Reshaping Data | The School of Data

Data Wrangling in R with dplyr and tidyr Reshaping Data

What is reshaping data?

Reshaping data is the process of changing the layout of your data to make it easier to analyze. Data can be in different formats, such as wide or long, depending on how it is structured. We can convert data from wide to long or from long to wide formats.

What we’ll learn in this section

Goals In this section, we'll:

understand the concepts of wide and long data formats
use pivot_longer() to reshape data from wide to long format
use pivot_wider() to reshape data from long to wide format

Wide Format

In wide format, each row represents a unique observation, and each column represents a variable. This format is useful for storing data where each variable has its own column.

Wide Format

Consider the following data frame students in wide format:

 # A tibble: 4 × 4
     id name  section study  play
  <int> <chr> <chr>   <dbl> <dbl>
1     1 Alia  A           2     5
2     2 Bala  B           8     5
3     3 Cara  A          NA    10
4     4 Dana  B           4    10

In this wide format, each student’s study and play hours are stored in separate columns.

Long Format

In long format, each row represents a unique observation. This format is useful for storing data where each variable is stored in a single column.

Long Format

Consider the following data frame activity_hours in long format:

 # A tibble: 8 × 4
     id name  section activity hours
  <dbl> <chr> <chr>   <chr>    <dbl>
1     1 Alia  A       study        2
2     1 Alia  A       play         5
3     2 Bala  B       study        8
4     2 Bala  B       play         5
5     3 Cara  A       study       NA
6     3 Cara  A       play        10
7     4 Dana  B       study        4
8     4 Dana  B       play        10

In this long format, each student’s study and play hours are stored in separate rows.

 activity_hours <- data.frame(
  id = c(1, 1, 2, 2, 3, 3, 4, 4),
  name = c("Alia", "Alia", "Bala", "Bala", "Cara", "Cara", "Dana", "Dana"),
  section = c("A", "A", "B", "B", "A", "A", "B", "B"),
  activity = c("study", "play", "study", "play", "study", "play", "study", "play"),
  hours = c(2, 5, 8, 5, NA, 10, 4, 10)
)

Reshaping Data with tidyr

The tidyr package in R provides functions to reshape data between wide and long formats. Two key functions for reshaping data are pivot_longer() and pivot_wider() .

Pivot Longer

The pivot_longer() function is used to reshape data from wide to long format.

Syntax

 pivot_longer(data, cols, names_to, values_to)

Example

Let’s use the pivot_longer() function to reshape the students data frame from wide to long format.

id	name	section	study	play
1	Alia	A	2	5
2	Bala	B	8	5
3	Cara	A	NA	10
4	Dana	B	4	10

 students |> 
  pivot_longer(cols = c(study, play), 
               names_to = "activity", 
               values_to = "hours")

Output:

 # A tibble: 8 × 5
     id name  section activity hours
  <int> <chr> <chr>   <chr>    <dbl>
1     1 Alia  A       study        2
2     1 Alia  A       play         5
3     2 Bala  B       study        8
4     2 Bala  B       play         5
5     3 Cara  A       study       NA
6     3 Cara  A       play        10
7     4 Dana  B       study        4
8     4 Dana  B       play        10

Before (Wide Format)

id	name	section	study	play
1	Alia	A	2	5
2	Bala	B	8	5
3	Cara	A	NA	10
4	Dana	B	4	10

After (Long Format)

id	name	section	activity	hours
1	Alia	A	study	2
1	Alia	A	play	5
2	Bala	B	study	8
2	Bala	B	play	5
3	Cara	A	study	NA
3	Cara	A	play	10
4	Dana	B	study	4
4	Dana	B	play	10

Exercise 5.1: Pivot Longer

Use pivot_longer() to reshape the following grades data frame from wide to long format. The new columns should be named “subject” and “score”.

student	math	science	history
Alice	85	92	78
Bob	91	85	89
Charlie	76	88	95

 grades |>
  pivot_longer(cols = c(math, science, history), 
               names_to = "subject", 
               values_to = "score")

Output:

 # A tibble: 9 × 3
  student subject score
  <chr>   <chr>   <dbl>
1 Alice   math       85
2 Alice   science    92
3 Alice   history    78
4 Bob     math       91
5 Bob     science    85
6 Bob     history    89
7 Charlie math       76
8 Charlie science    88
9 Charlie history    95

Pivot Wider

The pivot_wider() function is used to reshape data from long to wide format.

Syntax

 pivot_wider(data, names_from, values_from)

Example

Let’s use the pivot_wider() function to reshape a long format data frame into a wide format.

id	name	section	activity	hours
1	Alia	A	study	2
1	Alia	A	play	5
2	Bala	B	study	8
2	Bala	B	play	5
3	Cara	A	study	NA
3	Cara	A	play	10
4	Dana	B	study	4
4	Dana	B	play	10

 activity_hours |>
  pivot_wider(names_from = activity, values_from = hours)

Output:

 # A tibble: 4 × 5
     id name  section study  play
  <dbl> <chr> <chr>   <dbl> <dbl>
1     1 Alia  A           2     5
2     2 Bala  B           8     5
3     3 Cara  A          NA    10
4     4 Dana  B           4    10

Before (Long Format)

id	name	section	activity	hours
1	Alia	A	study	2
1	Alia	A	play	5
2	Bala	B	study	8
2	Bala	B	play	5
3	Cara	A	study	NA
3	Cara	A	play	10
4	Dana	B	study	4
4	Dana	B	play	10

After (Wide Format)

id	name	section	study	play
1	Alia	A	2	5
2	Bala	B	8	5
3	Cara	A	NA	10
4	Dana	B	4	10

Exercise 5.2: Pivot Wider

Use pivot_wider() to reshape the following grades_long data frame from long to wide format. The new columns should be named “math”, “science”, and “history”.

student	subject	score
Alice	math	85
Alice	science	92
Alice	history	78
Bob	math	91
Bob	science	85
Bob	history	89
Charlie	math	76
Charlie	science	88
Charlie	history	95

 grades_long |>
  pivot_wider(names_from = subject, values_from = score)

Output:

 # A tibble: 3 × 4
  student  math science history
  <chr>   <dbl>   <dbl>   <dbl>
1 Alice      85      92      78
2 Bob        91      85      89
3 Charlie    76      88      95

Review

In this section, we’ve learned about reshaping data between wide and long formats using the tidyr package in R.

Quiz

Summary In this section, we learned how to:

Reshape data from wide to long format using pivot_longer()
Reshape data from long to wide format using pivot_wider()

We are almost done with the course! In the next section, we will conclude our learning journey with a summary of what we’ve covered and some self-assessment tasks. 🎉

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On this page

- What is reshaping data?
- What we’ll learn in this section
- Pivot Longer
- Pivot Wider
- Review