11 Week 8 : The tidyr package
11.1 What is tidyr?
The same data can be organized (or structured) in multiple ways. However, one special structure of data, called tidy data, is particularly useful for data modeling and visualization. In fact, every function in
tidyverseexpects your data to be organized as tidy data. The term tidy data provide a framework for organizing your data that conform to standards that make data easier to use. Tidy data may still require further efforts for actual data modeling and visualization, but the job will be much easier. Hadley Wickham wrote a paper about tidy data, and you can find it here.The official tidyverse website (https://tidyr.tidyverse.org/) introduces the
tidyrpackage as follows:
"The goal of tidyr is to help you create tidy data. Tidy data is data where:
- Every column is variable.
- Every row is an observation.
- Every cell is a single value.
Tidy data describes a standard way of storing data that is used wherever possible throughout the tidyverse. If you ensure that your data is tidy, you’ll spend less time fighting with the tools and more time working on your analysis. Learn more about tidy data in vignette(“tidy-data”)."
- The definition of tidy data requires the definitions of variables and observations. Hadley’s paper present the definition as follows:
“A dataset is a collection of values, usually either numbers (if quantitative) or strings (if qualitative). Values are organised in two ways. Every value belongs to a variable and an observation. A variable contains all values that measure the same underlying attribute (like height, temperature, duration) across units. An observation contains all values measured on the same unit (like a person, or a day, or a race) across attributes.”
- It is important to keep in mind that the definition of a variable might be changed depending on your purpose (from A Tidyverse Cookbook):
“As you work with data, you will be surprised to realize that what is a variable (or observation) will depend less on the data itself and more on what you are trying to do with it. With enough mental flexibility, you can consider anything to be a variable. However, some variables will be more useful than others for any specific task.”
- This lecture focuses four key functions in the
tidyrpackage:pivot_longer()lengthens data, increasing the number of rows and decreasing the number of columns (i.e., turning columns into rows)pivot_wider()widens data, increasing the number of columns and decreasing the number of rows (i.e., turning rows into columns)separate()separates a character column into multiple columns with a regular expression or numeric locationsunite()unites multiple columns into one by pasting strings together
11.2 An example
iris“gives the measurements in centimeters of the variables sepal length and width and petal length and width, respectively, for 50 flowers from each of 3 species of iris. The species are Iris setosa, versicolor, and virginica.”
# as_tibble() convert a data frame into a tibble
# recall the definition of tidy data
# - Every column is variable
# - Every row is an observation
# - Every cell is a single value
# Does the iris data satisfy the three conditions?
as_tibble(iris)## # A tibble: 150 x 5
## Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## <dbl> <dbl> <dbl> <dbl> <fct>
## 1 5.1 3.5 1.4 0.2 setosa
## 2 4.9 3 1.4 0.2 setosa
## 3 4.7 3.2 1.3 0.2 setosa
## 4 4.6 3.1 1.5 0.2 setosa
## 5 5 3.6 1.4 0.2 setosa
## 6 5.4 3.9 1.7 0.4 setosa
## 7 4.6 3.4 1.4 0.3 setosa
## 8 5 3.4 1.5 0.2 setosa
## 9 4.4 2.9 1.4 0.2 setosa
## 10 4.9 3.1 1.5 0.1 setosa
## # ... with 140 more rows- Suppose that you need to create the following figure using
ggplot2.- Recall your
ggplot2class and sketch the code - Is there any issue in the
irisdata that prevents you from writing the code?
