5.2 Choices

The basic form of an if statement in R is as follows:

if (condition) true_action
if (condition) true_action else false_action

If condition is TRUE, true_action is evaluated; if condition is FALSE, the optional false_action is evaluated.

Typically the actions are compound statements contained within {:

grade <- function(x) {
  if (x > 90) {
    "A"
  } else if (x > 80) {
    "B"
  } else if (x > 50) {
    "C"
  } else {
    "F"
  }
}

if returns a value so that you can assign the results:

x1 <- if (TRUE) 1 else 2
x2 <- if (FALSE) 1 else 2

c(x1, x2)
#> [1] 1 2

(I recommend assigning the results of an if statement only when the entire expression fits on one line; otherwise it tends to be hard to read.)

When you use the single argument form without an else statement, if invisibly (Section 6.7.2) returns NULL if the condition is FALSE. Since functions like c() and paste() drop NULL inputs, this allows for a compact expression of certain idioms:

greet <- function(name, birthday = FALSE) {
  paste0(
    "Hi ", name,
    if (birthday) " and HAPPY BIRTHDAY"
  )
}
greet("Maria", FALSE)
#> [1] "Hi Maria"
greet("Jaime", TRUE)
#> [1] "Hi Jaime and HAPPY BIRTHDAY"

5.2.1 Invalid inputs

The condition should evaluate to a single TRUE or FALSE. Most other inputs will generate an error:

if ("x") 1
#> Error in if ("x") 1: argument is not interpretable as logical
if (logical()) 1
#> Error in if (logical()) 1: argument is of length zero
if (NA) 1
#> Error in if (NA) 1: missing value where TRUE/FALSE needed

The exception is a logical vector of length greater than 1, which generates a warning:

if (c(TRUE, FALSE)) 1
#> Warning in if (c(TRUE, FALSE)) 1: the condition has length > 1 and only the
#> first element will be used
#> [1] 1

In R 3.5.0 and greater, thanks to Henrik Bengtsson, you can turn this into an error by setting an environment variable:

Sys.setenv("_R_CHECK_LENGTH_1_CONDITION_" = "true")
if (c(TRUE, FALSE)) 1
#> Error in if (c(TRUE, FALSE)) 1: the condition has length > 1

I think this is good practice as it reveals a clear mistake that you might otherwise miss if it were only shown as a warning.

5.2.2 Vectorised if

Given that if only works with a single TRUE or FALSE, you might wonder what to do if you have a vector of logical values. Handling vectors of values is the job of ifelse(): a vectorised function with test, yes, and no vectors (that will be recycled to the same length):

x <- 1:10
ifelse(x %% 5 == 0, "XXX", as.character(x))
#>  [1] "1"   "2"   "3"   "4"   "XXX" "6"   "7"   "8"   "9"   "XXX"

ifelse(x %% 2 == 0, "even", "odd")
#>  [1] "odd"  "even" "odd"  "even" "odd"  "even" "odd"  "even" "odd"  "even"

Note that missing values will be propagated into the output.

I recommend using ifelse() only when the yes and no vectors are the same type as it is otherwise hard to predict the output type. See https://vctrs.r-lib.org/articles/stability.html#ifelse for additional discussion.

Another vectorised equivalent is the more general dplyr::case_when(). It uses a special syntax to allow any number of condition-vector pairs:

dplyr::case_when(
  x %% 35 == 0 ~ "fizz buzz",
  x %% 5 == 0 ~ "fizz",
  x %% 7 == 0 ~ "buzz",
  is.na(x) ~ "???",
  TRUE ~ as.character(x)
)
#>  [1] "1"    "2"    "3"    "4"    "fizz" "6"    "buzz" "8"    "9"    "fizz"

5.2.3 `switch()` statement

Closely related to if is the switch()-statement. It’s a compact, special purpose equivalent that lets you replace code like:

x_option <- function(x) {
  if (x == "a") {
    "option 1"
  } else if (x == "b") {
    "option 2" 
  } else if (x == "c") {
    "option 3"
  } else {
    stop("Invalid `x` value")
  }
}

with the more succinct:

x_option <- function(x) {
  switch(x,
    a = "option 1",
    b = "option 2",
    c = "option 3",
    stop("Invalid `x` value")
  )
}

The last component of a switch() should always throw an error, otherwise unmatched inputs will invisibly return NULL:

(switch("c", a = 1, b = 2))
#> NULL

If multiple inputs have the same output, you can leave the right hand side of = empty and the input will “fall through” to the next value. This mimics the behaviour of C’s switch statement:

legs <- function(x) {
  switch(x,
    cow = ,
    horse = ,
    dog = 4,
    human = ,
    chicken = 2,
    plant = 0,
    stop("Unknown input")
  )
}
legs("cow")
#> [1] 4
legs("dog")
#> [1] 4

It is also possible to use switch() with a numeric x, but is harder to read, and has undesirable failure modes if x is a not a whole number. I recommend using switch() only with character inputs.

5.2.4 Exercises

What type of vector does each of the following calls to ifelse() return?
```
ifelse(TRUE, 1, "no")
ifelse(FALSE, 1, "no")
ifelse(NA, 1, "no")
```
Read the documentation and write down the rules in your own words.

Why does the following code work?

x <- 1:10
if (length(x)) "not empty" else "empty"
#> [1] "not empty"

x <- numeric()
if (length(x)) "not empty" else "empty"
#> [1] "empty"

5.2 Choices

5.2.1 Invalid inputs

5.2.2 Vectorised if

5.2.3 switch() statement

5.2.4 Exercises

5.2.3 `switch()` statement