11 Scripts, algorithms and functions

The solutions assume the following packages are attached (other packages will be attached when needed):

library(sf)

E1. Read the script 11-centroid-alg.R in the code folder of the book’s GitHub repository.

• Which of the best practices covered in Section ?? does it follow?
• Create a version of the script on your computer in an IDE such as RStudio (preferably by typing the script line-by-line, in your own coding style and with your own comments, rather than copy-pasting — this will help you learn how to type scripts). Using the example of a square polygon (e.g., created with poly_mat = cbind(x = c(0, 9, 9, 0, 0), y = c(0, 0, 9, 9, 0))) execute the script line-by-line.
• What changes could be made to the script to make it more reproducible?
• How could the documentation be improved?

The script is stored in a logical location with a sensible file name. The script is well documented with comments and the code is well formatted. The script is reproducible.

Open a file and create a new script in RStudio, e.g., with the keyboard shortcut Ctrl + Shift + N (Windows) or Cmd + Shift + N (Mac), by clicking File > New File > R Script or by clicking the + icon in the top left of the Source pane. You can also create a new R script from the R console with the command file.create("11-centroid-alg.R").

The script is already reproducible, with a message stating that it needs an object called poly_mat to be present and, if none is present, it creates an example dataset at the outset for testing. For people new to R it could also contain a comment stating that R must be installed before running the script.

Documentation could be improved with a more detailed description of the algorithm, including a link to the relevant section of the book. Furthermore, the anonymous functions could be replaced with named functions and documented with Roxygen2 comments.

E2. In the geometric algorithms section, we calculated that the area of the polygon poly_mat was 245 units squared and that its centroid as at the coordinates (8.8, 9.2).

• Reproduce the results on your own computer with reference to the script 11-centroid-alg.R, an implementation of this algorithm (bonus: type the commands - try to avoid copy-pasting).
• Are the results correct? Verify them by converting poly_mat into an sfc object (named poly_sfc) with st_polygon() (hint: this function takes objects of class list()) and then using st_area() and st_centroid().

# We can verify the answer by converting `poly_mat` into a simple feature collection
# as follows, which shows the calculations match:
x_coords = c(10, 20, 12, 0, 0, 10)
y_coords = c(0, 15, 20, 10, 0, 0)
poly_mat = cbind(x_coords, y_coords)
poly_sfc = sf::st_polygon(list(poly_mat))
sf::st_area(poly_sfc)
sf::st_centroid(poly_sfc)
# By calling the script:
# source("https://github.com/geocompx/geocompr/raw/main/code/11-centroid-alg.R")

E3. It was stated that the algorithm we created only works for convex hulls. Define convex hulls (see the geometry operations chapter) and test the algorithm on a polygon that is not a convex hull.

x_coords = c(10, 20, 12, 0, 0, 5, 10)
y_coords = c(0, 15, 20, 10, 0, 5, 0)
plot(x_coords, y_coords, type = "l")
poly_mat = cbind(x_coords, y_coords)
# source("https://github.com/geocompx/geocompr/raw/main/code/11-centroid-alg.R")
# Area from our script: 270
poly_sfc = sf::st_polygon(list(poly_mat))
sf::st_area(poly_sfc) # Actual area: 220

• Bonus 1: Think about why the method only works for convex hulls and note changes that would need to be made to the algorithm to make it work for other types of polygon.
• Bonus 2: Building on the contents of 11-centroid-alg.R, write an algorithm only using base R functions that can find the total length of linestrings represented in matrix form.

The algorithm would need to be able to have negative as well as positive area values.

We leave Bonus 2 as an exercise for the reader.

E4. In the functions section, we created different versions of the poly_centroid() function that generated outputs of class sfg (poly_centroid_sfg()) and type-stable matrix outputs (poly_centroid_type_stable()). Further extend the function by creating a version (e.g., called poly_centroid_sf()) that is type stable (only accepts inputs of class sf) and returns sf objects (hint: you may need to convert the object x into a matrix with the command sf::st_coordinates(x)).

• Verify if it works by running poly_centroid_sf(sf::st_sf(sf::st_sfc(poly_sfc)))
• What error message do you get when you try to run poly_centroid_sf(poly_mat)?

poly_centroid_sf = function(x) {
  stopifnot(is(x, "sf"))
  xcoords = sf::st_coordinates(x)
  centroid_coords = poly_centroid(xcoords)
  centroid_sf = sf::st_sf(geometry = sf::st_sfc(sf::st_point(centroid_coords)))
  centroid_sf
}
poly_centroid_sf(sf::st_sf(sf::st_sfc(poly_sfc)))
poly_centroid_sf(poly_sfc)
poly_centroid_sf(poly_mat)