- Recall your
- In order to easily create the figure above, you need to reorganize (or restructure or reshape) the
irisdata as the following:
# STEP 1: pivot_longer() lengthens data
iris %>%
pivot_longer(cols = -Species, names_to = "Measures", values_to = "Values")## # A tibble: 600 x 3
## Species Measures Values
## <fct> <chr> <dbl>
## 1 setosa Sepal.Length 5.1
## 2 setosa Sepal.Width 3.5
## 3 setosa Petal.Length 1.4
## 4 setosa Petal.Width 0.2
## 5 setosa Sepal.Length 4.9
## 6 setosa Sepal.Width 3
## 7 setosa Petal.Length 1.4
## 8 setosa Petal.Width 0.2
## 9 setosa Sepal.Length 4.7
## 10 setosa Sepal.Width 3.2
## # ... with 590 more rows# STEP 2: `separate()` separates a character column into multiple columns
# recall the definition of tidy data
# - Every column is variable
# - Every row is an observation
# - Every cell is a single value
# Does this reorganized iris data satisfy the three conditions?
iris %>%
pivot_longer(cols = -Species, names_to = "Measures", values_to = "Values") %>%
separate(col = Measures, into = c("Part", "Measure"))## # A tibble: 600 x 4
## Species Part Measure Values
## <fct> <chr> <chr> <dbl>
## 1 setosa Sepal Length 5.1
## 2 setosa Sepal Width 3.5
## 3 setosa Petal Length 1.4
## 4 setosa Petal Width 0.2
## 5 setosa Sepal Length 4.9
## 6 setosa Sepal Width 3
## 7 setosa Petal Length 1.4
## 8 setosa Petal Width 0.2
## 9 setosa Sepal Length 4.7
## 10 setosa Sepal Width 3.2
## # ... with 590 more rows# STEP 3: `ggplot()` creates the plot
# Can you see the advantage of the reorganized iris data?
iris %>%
pivot_longer(cols = -Species, names_to = "Measures", values_to = "Values") %>%
separate(col = Measures, into = c("Part", "Measure")) %>%
ggplot(aes(x = Species, y = Values, color = Part)) +
geom_jitter() +
facet_grid(cols = vars(Measure)) +
theme_minimal()
11.3 pivot_longer()
“A common problem is a dataset where some of the column names are not names of variables, but values of a variable. Take table4a: the column names 1999 and 2000 represent values of the year variable, the values in the 1999 and 2000 columns represent values of the cases variable, and each row represents two observations, not one.” (from 12.3 Pivoting in R for Data Science)
## # A tibble: 3 x 3
## country `1999` `2000`
## * <chr> <int> <int>
## 1 Afghanistan 745 2666
## 2 Brazil 37737 80488
## 3 China 212258 213766"To tidy a dataset like this, we need to pivot the offending columns into a new pair of variables. To describe that operation we need three parameters:
- The set of columns whose names are values, not variables. In this example, those are the columns 1999 and 2000.
- The name of the variable to move the column names to. Here it is year.
- The name of the variable to move the column values to. Here it’s cases.
Together those parameters generate the call to pivot_longer():"
## # A tibble: 6 x 3
## country year cases
## <chr> <chr> <int>
## 1 Afghanistan 1999 745
## 2 Afghanistan 2000 2666
## 3 Brazil 1999 37737
## 4 Brazil 2000 80488
## 5 China 1999 212258
## 6 China 2000 213766- Usage
- pivot_longer(data, cols, names_to, values_to)
- cols =
columns to pivot into longer format - names_to = A string specifying the name of the column to create from the data stored in the column names of data
- values_to = A string specifying the name of the column to create from the data stored in cell values
- cols =
- check here for the full usage.
- pivot_longer(data, cols, names_to, values_to)
- Exercise 1
relig_income is a tibble in tidyr which contains the result of religion and income survey.
