriginal languages     Aboriginal langua… Vancou… 15           0           
##  4 Aboriginal languages     Aboriginal langua… Calgary 5            0           
##  5 Aboriginal languages     Aboriginal langua… Edmont… 10           0           
##  6 Non-Official & Non-Abor… Afrikaans          Toronto 265          0           
##  7 Non-Official & Non-Abor… Afrikaans          Montré… 10           0           
##  8 Non-Official & Non-Abor… Afrikaans          Vancou… 520          10          
##  9 Non-Official & Non-Abor… Afrikaans          Calgary 505          15          
## 10 Non-Official & Non-Abor… Afrikaans          Edmont… 300          0           
## # … with 1,060 more rows

Is this data set now tidy? If we recall the three criteria for tidy data:

  • each row is a single observation,
  • each column is a single variable, and
  • each value is a single cell.

We can see that this data now satisfies all three criteria, making it easier to analyze. But we aren’t done yet! Notice in the table above that the word <chr> appears beneath each of the column names. The word under the column name indicates the data type of each column. Here all of the variables are “character” data types. Recall, character data types are letter(s) or digits(s) surrounded by quotes. In the previous example in Section 3.4.2, the most_at_home and most_at_work variables were <dbl> (double)—you can verify this by looking at the tables in the previous sections—which is a type of numeric data. This change is due to the delimiter (/) when we read in this messy data set. R read these columns in as character types, and by default, separate will return columns as character data types.

It makes sense for region, category, and language to be stored as a character (or perhaps factor) type. However, suppose we want to apply any functions that treat the most_at_home and most_at_work columns as a number (e.g., finding rows above a numeric threshold of a column). In that case, it won’t be possible to do if the variable is stored as a character. Fortunately, the separate function provides a natural way to fix problems like this: we can set convert = TRUE to convert the most_at_home and most_at_work columns to the correct data type.

tidy_lang <- separate(lang_messy_longer,
  col = value,
  into = c("most_at_home", "most_at_work"),
  sep = "/", 
  convert = TRUE
)

tidy_lang
## # A tibble: 1,070 × 5
##    category                 language           region  most_at_home most_at_work
##    <chr>                    <chr>              <chr>          <int>        <int>
##  1 Aboriginal languages     Aboriginal langua… Toronto           50            0
##  2 Aboriginal languages     Aboriginal langua… Montré…           15            0
##  3 Aboriginal languages     Aboriginal langua… Vancou…           15            0
##  4 Aboriginal languages     Aboriginal langua… Calgary            5            0
##  5 Aboriginal languages     Aboriginal langua… Edmont…           10            0
##  6 Non-Official & Non-Abor… Afrikaans          Toronto          265            0
##  7 Non-Official & Non-Abor… Afrikaans          Montré…           10            0
##  8 Non-Official & Non-Abor… Afrikaans          Vancou…          520           10
##  9 Non-Official & Non-Abor… Afrikaans          Calgary          505           15
## 10 Non-Official & Non-Abor… Afrikaans          Edmont…          300            0
## # … with 1,060 more rows

Now we see <int> appears under the most_at_home and most_at_work columns, indicating they are integer data types (i.e., numbers)!

3.5 Using select to extract a range of columns

Now that the tidy_lang data is indeed tidy, we can start manipulating it using the powerful suite of functions from the tidyverse. For the first example, recall the select function from Chapter 1, which lets us create a subset of columns from a data frame. Suppose we wanted to select only the columns language, region, most_at_home and most_at_work from the tidy_lang data set. Using what we learned in Chapter 1, we would pass the tidy_lang data frame as well as all of these column names into the select function:

selected_columns <- select(tidy_lang, 
                           language, 
                           region, 
                           most_at_home, 
                           most_at_work)
selected_columns
## # A tibble: 1,070 × 4
##    language                     region    most_at_home most_at_work
##    <chr>                        <chr>            <int>        <int>
##  1 Aboriginal languages, n.o.s. Toronto             50            0
##  2 Aboriginal languages, n.o.s. Montréal            15            0
##  3 Aboriginal languages, n.o.s. Vancouver           15            0
##  4 Aboriginal languages, n.o.s. Calgary              5            0
##  5 Aboriginal languages, n.o.s. Edmonton            10            0
##  6 Afrikaans                    Toronto            265            0
##  7 Afrikaans                    Montréal            10            0
##  8 Afrikaans                    Vancouver          520           10
##  9 Afrikaans                    Calgary            505           15
## 10 Afrikaans                    Edmonton           300            0
## # … with 1,060 more rows

Here we wrote out the names of each of the columns. However, this method is time-consuming, especially if you have a lot of columns! Another approach is to use a “select helper.” Select helpers are operators that make it easier for us to select columns. For instance, we can use a select helper to choose a range of columns rather than typing each column name out. To do this, we use the colon (:) operator to denote the range. For example, to get all the columns in the tidy_lang data frame from language to most_at_work we pass language:most_at_work as the second argument to the select function.

column_range <- select(tidy_lang, language:most_at_work)
column_range
## # A tibble: 1,070 × 4
##    language                     region    most_at_home most_at_work
##    <chr>                        <chr>            <int>        <int>
##  1 Aboriginal languages, n.o.s. Toronto             50            0
##  2 Aboriginal languages, n.o.s. Montréal            15            0
##  3 Aboriginal languages, n.o.s. Vancouver           15            0
##  4 Aboriginal languages, n.o.s. Calgary              5            0
##  5 Aboriginal languages, n.o.s. Edmonton            10            0
##  6 Afrikaans                    Toronto            265            0
##  7 Afrikaans                    Montréal            10            0
##  8 Afrikaans                    Vancouver          520           10
##  9 Afrikaans                    Calgary            505           15
## 10 Afrikaans                    Edmonton           300            0
## # … with 1,060 more rows

Notice that we get the same output as we did above, but with less (and clearer!) code. This type of operator is especially handy for large data sets.

Suppose instead we wanted to extract columns that followed a particular pattern rather than just selecting a range. For example, let’s say we wanted only to select the columns most_at_home and most_at_work. There are other helpers that allow us to select variables based on their names. In particular, we can use the select helper starts_with to choose only the columns that start with the word “most”:

select(tidy_lang, starts_with("most"))
## # A tibble: 1,070 × 2
##    most_at_home most_at_work
##           <int>        <int>
##  1           50            0
##  2           15            0
##  3           15            0
##  4            5            0
##  5           10            0
##  6          265            0
##  7           10            0
##  8          520           10
##  9          505           15
## 10          300            0
## # … with 1,060 more rows

We could also have chosen the columns containing an underscore _ by adding contains("_") as the second argument in the select function, since we notice the columns we want contain underscores and the others don’t.

select(tidy_lang, contains("_"))
## # A tibble: 1,070 × 2
##    most_at_home most_at_work
##           <int>        <int>
##  1           50            0
##  2           15            0
##  3           15            0
##  4            5            0
##  5           10            0
##  6          265            0
##  7           10            0
##  8          520           10
##  9          505           15
## 10          300            0
## # … with 1,060 more rows

There are many different select helpers that select variables based on certain criteria. The additional resources section at the end of this chapter provides a comprehensive resource on select helpers.

