## 12.6 Advanced exercises

Here are some more advanced exercises on functional programming and applying functions to data structures (see Section 12.3):

### 12.6.1 Exercise A1

#### Punitive iterations revisited

This exercise asks to run and reflect upon some figures used in this chapter:

1. Run and explain the code shown in the loopy Bart memes of Figures 12.3 and 12.4.

2. Run and explain the code shown in the functional programming memes of Figures 12.5 and 12.6.

Note: The post Create Bart Simpson blackboard memes with R at the Learning Machines blog (by Holger K. von Jouanne-Diedrich) explains how to create your own memes.

### 12.6.2 Exercise A2

#### Star Wars creatures revisited

This exercise re-uses the starwars data of the dplyr package :

sws <- dplyr::starwars %>%
select(name:mass, species)

In Section 3.2.4, we learned how to use the mutate() function to compute someone’s height in feet (from a given height in centimeters) or their BMI (from given values of height and mass):

# Conversion factor (cm to feet):
factor_cm_2_feet <- 3.28084/100

# Using a mutate() pipe:
sws %>%
mutate(height_feet = factor_cm_2_feet * height,
BMI = mass / ((height / 100)  ^ 2),  # compute body mass index (kg/m^2)
BMI_low  = BMI < 18.5,               # classify low BMI values
BMI_high = BMI > 30,                 # classify high BMI values
BMI_norm = !BMI_low & !BMI_high      # classify normal BMI values
)
#> # A tibble: 87 × 9
#>    name         height  mass species height_feet   BMI BMI_low BMI_high BMI_norm
#>    <chr>         <int> <dbl> <chr>         <dbl> <dbl> <lgl>   <lgl>    <lgl>
#>  1 Luke Skywal…    172    77 Human          5.64  26.0 FALSE   FALSE    TRUE
#>  2 C-3PO           167    75 Droid          5.48  26.9 FALSE   FALSE    TRUE
#>  3 R2-D2            96    32 Droid          3.15  34.7 FALSE   TRUE     FALSE
#>  4 Darth Vader     202   136 Human          6.63  33.3 FALSE   TRUE     FALSE
#>  5 Leia Organa     150    49 Human          4.92  21.8 FALSE   FALSE    TRUE
#>  6 Owen Lars       178   120 Human          5.84  37.9 FALSE   TRUE     FALSE
#>  7 Beru Whites…    165    75 Human          5.41  27.5 FALSE   FALSE    TRUE
#>  8 R5-D4            97    32 Droid          3.18  34.0 FALSE   TRUE     FALSE
#>  9 Biggs Darkl…    183    84 Human          6.00  25.1 FALSE   FALSE    TRUE
#> 10 Obi-Wan Ken…    182    77 Human          5.97  23.2 FALSE   FALSE    TRUE
#> # … with 77 more rows

Using mutate() on the variables of a data table essentially allows computing variables on the fly. However, we often encounter situations in which the functions for computing variables have been defined elsewhere and only need to be applied to the variables in a table. The following steps simuluate this situation:

1. Create dedicated functions for computing:

• someone’s height in feet (from height in cm);
• someone’s body mass index (BMI, from their height and mass); and
• categorizing their BMI type (as in the mutate() command above).
1. Apply these functions to all individuals in sws by using appropriate variants of the apply() functions of base R.
1. Apply these functions to all individuals in sws by using appropriate map() functions from the purrr package .

This concludes our more advanced exercises on functional programming and applying functions to data structures.

### References

Henry, L., & Wickham, H. (2020). purrr: Functional programming tools. Retrieved from https://CRAN.R-project.org/package=purrr
Wickham, H., François, R., Henry, L., & Müller, K. (2022). Dplyr: A grammar of data manipulation. Retrieved from https://CRAN.R-project.org/package=dplyr