AE 09: Writing functions
Suggested answers
Application exercise
Answers
Packages
We will use the following packages in this application exercise.
- {tidyverse}: For data import, wrangling, and visualization.
Write a vector function
Your turn: Write a function that performs the Box-Cox power transformation using the value of (non-zero) lambda (\(\lambda\)) supplied.
\[ bc = \frac{x^{\lambda} - 1}{\lambda} \text{ for }\lambda \ne 0 \]
Set the default \(\lambda = 1\).
# write function
to_box_cox <- function(x, lambda = 1) {
(x^lambda - 1) / lambda
}
# test on data values
vals |>
mutate(x_bc = to_box_cox(x, lambda = 0.3))# A tibble: 10,000 × 2
x x_bc
<dbl> <dbl>
1 0.0843 -1.75
2 0.0577 -1.92
3 0.133 -1.51
4 0.00316 -2.74
5 0.00562 -2.63
6 0.0317 -2.15
7 0.0314 -2.15
8 0.0145 -2.40
9 0.273 -1.08
10 0.00292 -2.75
# ℹ 9,990 more rowsvals |>
mutate(x_bc = to_box_cox(x, lambda = 0.3)) |>
ggplot(mapping = aes(x = x_bc)) +
geom_histogram()`stat_bin()` using `bins = 30`. Pick better value with `binwidth`.
Your turn: Revise your function to check if \(\lambda \ne 0\). If \(\lambda = 0\), generate an error with an informative message.
TipGenerating conditions in R
Conditions in R are raised by three distinct functions:
Error in `mutate()`:
ℹ In argument: `x_bc = to_box_cox(x, lambda = 0)`.
Caused by error in `to_box_cox()`:
! Lambda set to 0. Re-run with a non-zero value for lambda.Demonstration: Revise your function for:
\[ bc = \begin{cases} \frac{x^{\lambda} - 1}{\lambda} & \text{for }\lambda \ne 0\\ \ln(x) & \text{for }\lambda = 0 \end{cases} \]
# A tibble: 10,000 × 2
x x_bc
<dbl> <dbl>
1 0.0843 -2.47
2 0.0577 -2.85
3 0.133 -2.02
4 0.00316 -5.76
5 0.00562 -5.18
6 0.0317 -3.45
7 0.0314 -3.46
8 0.0145 -4.23
9 0.273 -1.30
10 0.00292 -5.84
# ℹ 9,990 more rowsWrite a data frame function
Your turn: Write a function to calculate the median, maximum and minimum values of a variable grouped by another variable. Test it using the penguins data set.
# basic summary function
my_summary <- function(df, summary_var, group_var) {
df |>
group_by({{ group_var }}) |>
summarize(
median = median({{ summary_var }}, na.rm = TRUE),
minimum = min({{ summary_var }}, na.rm = TRUE),
maximum = max({{ summary_var }}, na.rm = TRUE),
.groups = "drop"
)
}
my_summary(df = penguins, summary_var = bill_len, group_var = species)# A tibble: 3 × 4
species median minimum maximum
<fct> <dbl> <dbl> <dbl>
1 Adelie 38.8 32.1 46
2 Chinstrap 49.6 40.9 58
3 Gentoo 47.3 40.9 59.6# default NULL for the grouping variable
my_summary <- function(df, summary_var, group_var = NULL) {
df |>
group_by({{ group_var }}) |>
summarize(
median = median({{ summary_var }}, na.rm = TRUE),
minimum = min({{ summary_var }}, na.rm = TRUE),
maximum = max({{ summary_var }}, na.rm = TRUE),
.groups = "drop"
)
}
my_summary(df = penguins, summary_var = bill_len)# A tibble: 1 × 3
median minimum maximum
<dbl> <dbl> <dbl>
1 44.4 32.1 59.6my_summary(df = penguins, summary_var = bill_len, group_var = species)# A tibble: 3 × 4
species median minimum maximum
<fct> <dbl> <dbl> <dbl>
1 Adelie 38.8 32.1 46
2 Chinstrap 49.6 40.9 58
3 Gentoo 47.3 40.9 59.6# use pick() to allow for multiple grouping variables
my_summary <- function(df, summary_var, group_var = NULL) {
df |>
group_by(pick({{ group_var }})) |>
summarize(
median = median({{ summary_var }}, na.rm = TRUE),
minimum = min({{ summary_var }}, na.rm = TRUE),
maximum = max({{ summary_var }}, na.rm = TRUE),
.groups = "drop"
)
}
my_summary(penguins, bill_len, c(species, island))# A tibble: 5 × 5
species island median minimum maximum
<fct> <fct> <dbl> <dbl> <dbl>
1 Adelie Biscoe 38.7 34.5 45.6
2 Adelie Dream 38.6 32.1 44.1
3 Adelie Torgersen 38.9 33.5 46
4 Chinstrap Dream 49.6 40.9 58
5 Gentoo Biscoe 47.3 40.9 59.6Acknowledgments
- Exercises are derived from From R User to R Programmer and licensed under CC BY 4.0.