3.6 Using filter to extract rows

Next, we revisit the filter function from Chapter 1, which lets us create a subset of rows from a data frame. Recall the two main arguments to the filter function: the first is the name of the data frame object, and the second is a logical statement to use when filtering the rows. filter works by returning the rows where the logical statement evaluates to TRUE. This section will highlight more advanced usage of the filter function. In particular, this section provides an in-depth treatment of the variety of logical statements one can use in the filter function to select subsets of rows.

3.6.1 Extracting rows that have a certain value with ==

Suppose we are only interested in the subset of rows in tidy_lang corresponding to the official languages of Canada (English and French). We can filter for these rows by using the equivalency operator (==) to compare the values of the category column with the value "Official languages". With these arguments, filter returns a data frame with all the columns of the input data frame but only the rows we asked for in the logical statement, i.e., those where the category column holds the value "Official languages". We name this data frame official_langs.

official_langs <- filter(tidy_lang, category == "Official languages")
official_langs
## # A tibble: 10 × 5
##    category           language region    most_at_home most_at_work
##    <chr>              <chr>    <chr>            <int>        <int>
##  1 Official languages English  Toronto        3836770      3218725
##  2 Official languages English  Montréal        620510       412120
##  3 Official languages English  Vancouver      1622735      1330555
##  4 Official languages English  Calgary        1065070       844740
##  5 Official languages English  Edmonton       1050410       792700
##  6 Official languages French   Toronto          29800        11940
##  7 Official languages French   Montréal       2669195      1607550
##  8 Official languages French   Vancouver         8630         3245
##  9 Official languages French   Calgary           8630         2140
## 10 Official languages French   Edmonton         10950         2520

3.6.2 Extracting rows that do not have a certain value with !=

What if we want all the other language categories in the data set except for those in the "Official languages" category? We can accomplish this with the != operator, which means “not equal to.” So if we want to find all the rows where the category does not equal "Official languages" we write the code below.

filter(tidy_lang, category != "Official languages")
## # A tibble: 1,060 × 5
##    category                 language           region  most_at_home most_at_work
##    <chr>                    <chr>              <chr>          <int>        <int>
##  1 Aboriginal languages     Aboriginal langua… Toronto           50            0
##  2 Aboriginal languages     Aboriginal langua… Montré…           15            0
##  3 Aboriginal languages     Aboriginal langua… Vancou…           15            0
##  4 Aboriginal languages     Aboriginal langua… Calgary            5            0
##  5 Aboriginal languages     Aboriginal langua… Edmont…           10            0
##  6 Non-Official & Non-Abor… Afrikaans          Toronto          265            0
##  7 Non-Official & Non-Abor… Afrikaans          Montré…           10            0
##  8 Non-Official & Non-Abor… Afrikaans          Vancou…          520           10
##  9 Non-Official & Non-Abor… Afrikaans          Calgary          505           15
## 10 Non-Official & Non-Abor… Afrikaans          Edmont…          300            0
## # … with 1,050 more rows

3.6.3 Extracting rows satisfying multiple conditions using , or &

Suppose now we want to look at only the rows for the French language in Montréal. To do this, we need to filter the data set to find rows that satisfy multiple conditions simultaneously. We can do this with the comma symbol (,), which in the case of filter is interpreted by R as “and.” We write the code as shown below to filter the official_langs data frame to subset the rows where region == "Montréal" and the language == "French".

filter(official_langs, region == "Montréal", language == "French")
## # A tibble: 1 × 5
##   category           language region   most_at_home most_at_work
##   <chr>              <chr>    <chr>           <int>        <int>
## 1 Official languages French   Montréal      2669195      1607550

We can also use the ampersand (&) logical operator, which gives us cases where both one condition and another condition are satisfied. You can use either comma (,) or ampersand (&) in the filter function interchangeably.

filter(official_langs, region == "Montréal" & language == "French")
## # A tibble: 1 × 5
##   category           language region   most_at_home most_at_work
##   <chr>              <chr>    <chr>           <int>        <int>
## 1 Official languages French   Montréal      2669195      1607550

3.6.4 Extracting rows satisfying at least one condition using |

Suppose we were interested in only those rows corresponding to cities in Alberta in the official_langs data set (Edmonton and Calgary). We can’t use , as we did above because region cannot be both Edmonton and Calgary simultaneously. Instead, we can use the vertical pipe (|) logical operator, which gives us the cases where one condition or another condition or both are satisfied. In the code below, we ask R to return the rows where the region columns are equal to “Calgary” or “Edmonton.”

filter(official_langs, region == "Calgary" | region == "Edmonton")
## # A tibble: 4 × 5
##   category           language region   most_at_home most_at_work
##   <chr>              <chr>    <chr>           <int>        <int>
## 1 Official languages English  Calgary       1065070       844740
## 2 Official languages English  Edmonton      1050410       792700
## 3 Official languages French   Calgary          8630         2140
## 4 Official languages French   Edmonton        10950         2520

3.6.5 Extracting rows with values in a vector using %in%

Next, suppose we want to see the populations of our five cities. Let’s read in the region_data.csv file that comes from the 2016 Canadian census, as it contains statistics for number of households, land area, population and number of dwellings for different regions.

region_data <- read_csv("data/region_data.csv")
region_data
## # A tibble: 35 × 5
##    region         households  area population dwellings
##    <chr>               <dbl> <dbl>      <dbl>     <dbl>
##  1 Belleville          43002 1355.     103472     45050
##  2 Lethbridge          45696 3047.     117394     48317
##  3 Thunder Bay         52545 2618.     121621     57146
##  4 Peterborough        50533 1637.     121721     55662
##  5 Saint John          52872 3793.     126202     58398
##  6 Brantford           52530 1086.     134203     54419
##  7 Moncton             61769 2625.     144810     66699
##  8 Guelph              59280  604.     151984     63324
##  9 Trois-Rivières      72502 1053.     156042     77734
## 10 Saguenay            72479 3079.     160980     77968
## # … with 25 more rows

To get the population of the five cities we can filter the data set using the %in% operator. The %in% operator is used to see if an element belongs to a vector. Here we are filtering for rows where the value in the region column matches any of the five cities we are intersted in: Toronto, Montréal, Vancouver, Calgary, and Edmonton.

city_names <- c("Toronto", "Montréal", "Vancouver", "Calgary", "Edmonton")
five_cities <- filter(region_data, 
                      region %in% city_names)
five_cities
## # A tibble: 5 × 5
##   region    households  area population dwellings
##   <chr>          <dbl> <dbl>      <dbl>     <dbl>
## 1 Edmonton      502143 9858.    1321426    537634
## 2 Calgary       519693 5242.    1392609    544870
## 3 Vancouver     960894 3040.    2463431   1027613
## 4 Montréal     1727310 4638.    4098927   1823281
## 5 Toronto      2135909 6270.    5928040   2235145

