Substrings with substr()

Part of the Logic & Flow section of Coddy's R journey. Lesson 1 of 64.

substr(x, start, stop) returns the characters of x from position start to position stop, both included. Positions start at 1, like vector indexes.

word <- "programming"
print(substr(word, 1, 7))
print(substr(word, 4, 6))

Output:

[1] "program"
[1] "gra"

To take characters from the end, compute the positions with nchar():

code <- "INV-2024-0042"
n <- nchar(code)
print(substr(code, n - 3, n))

Output:

[1] "0042"

A stop past the end is cut to the last character, and a start greater than stop gives an empty string:

print(substr("cat", 2, 10))
print(substr("cat", 3, 1))

Output:

[1] "at"
[1] ""

substr() works on every element of a character vector at once:

days <- c("Monday", "Tuesday", "Wednesday")
print(substr(days, 1, 3))

Output:

[1] "Mon" "Tue" "Wed"

Assigning to substr() overwrites characters in place. The string keeps its length:

id <- "user-0000"
substr(id, 6, 9) <- "1234"
print(id)

Output:

[1] "user-1234"

Challenge

Easy

Complete short_code(city) so it returns the first three letters of city in capital letters. For london it returns LON.

The supplied code reads the city from the input and prints the returned value on its own line.

Try it yourself

short_code <- function(city) {
  # Write your code here
  ""
}

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