library(dplyr)
# ggplot2 bundles the `mpg` dataset (MIT license, same as ggplot2 itself)
data(mpg, package = "ggplot2")14 Your First Data Wrangling
Learning Objectives: After this chapter you will be able to: - Use the five core
dplyrverbs:filter(),select(),mutate(),arrange(),summarise(). - Chain steps with the native base pipe|>. - Know where to go next after this book.
We use the native base pipe |> (standard since R 4.1.0; zero external dependencies). We do not use %>%.
14.1 The five verbs
mpg |>
filter(manufacturer == "audi") |>
select(manufacturer, model, displ, hwy) |>
mutate(km_per_litre = hwy * 0.425144) |>
arrange(desc(hwy))# A tibble: 18 Γ 5
manufacturer model displ hwy km_per_litre
<chr> <chr> <dbl> <int> <dbl>
1 audi a4 2 31 13.2
2 audi a4 2 30 12.8
3 audi a4 1.8 29 12.3
4 audi a4 1.8 29 12.3
5 audi a4 quattro 2 28 11.9
6 audi a4 3.1 27 11.5
7 audi a4 quattro 2 27 11.5
8 audi a4 2.8 26 11.1
9 audi a4 2.8 26 11.1
10 audi a4 quattro 1.8 26 11.1
11 audi a4 quattro 1.8 25 10.6
12 audi a4 quattro 2.8 25 10.6
13 audi a4 quattro 2.8 25 10.6
14 audi a4 quattro 3.1 25 10.6
15 audi a4 quattro 3.1 25 10.6
16 audi a6 quattro 3.1 25 10.6
17 audi a6 quattro 2.8 24 10.2
18 audi a6 quattro 4.2 23 9.78mpg |>
summarise(mean_hwy = mean(hwy), n = dplyr::n())# A tibble: 1 Γ 2
mean_hwy n
<dbl> <int>
1 23.4 234Common Mistake:
mean()on a vector withNAreturnsNA. Addna.rm = TRUEwhen missing values are present.
Try it in your browser
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14.2 Practice Check
Knowledge Check: Filter then select What does mpg |> filter(manufacturer == "audi") |> select(model, hwy) return?
Click to reveal answer
Answer: A data frame with two columns (model, hwy) containing only the rows where manufacturer is "audi".
filter() keeps matching rows; select() keeps matching columns. The |> pipe passes the left-hand result into the first argument of the right-hand function.
Knowledge Check: mutate() adds columns What does mpg |> mutate(km_per_litre = hwy * 0.425144) return, and how many rows does the result have compared to mpg?
Click to reveal answer
Answer: The full mpg data with one extra column (km_per_litre); the row count is unchanged.
mutate() adds (or modifies) columns row-wise β it never adds or removes rows. Use filter() to change row counts.
Knowledge Check: arrange() ordering After mpg |> arrange(desc(hwy)), which kind of car sits in row 1?
Click to reveal answer
Answer: The car with the highest highway mileage (hwy).
arrange() sorts ascending by default; wrapping in desc() flips it to descending, so the maximum comes first.
Knowledge Check: summarise() collapses rows How many rows does mpg |> summarise(mean_hwy = mean(hwy), n = dplyr::n()) return, and why?
Click to reveal answer
Answer: Exactly one row: the mean of hwy across the whole dataset and the total row count.
group_by(), summarise() collapses the entire data frame into a single summary row.
Knowledge Check: Reading a pipeline Read this pipeline aloud in order: mpg |> filter(manufacturer == "audi") |> select(model, hwy) |> arrange(desc(hwy)). What question does it answer?
Click to reveal answer
Answer: βAmong Audi models, which has the best (and worst) highway mileage?β β it keeps only Audis, keeps the model and mileage columns, and sorts best-first.
Explanation: Pipelines read top-to-bottom as a sequence of verbs; narrating each step in plain words is the fastest way to check you built the right one.14.3 Where to go next
Where to go next: - Book: R for Data Science (2e) β start at Chapter 1 - Courses: Rsquared Academy Data Wrangling series - More books: ebooks.rsquaredacademy.com