Note: What’s the difference between == and %in%? Suppose we have two vectors, vectorA and vectorB. If you type vectorA == vectorB into R it will compare the vectors element by element. R checks if the first element of vectorA equals the first element of vectorB, the second element of vectorA equals the second element of vectorB, and so on. On the other hand, vectorA %in% vectorB compares the first element of vectorA to all the elements in vectorB. Then the second element of vectorA is compared to all the elements in vectorB, and so on. Notice the difference between == and %in% in the example below.

c("Vancouver", "Toronto") == c("Toronto", "Vancouver")
## [1] FALSE FALSE
c("Vancouver", "Toronto") %in% c("Toronto", "Vancouver")
## [1] TRUE TRUE

3.6.6 Extracting rows above or below a threshold using > and <

We saw in Section 3.6.3 that 2,669,195 people reported speaking French in Montréal as their primary language at home. If we are interested in finding the official languages in regions with higher numbers of people who speak it as their primary language at home compared to French in Montréal, then we can use filter to obtain rows where the value of most_at_home is greater than 2,669,195.

filter(official_langs, most_at_home > 2669195)
## # A tibble: 1 × 5
##   category           language region  most_at_home most_at_work
##   <chr>              <chr>    <chr>          <int>        <int>
## 1 Official languages English  Toronto      3836770      3218725

filter returns a data frame with only one row, indicating that when considering the official languages, only English in Toronto is reported by more people as their primary language at home than French in Montréal according to the 2016 Canadian census.

3.7 Using mutate to modify or add columns

3.7.1 Using mutate to modify columns

In Section 3.4.3, when we first read in the "region_lang_top5_cities_messy.csv" data, all of the variables were “character” data types. During the tidying process, we used the convert argument from the separate function to convert the most_at_home and most_at_work columns to the desired integer (i.e., numeric class) data types. But suppose we didn’t use the convert argument, and needed to modify the column type some other way. Below we create such a situation so that we can demonstrate how to use mutate to change the column types of a data frame. mutate is a useful function to modify or create new data frame columns.

lang_messy <- read_csv("data/region_lang_top5_cities_messy.csv")
lang_messy_longer <- pivot_longer(lang_messy,
               cols = Toronto:Edmonton,
               names_to = "region",
               values_to = "value")
tidy_lang_chr <- separate(lang_messy_longer, col = value,
           into = c("most_at_home", "most_at_work"),
           sep = "/") 
official_langs_chr <- filter(tidy_lang_chr, category == "Official languages")

official_langs_chr 
## # A tibble: 10 × 5
##    category           language region    most_at_home most_at_work
##    <chr>              <chr>    <chr>     <chr>        <chr>       
##  1 Official languages English  Toronto   3836770      3218725     
##  2 Official languages English  Montréal  620510       412120      
##  3 Official languages English  Vancouver 1622735      1330555     
##  4 Official languages English  Calgary   1065070      844740      
##  5 Official languages English  Edmonton  1050410      792700      
##  6 Official languages French   Toronto   29800        11940       
##  7 Official languages French   Montréal  2669195      1607550     
##  8 Official languages French   Vancouver 8630         3245        
##  9 Official languages French   Calgary   8630         2140        
## 10 Official languages French   Edmonton  10950        2520

To use mutate, again we first specify the data set in the first argument, and in the following arguments, we specify the name of the column we want to modify or create (here most_at_home and most_at_work), an = sign, and then the function we want to apply (here as.numeric). In the function we want to apply, we refer directly to the column name upon which we want it to act (here most_at_home and most_at_work). In our example, we are naming the columns the same names as columns that already exist in the data frame (“most_at_home,” “most_at_work”) and this will cause mutate to overwrite those columns (also referred to as modifying those columns in-place). If we were to give the columns a new name, then mutate would create new columns with the names we specified. mutate’s general syntax is detailed in Figure 3.14.

Syntax for the mutate function.

Figure 3.14: Syntax for the mutate function.

Below we use mutate to convert the columns most_at_home and most_at_work to numeric data types in the official_langs data set as described in Figure 3.14:

official_langs_numeric <- mutate(official_langs_chr,
  most_at_home = as.numeric(most_at_home),
  most_at_work = as.numeric(most_at_work)
)

official_langs_numeric
## # A tibble: 10 × 5
##    category           language region    most_at_home most_at_work
##    <chr>              <chr>    <chr>            <dbl>        <dbl>
##  1 Official languages English  Toronto        3836770      3218725
##  2 Official languages English  Montréal        620510       412120
##  3 Official languages English  Vancouver      1622735      1330555
##  4 Official languages English  Calgary        1065070       844740
##  5 Official languages English  Edmonton       1050410       792700
##  6 Official languages French   Toronto          29800        11940
##  7 Official languages French   Montréal       2669195      1607550
##  8 Official languages French   Vancouver         8630         3245
##  9 Official languages French   Calgary           8630         2140
## 10 Official languages French   Edmonton         10950         2520

Now we see <dbl> appears under the most_at_home and most_at_work columns, indicating they are double data types (which is a numeric data type)!

3.7.2 Using mutate to create new columns

We can see in the table that 3,836,770 people reported speaking English in Toronto as their primary language at home, according to the 2016 Canadian census. What does this number mean to us? To understand this number, we need context. In particular, how many people were in Toronto when this data was collected? From the 2016 Canadian census profile, the population of Toronto was reported to be 5,928,040 people. The number of people who report that English is their primary language at home is much more meaningful when we report it in this context. We can even go a step further and transform this count to a relative frequency or proportion. We can do this by dividing the number of people reporting a given language as their primary language at home by the number of people who live in Toronto. For example, the proportion of people who reported that their primary language at home was English in the 2016 Canadian census was 0.65 in Toronto.

Let’s use mutate to create a new column in our data frame that holds the proportion of people who speak English for our five cities of focus in this chapter. To accomplish this, we will need to do two tasks beforehand:

  1. Create a vector containing the population values for the cities.
  2. Filter the official_langs data frame so that we only keep the rows where the language is English.