NoteSession information
sessioninfo::session_info()─ Session info ───────────────────────────────────────────────────────────────
setting value
version R version 4.5.1 (2025-06-13)
os macOS Tahoe 26.0.1
system aarch64, darwin20
ui X11
language (EN)
collate C.UTF-8
ctype C.UTF-8
tz America/New_York
date 2025-10-01
pandoc 3.6.3 @ /Applications/Positron.app/Contents/Resources/app/quarto/bin/tools/aarch64/ (via rmarkdown)
quarto 1.8.24 @ /Applications/quarto/bin/quarto
─ Packages ───────────────────────────────────────────────────────────────────
! package * version date (UTC) lib source
P cli 3.6.5 2025-04-23 [?] RSPM (R 4.5.0)
P digest 0.6.37 2024-08-19 [?] RSPM (R 4.5.0)
P dplyr * 1.1.4 2023-11-17 [?] RSPM (R 4.5.0)
P evaluate 1.0.4 2025-06-18 [?] RSPM (R 4.5.1)
P farver 2.1.2 2024-05-13 [?] RSPM (R 4.5.0)
P fastmap 1.2.0 2024-05-15 [?] RSPM (R 4.5.0)
P forcats * 1.0.0 2023-01-29 [?] RSPM (R 4.5.0)
P generics 0.1.4 2025-05-09 [?] RSPM (R 4.5.0)
P ggplot2 * 3.5.2 2025-04-09 [?] RSPM (R 4.5.0)
P glue 1.8.0 2024-09-30 [?] RSPM (R 4.5.0)
P gtable 0.3.6 2024-10-25 [?] RSPM (R 4.5.0)
P here 1.0.1 2020-12-13 [?] RSPM (R 4.5.0)
P hms 1.1.3 2023-03-21 [?] RSPM (R 4.5.0)
P htmltools 0.5.8.1 2024-04-04 [?] RSPM (R 4.5.0)
P htmlwidgets 1.6.4 2023-12-06 [?] RSPM (R 4.5.0)
P jsonlite 2.0.0 2025-03-27 [?] RSPM (R 4.5.0)
P knitr 1.50 2025-03-16 [?] RSPM (R 4.5.0)
P labeling 0.4.3 2023-08-29 [?] RSPM (R 4.5.0)
P lifecycle 1.0.4 2023-11-07 [?] RSPM (R 4.5.0)
P lubridate * 1.9.4 2024-12-08 [?] RSPM (R 4.5.0)
P magrittr 2.0.3 2022-03-30 [?] RSPM (R 4.5.1)
P pillar 1.11.0 2025-07-04 [?] RSPM (R 4.5.1)
P pkgconfig 2.0.3 2019-09-22 [?] RSPM (R 4.5.0)
P purrr * 1.1.0 2025-07-10 [?] RSPM (R 4.5.0)
P R6 2.6.1 2025-02-15 [?] RSPM (R 4.5.0)
P RColorBrewer 1.1-3 2022-04-03 [?] RSPM (R 4.5.0)
P readr * 2.1.5 2024-01-10 [?] RSPM (R 4.5.0)
P renv 1.1.5 2025-07-24 [?] RSPM
P rlang 1.1.6 2025-04-11 [?] RSPM (R 4.5.0)
P rmarkdown 2.29 2024-11-04 [?] RSPM
P rprojroot 2.1.0 2025-07-12 [?] RSPM (R 4.5.0)
P scales 1.4.0 2025-04-24 [?] RSPM (R 4.5.0)
P sessioninfo 1.2.3 2025-02-05 [?] RSPM (R 4.5.0)
P stringi 1.8.7 2025-03-27 [?] RSPM (R 4.5.0)
P stringr * 1.5.1 2023-11-14 [?] RSPM (R 4.5.1)
P tibble * 3.3.0 2025-06-08 [?] RSPM (R 4.5.0)
P tidyr * 1.3.1 2024-01-24 [?] RSPM (R 4.5.0)
P tidyselect 1.2.1 2024-03-11 [?] RSPM (R 4.5.0)
P tidyverse * 2.0.0 2023-02-22 [?] RSPM (R 4.5.0)
P timechange 0.3.0 2024-01-18 [?] RSPM (R 4.5.0)
P tzdb 0.5.0 2025-03-15 [?] RSPM (R 4.5.0)
P utf8 1.2.6 2025-06-08 [?] RSPM (R 4.5.0)
P vctrs 0.6.5 2023-12-01 [?] RSPM (R 4.5.0)
P withr 3.0.2 2024-10-28 [?] RSPM (R 4.5.0)
P xfun 0.52 2025-04-02 [?] RSPM (R 4.5.1)
P yaml 2.3.10 2024-07-26 [?] RSPM (R 4.5.0)
[1] /Users/bcs88/Projects/info-5001/course-site/renv/library/macos/R-4.5/aarch64-apple-darwin20
[2] /Users/bcs88/Library/Caches/org.R-project.R/R/renv/sandbox/macos/R-4.5/aarch64-apple-darwin20/4cd76b74
* ── Packages attached to the search path.
P ── Loaded and on-disk path mismatch.
──────────────────────────────────────────────────────────────────────────────