## # A tibble: 18 x 11
## religion `<$10k` `$10-20k` `$20-30k` `$30-40k` `$40-50k` `$50-75k` `$75-100k`
## <chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 Agnostic 27 34 60 81 76 137 122
## 2 Atheist 12 27 37 52 35 70 73
## 3 Buddhist 27 21 30 34 33 58 62
## 4 Catholic 418 617 732 670 638 1116 949
## 5 Don’t k~ 15 14 15 11 10 35 21
## 6 Evangel~ 575 869 1064 982 881 1486 949
## 7 Hindu 1 9 7 9 11 34 47
## 8 Histori~ 228 244 236 238 197 223 131
## 9 Jehovah~ 20 27 24 24 21 30 15
## 10 Jewish 19 19 25 25 30 95 69
## 11 Mainlin~ 289 495 619 655 651 1107 939
## 12 Mormon 29 40 48 51 56 112 85
## 13 Muslim 6 7 9 10 9 23 16
## 14 Orthodox 13 17 23 32 32 47 38
## 15 Other C~ 9 7 11 13 13 14 18
## 16 Other F~ 20 33 40 46 49 63 46
## 17 Other W~ 5 2 3 4 2 7 3
## 18 Unaffil~ 217 299 374 365 341 528 407
## # ... with 3 more variables: `$100-150k` <dbl>, `>150k` <dbl>, `Don't
## # know/refused` <dbl>Is relig_income a tidy data? why?
If not, tidy relig_income.
- Exercise 2
Let’s create an hypothetical stocks data which contains the prices of three stocks.
stocks <- tibble(
time = as.Date('2009-01-01') + 0:1,
Stock_X = c(1000,1200),
Stock_Y = c(900,1000),
Stock_Z = c(1400,1000)
)
stocks## # A tibble: 2 x 4
## time Stock_X Stock_Y Stock_Z
## <date> <dbl> <dbl> <dbl>
## 1 2009-01-01 1000 900 1400
## 2 2009-01-02 1200 1000 1000Is stocks tidy? why?
If not, tidy stocks.
## # A tibble: 6 x 3
## time stock value
## <date> <chr> <dbl>
## 1 2009-01-01 Stock_X 1000
## 2 2009-01-01 Stock_Y 900
## 3 2009-01-01 Stock_Z 1400
## 4 2009-01-02 Stock_X 1200
## 5 2009-01-02 Stock_Y 1000
## 6 2009-01-02 Stock_Z 1000- Exercise 3
billboard is a data set in tidyr. This is an typical example of a longitudinal data in a wide form.
## # A tibble: 317 x 79
## artist track date.entered wk1 wk2 wk3 wk4 wk5 wk6 wk7 wk8
## <chr> <chr> <date> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 2 Pac Baby~ 2000-02-26 87 82 72 77 87 94 99 NA
