Catching with tryCatch()

Part of the Logic & Flow section of Coddy's R journey. Lesson 53 of 64.

tryCatch(expr, error = function(e) ...) runs expr. If it raises an error, the handler runs instead and its value becomes the result, and the script continues:

safe_sqrt <- function(x) {
  tryCatch(sqrt(x), error = function(e) NA)
}
print(safe_sqrt(16))
print(safe_sqrt("a"))
print("still running")

Output:

[1] 4
[1] NA
[1] "still running"

The handler receives the error as e, and conditionMessage(e) returns its message text:

result <- tryCatch(
  stop("file not found"),
  error = function(e) paste("Problem:", conditionMessage(e))
)
print(result)

Output:

[1] "Problem: file not found"

Wrap only the call that can fail, and catch it where you can do something useful, such as a default value or a message for the user:

to_number <- function(s) {
  tryCatch({
    if (s == "") stop("empty input")
    as.integer(s)
  }, error = function(e) -1L)
}
print(to_number("42"))
print(to_number(""))

Output:

[1] 42
[1] -1

In a loop, tryCatch() lets one bad item fail while the others are still processed:

inputs <- list(4, "x", 9)
for (v in inputs) {
  r <- tryCatch(sqrt(v), error = function(e) "not a number")
  cat(format(v), "->", r, "\n")
}

Output:

4 -> 2 
x -> not a number 
9 -> 3

Challenge

Easy

Complete safe_divide(a, b). Use tryCatch(): inside it, raise the error division by zero when b is 0 and otherwise compute a / b. The handler returns Error: followed by the message. Return the result rounded to two decimals as text, 3.33, or the error text.

The supplied code reads pairs of numbers such as 10/3, separated by commas, and prints the result of each pair on its own line.

Try it yourself

safe_divide <- function(a, b) {
  # Write your code here
  ""
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
for (pair in strsplit(input[1], ",")[[1]]) {
  p <- as.numeric(strsplit(pair, "/")[[1]])
  cat(safe_divide(p[1], p[2]), "\n", sep = "")
}