Date: 2026-07-05

# Argument Matching

#### 2026-07-05

# Ignoring case

## `base`

The base function `match.arg()` is good, but it doesn’t offer the possiblity to ignore case during argument matching. Sometimes it’s good to ignore case; for example, if you’re matching the arguments `c("yes", "no")`, there’s no need to worry about case.

    base::match.arg("Y", c("yes", "no"))
    #> Error in base::match.arg("Y", c("yes", "no")): 'arg' should be one of "yes", "no"

## `strex`

The default behaviour of `strex::match_arg()` is to observe case, but case ignorance can be turned on with `ignore_case = TRUE`.

    strex::match_arg("Y", c("yes", "no"))
    #> Error in `str_match_arg_basic()`:
    #> ! `Y` must be a prefix of exactly one element of `choices`.
    #> ℹ Your `choices` are yes, no.
    #> ✖ Your `Y`, 'Y', is not a prefix of any of your `choices`.
    strex::match_arg("Y", c("yes", "no"), ignore_case = TRUE)
    #> [1] "yes"

# Error Messages

You can begin to see above that the error message from `strex::match_arg()` are more informative and nicely formatted. Here are a few more examples.

## No matches

    choices <- c("Apples", "Pears", "Bananas", "Oranges", "Avocados", "Apricots")
    match.arg("Q", choices)
    #> Error in match.arg("Q", choices): 'arg' should be one of "Apples", "Pears", "Bananas", "Oranges", "Avocados", "Apricots"
    strex::match_arg("Q", choices)
    #> Error in `str_match_arg_basic()`:
    #> ! `Q` must be a prefix of exactly one element of `choices`.
    #> ℹ Your `choices` are Apples, Pears, Bananas, Oranges, Avocados, Apricots.
    #> ✖ Your `Q`, 'Q', is not a prefix of any of your `choices`.

## Multiple matches

    match.arg("A", choices)
    #> Error in match.arg("A", choices): 'arg' should be one of "Apples", "Pears", "Bananas", "Oranges", "Avocados", "Apricots"
    strex::match_arg("A", choices)
    #> Error in `str_match_arg_basic()`:
    #> ! `arg` must be a prefix of exactly one element of `choices`.
    #> ✖ Your `arg`, 'A', is a prefix of two or more elements of `choices`.
    #> ℹ The first two of these are 'Apples' and 'Avocados'.

## Wrong `arg` length

    match.arg(c("A", "a"), choices)
    #> Error in match.arg(c("A", "a"), choices): 'arg' must be of length 1
    strex::match_arg(c("A", "a"), choices)
    #> Error in `str_match_arg_basic()`:
    #> ! `arg` must have length 1.
    #> ✖ Your `arg` has length 2.
    #> ℹ To use an `arg` with length greater than one, use `several_ok = TRUE`.

## Duplicate elements in `choices`

    choices <- c(choices, "Pears")
    match.arg("P", choices)
    #> Error in match.arg("P", choices): 'arg' should be one of "Apples", "Pears", "Bananas", "Oranges", "Avocados", "Apricots"
    strex::match_arg("P", choices)
    #> Error in `str_match_arg_basic()`:
    #> ! `choices` must not have duplicate elements.
    #> • Element 7 of your `choices`, 'Pears', is a duplicate.

## Not specifying `choices`

It’s OK not to specify choices in one circumstance: when `arg` is passed as a default argument of another function.

    myword <- function(w = c("abacus", "baseball", "candy")) {
      w <- strex::match_arg(w)
      w
    }
    myword()
    #> [1] "abacus"
    myword("b")
    #> [1] "baseball"
    myword("c")
    #> [1] "candy"

This is very strict though, only the symbol for the default argument can be passed, not any variant of it, not even something which evaluates to the same thing.

    myword <- function(w = c("abacus", "baseball", "candy")) {
      w <- strex::match_arg(identity(w))
      w
    }
    myword("b")
    #> Error in `strex::match_arg()`:
    #> ! You used `match_arg()` without specifying a `choices` argument.
    #> ℹ The only way to do this is from another function where the `arg` has a default setting. This is the same as `base::match.arg()`.
    #> ℹ See the man page for `match_arg()`.
    #> ℹ See the vignette on argument matching: enter `vignette('argument-matching', package = 'strex')` at the R console.
    myword <- function(w = c("abacus", "baseball", "candy")) {
      w <- strex::match_arg(as.character(w))
      w
    }
    myword("b")
    #> Error in `strex::match_arg()`:
    #> ! You used `match_arg()` without specifying a `choices` argument.
    #> ℹ The only way to do this is from another function where the `arg` has a default setting. This is the same as `base::match.arg()`.
    #> ℹ See the man page for `match_arg()`.
    #> ℹ See the vignette on argument matching: enter `vignette('argument-matching', package = 'strex')` at the R console.