## 2 2Ge+h~ The ~ 2000-09-02 91 87 92 NA NA NA NA NA
## 3 3 Doo~ Kryp~ 2000-04-08 81 70 68 67 66 57 54 53
## 4 3 Doo~ Loser 2000-10-21 76 76 72 69 67 65 55 59
## 5 504 B~ Wobb~ 2000-04-15 57 34 25 17 17 31 36 49
## 6 98^0 Give~ 2000-08-19 51 39 34 26 26 19 2 2
## 7 A*Tee~ Danc~ 2000-07-08 97 97 96 95 100 NA NA NA
## 8 Aaliy~ I Do~ 2000-01-29 84 62 51 41 38 35 35 38
## 9 Aaliy~ Try ~ 2000-03-18 59 53 38 28 21 18 16 14
## 10 Adams~ Open~ 2000-08-26 76 76 74 69 68 67 61 58
## # ... with 307 more rows, and 68 more variables: wk9 <dbl>, wk10 <dbl>,
## # wk11 <dbl>, wk12 <dbl>, wk13 <dbl>, wk14 <dbl>, wk15 <dbl>, wk16 <dbl>,
## # wk17 <dbl>, wk18 <dbl>, wk19 <dbl>, wk20 <dbl>, wk21 <dbl>, wk22 <dbl>,
## # wk23 <dbl>, wk24 <dbl>, wk25 <dbl>, wk26 <dbl>, wk27 <dbl>, wk28 <dbl>,
## # wk29 <dbl>, wk30 <dbl>, wk31 <dbl>, wk32 <dbl>, wk33 <dbl>, wk34 <dbl>,
## # wk35 <dbl>, wk36 <dbl>, wk37 <dbl>, wk38 <dbl>, wk39 <dbl>, wk40 <dbl>,
## # wk41 <dbl>, wk42 <dbl>, wk43 <dbl>, wk44 <dbl>, wk45 <dbl>, wk46 <dbl>,
## # wk47 <dbl>, wk48 <dbl>, wk49 <dbl>, wk50 <dbl>, wk51 <dbl>, wk52 <dbl>,
## # wk53 <dbl>, wk54 <dbl>, wk55 <dbl>, wk56 <dbl>, wk57 <dbl>, wk58 <dbl>,
## # wk59 <dbl>, wk60 <dbl>, wk61 <dbl>, wk62 <dbl>, wk63 <dbl>, wk64 <dbl>,
## # wk65 <dbl>, wk66 <lgl>, wk67 <lgl>, wk68 <lgl>, wk69 <lgl>, wk70 <lgl>,
## # wk71 <lgl>, wk72 <lgl>, wk73 <lgl>, wk74 <lgl>, wk75 <lgl>, wk76 <lgl>The following code makes billboard longer.
billboard %>%
pivot_longer(
cols = starts_with("wk"),
names_to = "week",
values_to = "rank",
values_drop_na = TRUE
)## # A tibble: 5,307 x 5
## artist track date.entered week rank
## <chr> <chr> <date> <chr> <dbl>
## 1 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk1 87
## 2 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk2 82
## 3 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk3 72
## 4 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk4 77
## 5 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk5 87
## 6 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk6 94
## 7 2 Pac Baby Don't Cry (Keep... 2000-02-26 wk7 99
## 8 2Ge+her The Hardest Part Of ... 2000-09-02 wk1 91
## 9 2Ge+her The Hardest Part Of ... 2000-09-02 wk2 87
## 10 2Ge+her The Hardest Part Of ... 2000-09-02 wk3 92
## # ... with 5,297 more rows11.4 pivot_wider()
“pivot_wider() is the opposite of pivot_longer(). You use it when an observation is scattered across multiple rows. For example, take table2: an observation is a country in a year, but each observation is spread across two rows.”
## # A tibble: 12 x 4
## country year type count
## <chr> <int> <chr> <int>
## 1 Afghanistan 1999 cases 745
## 2 Afghanistan 1999 population 19987071
## 3 Afghanistan 2000 cases 2666
## 4 Afghanistan 2000 population 20595360
## 5 Brazil 1999 cases 37737
## 6 Brazil 1999 population 172006362
## 7 Brazil 2000 cases 80488
## 8 Brazil 2000 population 174504898
## 9 China 1999 cases 212258
## 10 China 1999 population 1272915272
## 11 China 2000 cases 213766
## 12 China 2000 population 1280428583To tidy this up, we first analyse the representation in similar way to pivot_longer(). This time, however, we only need two parameters: * The column to take variable names from. Here, it’s type. * The column to take values from. Here it’s count. Once we’ve figured that out, we can use pivot_wider(), as shown programmatically below, and visually in Figure 12.3.
## # A tibble: 6 x 4
## country year cases population
## <chr> <int> <int> <int>
## 1 Afghanistan 1999 745 19987071
## 2 Afghanistan 2000 2666 20595360
## 3 Brazil 1999 37737 172006362
## 4 Brazil 2000 80488 174504898
## 5 China 1999 212258 1272915272
## 6 China 2000 213766 1280428583- Usage
- pivot_wider(data, names_from, values_from)
- names_from = A string specifying the name of the column to get the name of the output column
- values_from = A string specifying the name of the column to get the cell values from
- check here for the full usage.
- pivot_wider(data, names_from, values_from)
11.5 separate()
separate() separates a single column into multiple columns. By default, separate() will split values whereever it sees a non-alphanumeric characters.
