Match/validate a set of conditions for an object and continue with the action associated with the first valid match.

Description

when is a flavour of pattern matching (or an if-else abstraction) in which a value is matched against a sequence of condition-action sets. When a valid match/condition is found the action is executed and the result of the action is returned.

Usage

when(., ...)

Arguments

Value

The value resulting from the action of the first valid match/condition is returned. If no matches are found, and no default is given, NULL will be returned.

Validity of the conditions are tested with isTRUE, or equivalently with identical(condition, TRUE). In other words conditions resulting in more than one logical will never be valid. Note that the input value is always treated as a single object, as opposed to the ifelse function.

Examples

1:10 %>%
  when(
    sum(.) <=  50 ~ sum(.),
    sum(.) <= 100 ~ sum(.)/2,
    ~ 0
  )

1:10 %>%
  when(
    sum(.) <=   x ~ sum(.),
    sum(.) <= 2*x ~ sum(.)/2,
    ~ 0,
    x = 60
  )

iris %>%
  subset(Sepal.Length > 10) %>%
  when(
    nrow(.) > 0 ~ .,
    ~ iris %>% head(10)
  )

iris %>%
  head %>%
  when(nrow(.) < 10 ~ .,
       ~ stop("Expected fewer than 10 rows."))