To create a vector containing the population values for the five cities (Toronto, Montréal, Vancouver, Calgary, Edmonton), we will use the c function (recall that c stands for “concatenate”):

city_pops <- c(5928040, 4098927, 2463431, 1392609, 1321426)
city_pops
## [1] 5928040 4098927 2463431 1392609 1321426

And next, we will filter the official_langs data frame so that we only keep the rows where the language is English. We will name the new data frame we get from this english_langs:

english_langs <- filter(official_langs, language == "English")
english_langs
## # A tibble: 5 × 5
##   category           language region    most_at_home most_at_work
##   <chr>              <chr>    <chr>            <int>        <int>
## 1 Official languages English  Toronto        3836770      3218725
## 2 Official languages English  Montréal        620510       412120
## 3 Official languages English  Vancouver      1622735      1330555
## 4 Official languages English  Calgary        1065070       844740
## 5 Official languages English  Edmonton       1050410       792700

Finally, we can use mutate to create a new column, named most_at_home_proportion, that will have value that corresponds to the proportion of people reporting English as their primary language at home. We will compute this by dividing the column by our vector of city populations.

english_langs <- mutate(english_langs, 
                         most_at_home_proportion = most_at_home / city_pops)

english_langs
## # A tibble: 5 × 6
##   category           language region  most_at_home most_at_work most_at_home_pr…
##   <chr>              <chr>    <chr>          <int>        <int>            <dbl>
## 1 Official languages English  Toronto      3836770      3218725            0.647
## 2 Official languages English  Montré…       620510       412120            0.151
## 3 Official languages English  Vancou…      1622735      1330555            0.659
## 4 Official languages English  Calgary      1065070       844740            0.765
## 5 Official languages English  Edmont…      1050410       792700            0.795

In the computation above, we had to ensure that we ordered the city_pops vector in the same order as the cities were listed in the english_langs data frame. This is because R will perform the division computation we did by dividing each element of the most_at_home column by each element of the city_pops vector, matching them up by position. Failing to do this would have resulted in the incorrect math being performed.

Note: In more advanced data wrangling, one might solve this problem in a less error-prone way though using a technique called “joins.” We link to resources that discuss this in the additional resources at the end of this chapter.

3.8 Combining functions using the pipe operator, |>

In R, we often have to call multiple functions in a sequence to process a data frame. The basic ways of doing this can become quickly unreadable if there are many steps. For example, suppose we need to perform three operations on a data frame called data:

  1. add a new column new_col that is double another old_col,
  2. filter for rows where another column, other_col, is more than 5, and
  3. select only the new column new_col for those rows.

One way of performing these three steps is to just write multiple lines of code, storing temporary objects as you go:

output_1 <- mutate(data, new_col = old_col * 2)
output_2 <- filter(output_1, other_col > 5)
output <- select(output_2, new_col)

This is difficult to understand for multiple reasons. The reader may be tricked into thinking the named output_1 and output_2 objects are important for some reason, while they are just temporary intermediate computations. Further, the reader has to look through and find where output_1 and output_2 are used in each subsequent line.

Another option for doing this would be to compose the functions:

output <- select(filter(mutate(data, new_col = old_col * 2), 
                        other_col > 5), 
                 new_col)

Code like this can also be difficult to understand. Functions compose (reading from left to right) in the opposite order in which they are computed by R (above, mutate happens first, then filter, then select). It is also just a really long line of code to read in one go.

The pipe operator (|>) solves this problem, resulting in cleaner and easier-to-follow code. |> is built into R so you don’t need to load any packages to use it. You can think of the pipe as a physical pipe. It takes the output from the function on the left-hand side of the pipe, and passes it as the first argument to the function on the right-hand side of the pipe. The code below accomplishes the same thing as the previous two code blocks:

output <- data |>
  mutate(new_col = old_col * 2) |>
  filter(other_col > 5) |>
  select(new_col)

Note: You might also have noticed that we split the function calls across lines after the pipe, similar to when we did this earlier in the chapter for long function calls. Again, this is allowed and recommended, especially when the piped function calls create a long line of code. Doing this makes your code more readable. When you do this, it is important to end each line with the pipe operator |> to tell R that your code is continuing onto the next line.

Note: In this textbook, we will be using the base R pipe operator syntax, |>. This base R |> pipe operator was inspired by a previous version of the pipe operator, %>%. The %>% pipe operator is not built into R and is from the magrittr R package. The tidyverse metapackage imports the %>% pipe operator via dplyr (which in turn imports the magrittr R package). There are some other differences between %>% and |> related to more advanced R uses, such as sharing and distributing code as R packages, however, these are beyond the scope of this textbook. We have this note in the book to make the reader aware that %>% exists as it is still commonly used in data analysis code and in many data science books and other resources. In most cases these two pipes are interchangeable and either can be used.

3.8.1 Using |> to combine filter and select

Let’s work with the tidy tidy_lang data set from Section 3.4.3, which contains the number of Canadians reporting their primary language at home and work for five major cities (Toronto, Montréal, Vancouver, Calgary, and Edmonton):

tidy_lang
## # A tibble: 1,070 × 5
##    category                 language           region  most_at_home most_at_work
##    <chr>                    <chr>              <chr>          <int>        <int>
##  1 Aboriginal languages     Aboriginal langua… Toronto           50            0
##  2 Aboriginal languages     Aboriginal langua… Montré…           15            0
##  3 Aboriginal languages     Aboriginal langua… Vancou…           15            0
##  4 Aboriginal languages     Aboriginal langua… Calgary            5            0
##  5 Aboriginal languages     Aboriginal langua… Edmont…           10            0
##  6 Non-Official & Non-Abor… Afrikaans          Toronto          265            0
##  7 Non-Official & Non-Abor… Afrikaans          Montré…           10            0
##  8 Non-Official & Non-Abor… Afrikaans          Vancou…          520           10
##  9 Non-Official & Non-Abor… Afrikaans          Calgary          505           15
## 10 Non-Official & Non-Abor… Afrikaans          Edmont…          300            0
## # … with 1,060 more rows

Suppose we want to create a subset of the data with only the languages and counts of each language spoken most at home for the city of Vancouver. To do this, we can use the functions filter and select. First, we use filter to create a data frame called van_data that contains only values for Vancouver.

van_data <- filter(tidy_lang, region == "Vancouver")
van_data
## # A tibble: 214 × 5
##    category                language            region  most_at_home most_at_work
##    <chr>                   <chr>               <chr>          <int>        <int>
##  1 Aboriginal languages    Aboriginal languag… Vancou…           15            0
##  2 Non-Official & Non-Abo… Afrikaans           Vancou…          520           10
##  3 Non-Official & Non-Abo… Afro-Asiatic langu… Vancou…           10            0
##  4 Non-Official & Non-Abo… Akan (Twi)          Vancou…          125           10
##  5 Non-Official & Non-Abo… Albanian            Vancou…          530           10
##  6 Aboriginal languages    Algonquian languag… Vancou…            0            0
##  7 Aboriginal languages    Algonquin           Vancou…            0            0
##  8 Non-Official & Non-Abo… American Sign Lang… Vancou…          300          140
##  9 Non-Official & Non-Abo… Amharic             Vancou…          540           10
## 10 Non-Official & Non-Abo… Arabic              Vancou…         8680          275
## # … with 204 more rows

We then use select on this data frame to keep only the variables we want:

van_data_selected <- select(van_data, language, most_at_home)
van_data_selected
## # A tibble: 214 × 2
##    language                       most_at_home
##    <chr>                                 <int>
##  1 Aboriginal languages, n.o.s.             15
##  2 Afrikaans                               520
##  3 Afro-Asiatic languages, n.i.e.           10
##  4 Akan (Twi)                              125
##  5 Albanian                                530
##  6 Algonquian languages, n.i.e.              0
##  7 Algonquin                                 0
##  8 American Sign Language                  300
##  9 Amharic                                 540
## 10 Arabic                                 8680
## # … with 204 more rows

