case_match: A general vectorised 'switch()'
In hadley/dplyr: A Grammar of Data Manipulation

case_match

R Documentation

A general vectorised `switch()`

Description

This function allows you to vectorise multiple switch() statements. Each case is evaluated sequentially and the first match for each element determines the corresponding value in the output vector. If no cases match, the .default is used.

case_match() is an R equivalent of the SQL "simple" ⁠CASE WHEN⁠ statement.

Connection to `case_when()`

While case_when() uses logical expressions on the left-hand side of the formula, case_match() uses values to match against .x with. The following two statements are roughly equivalent:

case_when(
  x %in% c("a", "b") ~ 1,
  x %in% "c" ~ 2,
  x %in% c("d", "e") ~ 3
)

case_match(
  x,
  c("a", "b") ~ 1,
  "c" ~ 2,
  c("d", "e") ~ 3
)

Usage

case_match(.x, ..., .default = NULL, .ptype = NULL)

Arguments

`.x`	A vector to match against.
`...`	<`dynamic-dots`> A sequence of two-sided formulas: `old_values ~ new_value`. The right hand side (RHS) determines the output value for all values of `.x` that match the left hand side (LHS). The LHS must evaluate to the same type of vector as `.x`. It can be any length, allowing you to map multiple `.x` values to the same RHS value. If a value is repeated in the LHS, i.e. a value in `.x` matches to multiple cases, the first match is used. The RHS inputs will be coerced to their common type. Each RHS input will be recycled to the size of `.x`.
`.default`	The value used when values in `.x` aren't matched by any of the LHS inputs. If `NULL`, the default, a missing value will be used. `.default` is recycled to the size of `.x`.
`.ptype`	An optional prototype declaring the desired output type. If not supplied, the output type will be taken from the common type of all RHS inputs and `.default`.

Value

A vector with the same size as .x and the same type as the common type of the RHS inputs and .default (if not overridden by .ptype).

Examples

x <- c("a", "b", "a", "d", "b", NA, "c", "e")

# `case_match()` acts like a vectorized `switch()`.
# Unmatched values "fall through" as a missing value.
case_match(
  x,
  "a" ~ 1,
  "b" ~ 2,
  "c" ~ 3,
  "d" ~ 4
)

# Missing values can be matched exactly, and `.default` can be used to
# control the value used for unmatched values of `.x`
case_match(
  x,
  "a" ~ 1,
  "b" ~ 2,
  "c" ~ 3,
  "d" ~ 4,
  NA ~ 0,
  .default = 100
)

# Input values can be grouped into the same expression to map them to the
# same output value
case_match(
  x,
  c("a", "b") ~ "low",
  c("c", "d", "e") ~ "high"
)

# `case_match()` isn't limited to character input:
y <- c(1, 2, 1, 3, 1, NA, 2, 4)

case_match(
  y,
  c(1, 3) ~ "odd",
  c(2, 4) ~ "even",
  .default = "missing"
)

# Setting `.default` to the original vector is a useful way to replace
# selected values, leaving everything else as is
case_match(y, NA ~ 0, .default = y)

starwars %>%
  mutate(
    # Replace missings, but leave everything else alone
    hair_color = case_match(hair_color, NA ~ "unknown", .default = hair_color),
    # Replace some, but not all, of the species
    species = case_match(
      species,
      "Human" ~ "Humanoid",
      "Droid" ~ "Robot",
      c("Wookiee", "Ewok") ~ "Hairy",
      .default = species
    ),
    .keep = "used"
  )

hadley/dplyr documentation built on Nov. 6, 2024, 4:48 p.m.