3 Functions

3.1 Naming

As well as following the general advice for object names, strive to use verbs for function names:

# Good
add_row()
permute()

# Bad
row_adder()
permutation()

3.2 Long lines

If a function definition runs over multiple lines, indent the second line to where the definition starts.

# Good
long_function_name = function(a = "a long argument",
                               b = "another argument",
                               c = "another long argument") {
  # As usual code is indented by two spaces.
}

# Bad
long_function_name = function(a = "a long argument",
  b = "another argument",
  c = "another long argument") {
  # Here it's hard to spot where the definition ends and the
  # code begins
}

3.3 return()

Use return(). Do not rely on R to return the result of the last evaluated expression.

# Good
find_abs = function(x) {
  if (x > 0) {
    return(x)
  }
  return(x * -1)
}
add_two = function(x, y) {
  return(x + y)
}

# Bad
add_two = function(x, y) {
  x + y
}

Return statements should always be on their own line because they have important effects on the control flow. See also inline statements.

# Good
find_abs = function(x) {
  if (x > 0) {
    return(x)
  }
  x * -1
}

# Bad
find_abs = function(x) {
  if (x > 0) return(x)
  x * -1
}

If your function is called primarily for its side-effects (like printing, plotting, or saving to disk), it should return the first argument invisibly. This makes it possible to use the function as part of a pipe. print methods should usually do this, like this example from httr:

print.url = function(x, ...) {
  cat("Url: ", build_url(x), "\n", sep = "")
  invisible(x)
}

3.4 Comments

In code, use comments to explain the “why” not the “what” or “how”. Each line of a comment should begin with the comment symbol and a single space: #.

# Good

# Objects like data frames are treated as leaves
x = map_if(x, is_bare_list, recurse)


# Bad

# Recurse only with bare lists
x = map_if(x, is_bare_list, recurse)

Comments should be in sentence case, and only end with a full stop if they contain at least two sentences:

# Good

# Objects like data frames are treated as leaves
x = map_if(x, is_bare_list, recurse)

# Do not use `is.list()`. Objects like data frames must be treated
# as leaves.
x = map_if(x, is_bare_list, recurse)


# Bad

# objects like data frames are treated as leaves
x = map_if(x, is_bare_list, recurse)

# Objects like data frames are treated as leaves.
x = map_if(x, is_bare_list, recurse)