Set Operations

Part of the Logic & Flow section of Coddy's R journey. Lesson 13 of 64.

Three functions combine two vectors as sets. Each returns values without repeats:

union(a, b) has the values in either vector, intersect(a, b) the values in both, and setdiff(a, b) the values of a that are not in b.

mon <- c("ana", "ben", "cy")
tue <- c("ben", "dee", "cy", "ben")
print(union(mon, tue))
print(intersect(mon, tue))
print(setdiff(mon, tue))

Output:

[1] "ana" "ben" "cy"  "dee"
[1] "ben" "cy" 
[1] "ana"

setdiff() depends on the order of its arguments. Swap them to get the values that are only in the second vector:

mon <- c("ana", "ben", "cy")
tue <- c("ben", "dee", "cy")
print(setdiff(tue, mon))

Output:

[1] "dee"

When nothing is left, the result is an empty vector, character(0) for text:

print(intersect(c("a", "b"), c("x", "y")))

Output:

character(0)

The results keep the order of the first vector. Sort them when the output needs a predictable order, and check for an empty result before joining:

a <- c("pear", "fig", "kiwi")
b <- c("kiwi", "pear")
both <- sort(intersect(a, b))
print(paste(both, collapse = ", "))
print(length(setdiff(b, a)) == 0)

Output:

[1] "kiwi, pear"
[1] TRUE

Challenge

Easy

Complete compare_courses(a, b). a and b are the students of two courses. Return three lines:

Both: followed by the students in both courses
Only first: followed by the students only in a
Total: followed by the number of different students across both courses

Sort the names in each list and join them with , . An empty list is written none.

The supplied code reads the two courses as comma-separated lines and prints each returned line.

Try it yourself

compare_courses <- function(a, b) {
  # Write your code here
  c("Both: ", "Only first: ", "Total: 0")
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
cat(compare_courses(strsplit(input[1], ",")[[1]], strsplit(input[2], ",")[[1]]), sep = "\n")