Although this is valid code, there is a more readable approach we could take by using the pipe, |>. With the pipe, we do not need to create an intermediate object to store the output from filter. Instead, we can directly send the output of filter to the input of select:

van_data_selected <- tidy_lang |>
        filter(region == "Vancouver") |> 
        select(language, most_at_home)

van_data_selected
## # A tibble: 214 × 2
##    language                       most_at_home
##    <chr>                                 <int>
##  1 Aboriginal languages, n.o.s.             15
##  2 Afrikaans                               520
##  3 Afro-Asiatic languages, n.i.e.           10
##  4 Akan (Twi)                              125
##  5 Albanian                                530
##  6 Algonquian languages, n.i.e.              0
##  7 Algonquin                                 0
##  8 American Sign Language                  300
##  9 Amharic                                 540
## 10 Arabic                                 8680
## # … with 204 more rows

But wait…Why do the select and filter function calls look different in these two examples? Remember: when you use the pipe, the output of the first function is automatically provided as the first argument for the function that comes after it. Therefore you do not specify the first argument in that function call. In the code above, the first line is just the tidy_lang data frame with a pipe. The pipe passes the left-hand side (tidy_lang) to the first argument of the function on the right (filter), so in the filter function you only see the second argument (and beyond). Then again after filter there is a pipe, which passes the result of the filter step to the first argument of the select function. As you can see, both of these approaches—with and without pipes—give us the same output, but the second approach is clearer and more readable.

3.8.2 Using |> with more than two functions

The pipe operator (|>) can be used with any function in R. Additionally, we can pipe together more than two functions. For example, we can pipe together three functions to:

  • filter rows to include only those where the counts of the language most spoken at home are greater than 10,000,
  • select only the columns corresponding to region, language and most_at_home, and
  • arrange the data frame rows in order by counts of the language most spoken at home from smallest to largest.

As we saw in Chapter 1, we can use the tidyverse arrange function to order the rows in the data frame by the values of one or more columns. Here we pass the column name most_at_home to arrange the data frame rows by the values in that column, in ascending order.

large_region_lang <- filter(tidy_lang, most_at_home > 10000) |>
  select(region, language, most_at_home) |>
  arrange(most_at_home)

large_region_lang
## # A tibble: 67 × 3
##    region    language most_at_home
##    <chr>     <chr>           <int>
##  1 Edmonton  Arabic          10590
##  2 Montréal  Tamil           10670
##  3 Vancouver Russian         10795
##  4 Edmonton  Spanish         10880
##  5 Edmonton  French          10950
##  6 Calgary   Arabic          11010
##  7 Calgary   Urdu            11060
##  8 Vancouver Hindi           11235
##  9 Montréal  Armenian        11835
## 10 Toronto   Romanian        12200
## # … with 57 more rows

You will notice above that we passed tidy_lang as the first argument of the filter function. We can also pipe the data frame into the same sequence of functions rather than using it as the first argument of the first function. These two choices are equivalent, and we get the same result.

large_region_lang <- tidy_lang |> 
  filter(most_at_home > 10000) |>
  select(region, language, most_at_home) |>
  arrange(most_at_home)

large_region_lang
## # A tibble: 67 × 3
##    region    language most_at_home
##    <chr>     <chr>           <int>
##  1 Edmonton  Arabic          10590
##  2 Montréal  Tamil           10670
##  3 Vancouver Russian         10795
##  4 Edmonton  Spanish         10880
##  5 Edmonton  French          10950
##  6 Calgary   Arabic          11010
##  7 Calgary   Urdu            11060
##  8 Vancouver Hindi           11235
##  9 Montréal  Armenian        11835
## 10 Toronto   Romanian        12200
## # … with 57 more rows

Now that we’ve shown you the pipe operator as an alternative to storing temporary objects and composing code, does this mean you should never store temporary objects or compose code? Not necessarily! There are times when you will still want to do these things. For example, you might store a temporary object before feeding it into a plot function so you can iteratively change the plot without having to redo all of your data transformations. Additionally, piping many functions can be overwhelming and difficult to debug; you may want to store a temporary object midway through to inspect your result before moving on with further steps.

3.9 Aggregating data with summarize and map

3.9.1 Calculating summary statistics on whole columns

As a part of many data analyses, we need to calculate a summary value for the data (a summary statistic). Examples of summary statistics we might want to calculate are the number of observations, the average/mean value for a column, the minimum value, etc. Oftentimes, this summary statistic is calculated from the values in a data frame column, or columns, as shown in Figure 3.15.

summarize is useful for calculating summary statistics on one or more column(s). In its simplest use case, it creates a new data frame with a single row containing the summary statistic(s) for each column being summarized. The darker, top row of each table represents the column headers.

Figure 3.15: summarize is useful for calculating summary statistics on one or more column(s). In its simplest use case, it creates a new data frame with a single row containing the summary statistic(s) for each column being summarized. The darker, top row of each table represents the column headers.

A useful dplyr function for calculating summary statistics is summarize, where the first argument is the data frame and subsequent arguments are the summaries we want to perform. Here we show how to use the summarize function to calculate the minimum and maximum number of Canadians reporting a particular language as their primary language at home. First a reminder of what region_lang looks like:

region_lang
## # A tibble: 7,490 × 7
##    region  category language  mother_tongue most_at_home most_at_work lang_known
##    <chr>   <chr>    <chr>             <dbl>        <dbl>        <dbl>      <dbl>
##  1 St. Jo… Aborigi… Aborigin…             5            0            0          0
##  2 Halifax Aborigi… Aborigin…             5            0            0          0
##  3 Moncton Aborigi… Aborigin…             0            0            0          0
##  4 Saint … Aborigi… Aborigin…             0            0            0          0
##  5 Saguen… Aborigi… Aborigin…             5            5            0          0
##  6 Québec  Aborigi… Aborigin…             0            5            0         20
##  7 Sherbr… Aborigi… Aborigin…             0            0            0          0
##  8 Trois-… Aborigi… Aborigin…             0            0            0          0
##  9 Montré… Aborigi… Aborigin…            30           15            0         10
## 10 Kingst… Aborigi… Aborigin…             0            0            0          0
## # … with 7,480 more rows

We apply summarize to calculate the minimum and maximum number of Canadians reporting a particular language as their primary language at home, for any region:

summarize(region_lang,
          min_most_at_home = min(most_at_home),
          max_most_at_home = max(most_at_home))
## # A tibble: 1 × 2
##   min_most_at_home max_most_at_home
##              <dbl>            <dbl>
## 1                0          3836770

From this we see that there are some languages in the data set that no one speaks as their primary language at home. We also see that the most commonly spoken primary language at home is spoken by 3,836,770 people.

