Blackjack

The switch() function can be a concise replacement for a long series of if ... else if tests. The variable being switched on is most commonly a string, and if so the quotes can be omitted from the selector.

star <- function(type) {
  switch(type,
    M = , # will "fall through" if no value given
    K = "red dwarf",
    G = "Earth-like",
    "bigger star" # only correct for O,B,A,F
  )
}

> star("M")
[1] "red dwarf"

Note that options will only fall through if the value is left blank, as with M in the example above. There is no need to include break statements as with some other languages.

When switching on character types, the final value is a default, as here. With integer types, no default can be given.

Functions in dplyr

The switch in base R is quite limited: it will only do an exact match to a single input. This seemed reasonable when R was first released in 1993, but needs improvement for modern usage.

The dplyr library provides two extra functions related to switch.

The recode_values and replace_values functions

These two related functions allow a vectorized switch-like mapping of old values to new values.

The main difference between them is that recode_values() creates an entirely new vector, while replace_values() allows partial updates of an existing vector.

Matching is still exact, but:

• The options on the left can be vectors, and the input matches if any element matches.
• Options on the left and results on the right are linked with a tilde ~ instead of =.
• A default can be specified in all cases.

library(dplyr)

x <- c("a", "b", "a", "d", "b", NA, "c", "e", "z")
recode_values(
  x,
  "a" ~ 1,
  "b" ~ 2,
  "c" ~ 3,
  c("d", "e") ~ 4, # either "d' or "e" will match
  NA ~ 0,          # matches missing values
  default = 100   # note the different syntax for the default
)
#> [1]   1   2   1   4   2   0   3   4 100

The case_when function

case_when takes case_match syntax a stage further, by allowing any logical expression on the left of the ~.

The input vector (x in the example below) is not supplied as an argument, it just needs to be already defined.

x <- 1:10
case_when(
  x < 3 ~ "low",
  between(x, 3, 5) ~ "mid",
  between(x, 6, 8) ~ "high",
  .default = "what?"
)
#>  [1] "low"   "low"   "mid"   "mid"   "mid"   "high"  "high"  "high"  "what?" "what?"

The between() function is also part of dplyr.