## # A tibble: 6 x 3
## country year rate
## * <chr> <int> <chr>
## 1 Afghanistan 1999 745/19987071
## 2 Afghanistan 2000 2666/20595360
## 3 Brazil 1999 37737/172006362
## 4 Brazil 2000 80488/174504898
## 5 China 1999 212258/1272915272
## 6 China 2000 213766/1280428583# By default, any non-alphanumeric character will be a delimiter.
table3 %>%
separate(rate, into = c("cases", "population"))## # A tibble: 6 x 4
## country year cases population
## <chr> <int> <chr> <chr>
## 1 Afghanistan 1999 745 19987071
## 2 Afghanistan 2000 2666 20595360
## 3 Brazil 1999 37737 172006362
## 4 Brazil 2000 80488 174504898
## 5 China 1999 212258 1272915272
## 6 China 2000 213766 1280428583# you can specify your own delimiter using sep
# note that the types of cases and population are characters
# separate() keeps the original type
table3 %>%
separate(rate, into = c("cases", "population"), sep = "/")## # A tibble: 6 x 4
## country year cases population
## <chr> <int> <chr> <chr>
## 1 Afghanistan 1999 745 19987071
## 2 Afghanistan 2000 2666 20595360
## 3 Brazil 1999 37737 172006362
## 4 Brazil 2000 80488 174504898
## 5 China 1999 212258 1272915272
## 6 China 2000 213766 1280428583# convert = TRUE will convert to better type
table3 %>%
separate(rate, into = c("cases", "population"), convert = TRUE)## # A tibble: 6 x 4
## country year cases population
## <chr> <int> <int> <int>
## 1 Afghanistan 1999 745 19987071
## 2 Afghanistan 2000 2666 20595360
## 3 Brazil 1999 37737 172006362
## 4 Brazil 2000 80488 174504898
## 5 China 1999 212258 1272915272
## 6 China 2000 213766 1280428583# separate() will interpret the integers as positions to split at
table3 %>%
separate(year, into = c("century", "year"), sep = 2)## # A tibble: 6 x 4
## country century year rate
## <chr> <chr> <chr> <chr>
## 1 Afghanistan 19 99 745/19987071
## 2 Afghanistan 20 00 2666/20595360
## 3 Brazil 19 99 37737/172006362
## 4 Brazil 20 00 80488/174504898
## 5 China 19 99 212258/1272915272
## 6 China 20 00 213766/128042858311.6 unite()
unite() combines multiple columns into a single column.
## # A tibble: 6 x 4
## country century year rate
## * <chr> <chr> <chr> <chr>
## 1 Afghanistan 19 99 745/19987071
## 2 Afghanistan 20 00 2666/20595360
## 3 Brazil 19 99 37737/172006362
## 4 Brazil 20 00 80488/174504898
## 5 China 19 99 212258/1272915272
## 6 China 20 00 213766/1280428583## # A tibble: 6 x 3
## country new rate
## <chr> <chr> <chr>
## 1 Afghanistan 19_99 745/19987071
## 2 Afghanistan 20_00 2666/20595360
## 3 Brazil 19_99 37737/172006362
## 4 Brazil 20_00 80488/174504898
## 5 China 19_99 212258/1272915272
## 6 China 20_00 213766/1280428583## # A tibble: 6 x 3
## country new rate
## <chr> <chr> <chr>
## 1 Afghanistan 1999 745/19987071
## 2 Afghanistan 2000 2666/20595360
## 3 Brazil 1999 37737/172006362
## 4 Brazil 2000 80488/174504898
## 5 China 1999 212258/1272915272
## 6 China 2000 213766/1280428583Cormen, Thomas H, Charles E Leiserson, Ronald L Rivest, and Clifford Stein. 2009. Introduction to Algorithms. MIT press.
Wickham, Hadley. 2016. Ggplot2: Elegant Graphics for Data Analysis. springer.
Wickham, H., and G. Grolemund. 2016. R for Data Science: Import, Tidy, Transform, Visualize, and Model Data. O’Reilly Media. https://books.google.co.kr/books?id=vfi3DQAAQBAJ.