3.9.2 Calculating summary statistics when there are NAs

In data frames in R, the value NA is often used to denote missing data. Many of the base R statistical summary functions (e.g., max, min, mean, sum, etc) will return NA when applied to columns containing NA values. Usually that is not what we want to happen; instead, we would usually like R to ignore the missing entries and calculate the summary statistic using all of the other non-NA values in the column. Fortunately many of these functions provide an argument na.rm that lets us tell the function what to do when it encounters NA values. In particular, if we specify na.rm = TRUE, the function will ignore missing values and return a summary of all the non-missing entries. We show an example of this combined with summarize below.

First we create a new version of the region_lang data frame, named region_lang_na, that has a seemingly innocuous NA in the first row of the most_at_home column:

region_lang_na
## # A tibble: 7,490 × 7
##    region  category language  mother_tongue most_at_home most_at_work lang_known
##    <chr>   <chr>    <chr>             <dbl>        <dbl>        <dbl>      <dbl>
##  1 St. Jo… Aborigi… Aborigin…             5           NA            0          0
##  2 Halifax Aborigi… Aborigin…             5            0            0          0
##  3 Moncton Aborigi… Aborigin…             0            0            0          0
##  4 Saint … Aborigi… Aborigin…             0            0            0          0
##  5 Saguen… Aborigi… Aborigin…             5            5            0          0
##  6 Québec  Aborigi… Aborigin…             0            5            0         20
##  7 Sherbr… Aborigi… Aborigin…             0            0            0          0
##  8 Trois-… Aborigi… Aborigin…             0            0            0          0
##  9 Montré… Aborigi… Aborigin…            30           15            0         10
## 10 Kingst… Aborigi… Aborigin…             0            0            0          0
## # … with 7,480 more rows

Now if we apply the summarize function as above, we see that we no longer get the minimum and maximum returned, but just an NA instead!

summarize(region_lang_na,
          min_most_at_home = min(most_at_home),
          max_most_at_home = max(most_at_home))
## # A tibble: 1 × 2
##   min_most_at_home max_most_at_home
##              <dbl>            <dbl>
## 1               NA               NA

We can fix this by adding the na.rm = TRUE as explained above:

summarize(region_lang_na,
          min_most_at_home = min(most_at_home, na.rm = TRUE),
          max_most_at_home = max(most_at_home, na.rm = TRUE))
## # A tibble: 1 × 2
##   min_most_at_home max_most_at_home
##              <dbl>            <dbl>
## 1                0          3836770

3.9.3 Calculating summary statistics for groups of rows

A common pairing with summarize is group_by. Pairing these functions together can let you summarize values for subgroups within a data set, as illustrated in Figure 3.16. For example, we can use group_by to group the regions of the tidy_lang data frame and then calculate the minimum and maximum number of Canadians reporting the language as the primary language at home for each of the regions in the data set.

summarize and group_by is useful for calculating summary statistics on one or more column(s) for each group. It creates a new data frame—with one row for each group—containing the summary statistic(s) for each column being summarized. It also creates a column listing the value of the grouping variable. The darker, top row of each table represents the column headers. The gray, blue, and green colored rows correspond to the rows that belong to each of the three groups being represented in this cartoon example.

Figure 3.16: summarize and group_by is useful for calculating summary statistics on one or more column(s) for each group. It creates a new data frame—with one row for each group—containing the summary statistic(s) for each column being summarized. It also creates a column listing the value of the grouping variable. The darker, top row of each table represents the column headers. The gray, blue, and green colored rows correspond to the rows that belong to each of the three groups being represented in this cartoon example.

The group_by function takes at least two arguments. The first is the data frame that will be grouped, and the second and onwards are columns to use in the grouping. Here we use only one column for grouping (region), but more than one can also be used. To do this, list additional columns separated by commas.

group_by(region_lang, region) |>
  summarize(
    min_most_at_home = min(most_at_home),
    max_most_at_home = max(most_at_home)
    )
## # A tibble: 35 × 3
##    region               min_most_at_home max_most_at_home
##    <chr>                           <dbl>            <dbl>
##  1 Abbotsford - Mission                0           137445
##  2 Barrie                              0           182390
##  3 Belleville                          0            97840
##  4 Brantford                           0           124560
##  5 Calgary                             0          1065070
##  6 Edmonton                            0          1050410
##  7 Greater Sudbury                     0           133960
##  8 Guelph                              0           130950
##  9 Halifax                             0           371215
## 10 Hamilton                            0           630380
## # … with 25 more rows

Notice that group_by on its own doesn’t change the way the data looks. In the output below, the grouped data set looks the same, and it doesn’t appear to be grouped by region. Instead, group_by simply changes how other functions work with the data, as we saw with summarize above.

group_by(region_lang, region)
## # A tibble: 7,490 × 7
## # Groups:   region [35]
##    region  category language  mother_tongue most_at_home most_at_work lang_known
##    <chr>   <chr>    <chr>             <dbl>        <dbl>        <dbl>      <dbl>
##  1 St. Jo… Aborigi… Aborigin…             5            0            0          0
##  2 Halifax Aborigi… Aborigin…             5            0            0          0
##  3 Moncton Aborigi… Aborigin…             0            0            0          0
##  4 Saint … Aborigi… Aborigin…             0            0            0          0
##  5 Saguen… Aborigi… Aborigin…             5            5            0          0
##  6 Québec  Aborigi… Aborigin…             0            5            0         20
##  7 Sherbr… Aborigi… Aborigin…             0            0            0          0
##  8 Trois-… Aborigi… Aborigin…             0            0            0          0
##  9 Montré… Aborigi… Aborigin…            30           15            0         10
## 10 Kingst… Aborigi… Aborigin…             0            0            0          0
## # … with 7,480 more rows

3.9.4 Calculating summary statistics on many columns

Sometimes we need to summarize statistics across many columns. An example of this is illustrated in Figure 3.17. In such a case, using summarize alone means that we have to type out the name of each column we want to summarize. In this section we will meet two strategies for performing this task. First we will see how we can do this using summarize + across. Then we will also explore how we can use a more general iteration function, map, to also accomplish this.

summarize + across or map is useful for efficiently calculating summary statistics on many columns at once. The darker, top row of each table represents the column headers.

Figure 3.17: summarize + across or map is useful for efficiently calculating summary statistics on many columns at once. The darker, top row of each table represents the column headers.

summarize and across for calculating summary statistics on many columns

To summarize statistics across many columns, we can use the summarize function we have just recently learned about. However, in such a case, using summarize alone means that we have to type out the name of each column we want to summarize. To do this more efficiently, we can pair summarize with across and use a colon : to specify a range of columns we would like to perform the statistical summaries on. Here we demonstrate finding the maximum value of each of the numeric columns of the region_lang data set.

region_lang |>
  summarize(across(mother_tongue:lang_known, max))
## # A tibble: 1 × 4
##   mother_tongue most_at_home most_at_work lang_known
##           <dbl>        <dbl>        <dbl>      <dbl>
## 1       3061820      3836770      3218725    5600480

Note: Similar to when we use base R statistical summary functions (e.g., max, min, mean, sum, etc) with summarize alone, the use of the summarize + across functions paired with base R statistical summary functions also return NAs when we apply them to columns that contain NAs in the data frame.

To avoid this, again we need to add the argument na.rm = TRUE, but in this case we need to use it a little bit differently. In this case, we need to add a , and then na.rm = TRUE, after specifying the function we want summarize + across to apply, as illustrated below:

region_lang_na |>
  summarize(across(mother_tongue:lang_known, max, na.rm = TRUE))
## # A tibble: 1 × 4
##   mother_tongue most_at_home most_at_work lang_known
##           <dbl>        <dbl>        <dbl>      <dbl>
## 1       3061820      3836770      3218725    5600480

map for calculating summary statistics on many columns

An alternative to summarize and across for applying a function to many columns is the map family of functions. Let’s again find the maximum value of each column of the region_lang data frame, but using map with the max function this time. map takes two arguments: an object (a vector, data frame or list) that you want to apply the function to, and the function that you would like to apply to each column. Note that map does not have an argument to specify which columns to apply the function to. Therefore, we will use the select function before calling map to choose the columns for which we want the maximum.

region_lang |>
  select(mother_tongue:lang_known) |>
  map(max)
## $mother_tongue
## [1] 3061820
## 
## $most_at_home
## [1] 3836770
## 
## $most_at_work
## [1] 3218725
## 
## $lang_known
## [1] 5600480

Note: The map function comes from the purrr package. But since purrr is part of the tidyverse, once we call library(tidyverse) we do not need to load the purrr package separately.

The output looks a bit weird… we passed in a data frame, but the output doesn’t look like a data frame. As it so happens, it is not a data frame, but rather a plain list:

region_lang |>
  select(mother_tongue:lang_known) |>
  map(max) |>
  typeof()
## [1] "list"

So what do we do? Should we convert this to a data frame? We could, but a simpler alternative is to just use a different map function. There are quite a few to choose from, they all work similarly, but their name reflects the type of output you want from the mapping operation. Table 3.3 lists the commonly used map functions as well as their output type.

Table 3.3: The map functions in R.
map function Output
map list
map_lgl logical vector
map_int integer vector
map_dbl double vector
map_chr character vector
map_dfc data frame, combining column-wise
map_dfr data frame, combining row-wise

Let’s get the columns’ maximums again, but this time use the map_dfr function to return the output as a data frame:

region_lang |>
  select(mother_tongue:lang_known) |>
  map_dfr(max)
## # A tibble: 1 × 4
##   mother_tongue most_at_home most_at_work lang_known
##           <dbl>        <dbl>        <dbl>      <dbl>
## 1       3061820      3836770      3218725    5600480

Note: Similar to when we use base R statistical summary functions (e.g., max, min, mean, sum, etc.) with summarize, map functions paired with base R statistical summary functions also return NA values when we apply them to columns that contain NA values.

To avoid this, again we need to add the argument na.rm = TRUE. When we use this with map, we do this by adding a , and then na.rm = TRUE after specifying the function, as illustrated below:

region_lang_na |>
  select(mother_tongue:lang_known) |>
  map_dfr(max, na.rm = TRUE)
## # A tibble: 1 × 4
##   mother_tongue most_at_home most_at_work lang_known
##           <dbl>        <dbl>        <dbl>      <dbl>
## 1       3061820      3836770      3218725    5600480

The map functions are generally quite useful for solving many problems involving repeatedly applying functions in R. Additionally, their use is not limited to columns of a data frame; map family functions can be used to apply functions to elements of a vector, or a list, and even to lists of (nested!) data frames. To learn more about the map functions, see the additional resources section at the end of this chapter.

3.10 Apply functions across many columns with mutate and across

Sometimes we need to apply a function to many columns in a data frame. For example, we would need to do this when converting units of measurements across many columns. We illustrate such a data transformation in Figure 3.18.

mutate and across is useful for applying functions across many columns. The darker, top row of each table represents the column headers.

Figure 3.18: mutate and across is useful for applying functions across many columns. The darker, top row of each table represents the column headers.

For example, imagine that we wanted to convert all the numeric columns in the region_lang data frame from double type to integer type using the as.integer function. When we revisit the region_lang data frame, we can see that this would be the columns from mother_tongue to lang_known.

region_lang
## # A tibble: 7,490 × 7
##    region  category language  mother_tongue most_at_home most_at_work lang_known
##    <chr>   <chr>    <chr>             <dbl>        <dbl>        <dbl>      <dbl>
##  1 St. Jo… Aborigi… Aborigin…             5            0            0          0
##  2 Halifax Aborigi… Aborigin…             5            0            0          0
##  3 Moncton Aborigi… Aborigin…             0            0            0          0
##  4 Saint … Aborigi… Aborigin…             0            0            0          0
##  5 Saguen… Aborigi… Aborigin…             5            5            0          0
##  6 Québec  Aborigi… Aborigin…             0            5            0         20
##  7 Sherbr… Aborigi… Aborigin…             0            0            0          0
##  8 Trois-… Aborigi… Aborigin…             0            0            0          0
##  9 Montré… Aborigi… Aborigin…            30           15            0         10
## 10 Kingst… Aborigi… Aborigin…             0            0            0          0
## # … with 7,480 more rows

To accomplish such a task, we can use mutate paired with across. This works in a similar way for column selection, as we saw when we used summarize + across earlier. As we did above, we again use across to specify the columns using select syntax as well as the function we want to apply on the specified columns. However, a key difference here is that we are using mutate, which means that we get back a data frame with the same number of rows.

region_lang |> 
  mutate(across(mother_tongue:lang_known, as.integer))
## # A tibble: 7,490 × 7
##    region  category language  mother_tongue most_at_home most_at_work lang_known
##    <chr>   <chr>    <chr>             <int>        <int>        <int>      <int>
##  1 St. Jo… Aborigi… Aborigin…             5            0            0          0
##  2 Halifax Aborigi… Aborigin…             5            0            0          0
##  3 Moncton Aborigi… Aborigin…             0            0            0          0
##  4 Saint … Aborigi… Aborigin…             0            0            0          0
##  5 Saguen… Aborigi… Aborigin…             5            5            0          0
##  6 Québec  Aborigi… Aborigin…             0            5            0         20
##  7 Sherbr… Aborigi… Aborigin…             0            0            0          0
##  8 Trois-… Aborigi… Aborigin…             0            0            0          0
##  9 Montré… Aborigi… Aborigin…            30           15            0         10
## 10 Kingst… Aborigi… Aborigin…             0            0            0          0
## # … with 7,480 more rows

We see that we get back a data frame with the same number of columns and rows. The only thing that changes is the transformation we applied to the specified columns (here mother_tongue to lang_known).

3.11 Apply functions across columns within one row with rowwise and mutate

What if you want to apply a function across columns but within one row? We illustrate such a data transformation in Figure 3.19.

rowwise and mutate is useful for applying functions across columns within one row. The darker, top row of each table represents the column headers.

Figure 3.19: rowwise and mutate is useful for applying functions across columns within one row. The darker, top row of each table represents the column headers.

For instance, suppose we want to know the maximum value between mother_tongue, most_at_home, most_at_work and lang_known for each language and region in the region_lang data set. In other words, we want to apply the max function row-wise. We will use the (aptly named) rowwise function in combination with mutate to accomplish this task.

Before we apply rowwise, we will select only the count columns so we can see all the columns in the data frame’s output easily in the book. So for this demonstration, the data set we are operating on looks like this:

region_lang |> 
  select(mother_tongue:lang_known)
## # A tibble: 7,490 × 4
##    mother_tongue most_at_home most_at_work lang_known
##            <dbl>        <dbl>        <dbl>      <dbl>
##  1             5            0            0          0
##  2             5            0            0          0
##  3             0            0            0          0
##  4             0            0            0          0
##  5             5            5            0          0
##  6             0            5            0         20
##  7             0            0            0          0
##  8             0            0            0          0
##  9            30           15            0         10
## 10             0            0            0          0
## # … with 7,480 more rows

Now we apply rowwise before mutate, to tell R that we would like the mutate function to be applied across, and within, a row, as opposed to being applied on a column (which is the default behavior of mutate):

region_lang |> 
  select(mother_tongue:lang_known) |>
  rowwise() |> 
  mutate(maximum = max(c(mother_tongue, 
                         most_at_home, 
                         most_at_work, 
                         lang_known)))
## # A tibble: 7,490 × 5
## # Rowwise: 
##    mother_tongue most_at_home most_at_work lang_known maximum
##            <dbl>        <dbl>        <dbl>      <dbl>   <dbl>
##  1             5            0            0          0       5
##  2             5            0            0          0       5
##  3             0            0            0          0       0
##  4             0            0            0          0       0
##  5             5            5            0          0       5
##  6             0            5            0         20      20
##  7             0            0            0          0       0
##  8             0            0            0          0       0
##  9            30           15            0         10      30
## 10             0            0            0          0       0
## # … with 7,480 more rows

We see that we get an additional column added to the data frame, named maximum, which is the maximum value between mother_tongue, most_at_home, most_at_work and lang_known for each language and region.

Similar to group_by, rowwise doesn’t appear to do anything when it is called by itself. However, we can apply rowwise in combination with other functions to change how these other functions operate on the data. Notice if we used mutate without rowwise, we would have computed the maximum value across all rows rather than the maximum value for each row. Below we show what would have happened had we not used rowwise. In particular, the same maximum value is reported in every single row; this code does not provide the desired result.

region_lang |> 
  select(mother_tongue:lang_known) |>
  mutate(maximum = max(c(mother_tongue, 
                         most_at_home, 
                         most_at_home, 
                         lang_known)))
## # A tibble: 7,490 × 5
##    mother_tongue most_at_home most_at_work lang_known maximum
##            <dbl>        <dbl>        <dbl>      <dbl>   <dbl>
##  1             5            0            0          0 5600480
##  2             5            0            0          0 5600480
##  3             0            0            0          0 5600480
##  4             0            0            0          0 5600480
##  5             5            5            0          0 5600480
##  6             0            5            0         20 5600480
##  7             0            0            0          0 5600480
##  8             0            0            0          0 5600480
##  9            30           15            0         10 5600480
## 10             0            0            0          0 5600480
## # … with 7,480 more rows

3.12 Summary

Cleaning and wrangling data can be a very time-consuming process. However, it is a critical step in any data analysis. We have explored many different functions for cleaning and wrangling data into a tidy format. Table 3.4 summarizes some of the key wrangling functions we learned in this chapter. In the following chapters, you will learn how you can take this tidy data and do so much more with it to answer your burning data science questions!

Table 3.4: Summary of wrangling functions
Function Description
across allows you to apply function(s) to multiple columns
filter subsets rows of a data frame
group_by allows you to apply function(s) to groups of rows
mutate adds or modifies columns in a data frame
map general iteration function
pivot_longer generally makes the data frame longer and narrower
pivot_wider generally makes a data frame wider and decreases the number of rows
rowwise applies functions across columns within one row
separate splits up a character column into multiple columns
select subsets columns of a data frame
summarize calculates summaries of inputs

3.13 Exercises

Practice exercises for the material covered in this chapter can be found in the accompanying worksheets repository in the “Cleaning and wrangling data” row. You can launch an interactive version of the worksheet in your browser by clicking the “launch binder” button. You can also preview a non-interactive version of the worksheet by clicking “view worksheet.” If you instead decide to download the worksheet and run it on your own machine, make sure to follow the instructions for computer setup found in Chapter 13. This will ensure that the automated feedback and guidance that the worksheets provide will function as intended.

3.14 Additional resources

  • As we mentioned earlier, tidyverse is actually an R meta package: it installs and loads a collection of R packages that all follow the tidy data philosophy we discussed above. One of the tidyverse packages is dplyr—a data wrangling workhorse. You have already met many of dplyr’s functions (select, filter, mutate, arrange, summarize, and group_by). To learn more about these functions and meet a few more useful functions, we recommend you check out Chapters 5-9 of the STAT545 online notes. of the data wrangling, exploration, and analysis with R book.
  • The dplyr R package documentation (Wickham, François, et al. 2021) is another resource to learn more about the functions in this chapter, the full set of arguments you can use, and other related functions. The site also provides a very nice cheat sheet that summarizes many of the data wrangling functions from this chapter.
  • Check out the tidyselect R package page (Henry and Wickham 2021) for a comprehensive list of select helpers. These helpers can be used to choose columns in a data frame when paired with the select function (and other functions that use the tidyselect syntax, such as pivot_longer). The documentation for select helpers is a useful reference to find the helper you need for your particular problem.
  • R for Data Science (Wickham and Grolemund 2016) has a few chapters related to data wrangling that go into more depth than this book. For example, the tidy data chapter covers tidy data, pivot_longer/pivot_wider and separate, but also covers missing values and additional wrangling functions (like unite). The data transformation chapter covers select, filter, arrange, mutate, and summarize. And the map functions chapter provides more about the map functions.
  • You will occasionally encounter a case where you need to iterate over items in a data frame, but none of the above functions are flexible enough to do what you want. In that case, you may consider using a for loop.

References

Henry, Lionel, and Hadley Wickham. 2021. tidyselect R package. https://tidyselect.r-lib.org/.
Wickham, Hadley. 2014. “Tidy Data.” Journal of Statistical Software 59 (10): 1–23.
———. 2019. Advanced R. CRC Press. https://adv-r.hadley.nz/.
Wickham, Hadley, Romain François, Lionel Henry, and Kirill Müller. 2021. dplyr R package. https://dplyr.tidyverse.org/.
Wickham, Hadley, and Garrett Grolemund. 2016. R for Data Science: Import, Tidy, Transform, Visualize, and Model Data. O’Reilly. https://r4ds.had.co.nz/.